Power Resilience Enhancement of a Residential Electricity User Using Photovoltaics and a Battery Energy Storage System Under Uncertainty Conditions

energies

Article Power Resilience Enhancement of a Residential Electricity User Using Photovoltaics and a Battery Energy Storage System under Uncertainty Conditions

Nallapaneni Manoj Kumar 1 , Aritra Ghosh 2,3,* and Shauhrat S. Chopra 1,* 1 School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong; [email protected] 2 Environment and Sustainability Institute, University of Exeter, Penryn, Cornwall TR10 9FE, UK 3 College of Engineering, Mathematics and Physical Sciences, Renewable Energy, University of Exeter, Cornwall TR10 9FE, UK * Correspondence: [email protected] (A.G.); [email protected] (S.S.C.)

Received: 30 June 2020; Accepted: 10 August 2020; Published: 13 August 2020

Abstract: Even in today’s modern electric grid infrastructure, the uncertainty in the power supply is more often seen and is mainly due to power outages. The reasons for power outages might be any of the following: extreme weather events, asset failure, natural disasters, power surges, acute accidents, and even operational errors by the workforce. Such uncertain situations are permitting us to think of it as a resilience problem. In most cases, the power outages may last from a few minutes to a few weeks, depending on the nature of the resilience issue and the power supply system (PSS) configuration. Therefore, it is imperative to understand and improve the resilience of a PSS. In this paper, a four-component resilience framework is proposed to study and compare the resilience of three different PSS configurations of residential electricity users (REUs) considering the realistic power outage conditions in the humid subtropical ecosystem. The proposed PSS configurations contain electric grid (EG), natural gas power generator (NGPG), battery energy storage (BES), and photovoltaics (PV) as the assets. The three PSS configurations of a REUs are EG + BES, EG + NGPG + BES, and EG + PV + BES, respectively, and in these, one REU is only the consumer and the other two REUs are prosumers. By using the proposed framework, simulations are performed on the three PSS configuration to understand the increasing load resiliency in the event of a power outage. Also, a comparative techno-economic and life cycle based environmental assessment is performed to select the most resilient PSS configuration among the EG + BES, EG + NGPG + BES, and EG + PV + BES for an REU. From the results, it was established that EG + PV + BES configuration would enhance the power resilience of an REU better than the other two PSS configurations. Besides, it is also observed that the identified resilient PSS configuration is cost-effective and environmentally efficient. Overall, the proposed framework will enable the REUs to opt for the PSS configuration that is resilient and affordable.

Keywords: energy resilience; four components of resilience; power outages; power supply system; photovoltaics; battery energy storage; techno-economic modeling; environmental analysis; microgrid; prosumer; resilience framework

1. Introduction In general, the conventional electric grid is a centralized system that connects the power output from many fossil and non-fossil fuel-based power plants to one and transmits the power steadily to electricity users. The most commonly seen electricity users are residential ones. The electric grid (EG) transmits power from remote areas (where the power plants are located) over a long distance to

Energies 2020, 13, 4193; doi:10.3390/en13164193 www.mdpi.com/journal/energies Energies 2020, 13, 4193 2 of 27 the residential electricity demand centers. This allows power flow only in one direction from high voltage to a low voltage level, and then it is distributed to the electricity consumers, who are typically called residential electricity users (REUs), but in today’s trend, the role of renewable energy (RE) has become very crucial in EG. RE, in most cases, is localized, and the power generation capacities vary from small to megawatt-scale. In most cases, the generated voltages from the RE-based power plants are small relative to those of conventional power plants. Even though RE-based power plants are feasible for local power generation, their integration into a large and highly centralized grid poses a limitation to their use and overall, it is understood that this is challenging [1]. This is due to the frequent fluctuation and, as a result, frequent loss of generation occasioned by the intermittent nature and variability of RE. Besides, it is further compounded by the reverse power flow that may occur in a conventional EG that was originally designed to allow power flow only in one direction, but later, with the advancements seen in distributed energy resources (DERs) and grid integration regulations, the deployment of RE into EG has become a feasible option [1,2]. Today, the trend for the use of DERs, particularly RE for electric power generation both at the off-site and on-site load centers, has increased exponentially across the globe. The growth in RE use is due to the need for decarbonization in the power sector. Besides, environmental friendliness and energy security, as well as consistent improvement in technology and falling cost of RE are also favoring this growth [3]. Now with the support of DERs and other improvements seen in energy-related technologies like power electronics and power system control and operations, the conventional EG has taken a paradigm shift towards modernization. Even after modernization, the EG continues to experience uncertainties that directly or indirectly affect the energy consumption patterns of REUs [4]. The uncertainty in supplying power to REUs is more often seen and is mainly due to power outages. The reasons for power outages might be any of the following: extreme weather events, asset failure, natural disasters, power surges, acute accidents, and even the operational errors by the workforce [5]. Such uncertain situations are permitting us to think of this as a resilience problem. In most cases, the power outages may last from a few minutes to weeks, depending on the nature of the resilience issue and the power supply system (PSS) configuration. Thus, the power outage situations result in the complete unavailability of the power to REUs. From the resilient PSS point of view during the power outages, power availability has to be ensured, and it can be done in many ways. Indeed, there are numerous ways to understand resilience, and these would depend on the nature of the system. Recent studies, for this reason, have called for engineering greater resilience especially in the power and other industrial systems. A few studies have employed network analysis to understand the resilience and sustainability of industrial symbiosis system that facilitates energy, water and material flows [6,7]. The network analysis is further extended and applied in few critical infrastructures in the United States of America (USA) and United Kingdom (UK) (for e.g.,: energy resources and power sector, information technology and communication, finance, healthcare and public health, transportation, and food and agriculture) to understand the implications of interconnectedness and interdependencies on resilience [8,9]. But when it comes to power sector, metrics-based approaches are used [10]. More often the mitigation strategies for enhancing the resilience of power systems are based on short term and long-term measures adopted [11]. However, in the literature, many authors have varying opinions on the way to ensure resilience, and it is often considered as an issue of reliability. Few studies exist in the literature where researchers have discussed the difference between reliability and resilience of the power supply system (PSS). Reliability more often deals with component failure, but the resilience of PSS is different, and it often deals with the capability of a PSS to sustain and bounce back to normal operation after any unseen or unexpected uncertainty [12]. This means the resilience assessment admits the possibility of PSS failure and focuses on its recovery and adaptation, thereby ensures continuous power supply to the REUs. The U.S. National Academy of Sciences defines resilience as the ability to plan for, recover from, and adapt to adverse events over time [13], but the most recent definition of resilience from the literature is the ability of a system to sustain, to rapidly recover, and learn to adapt its structure to unexpected Energies 2020, 13, 4193 3 of 27 Energies 2020, 13, x 3 of 26 Energies 2020, 13, x 3 of 26

disruptiveunexpected events disruptive [14]. events Based [14]. on the Based above on the deﬁnitions above definitions of resilience, of resilience, it is clear it that, is clear for that, any for sought any unexpected disruptive events [14]. Based on the above definitions of resilience, it is clear that, for any ofsought disruption, of disruption, the PSS the should PSS beshould able tobe recover able to soonrecover and soon provide and theprovide power the to power REUs, butto REUs, ensuring but sought of disruption, the PSS should be able to recover soon and provide the power to REUs, but resilienceensuring isresilience quite di ﬃiscult quite with difficult a conventional with a PSS.conventional Even with modernPSS. Even electricity with modern infrastructure, electricity the ensuring resilience is quite difficult with a conventional PSS. Even with modern electricity disruptiveinfrastructure, events the aredisruptive still occurring, events are and still resilience occurring, is aand big resilience question. is Fora big instance, question. irrespective For instance, of infrastructure, the disruptive events are still occurring, and resilience is a big question. For instance, whetherirrespective a nation of whether is developed a nation or developing, is developed the or power developing, outage incidents the power are happeningoutage incidents across theare irrespective of whether a nation is developed or developing, the power outage incidents are globe.happening From across 1960 the to 2019, globe. across From the 1960 globe, to 2019, thousands across the of powerglobe, thousands outage events of power were recorded,outage events and happening across the globe. From 1960 to 2019, across the globe, thousands of power outage events amongwere recorded, these, a fewand are among very these, massive a few events are that very lasted mass forive moreevents than that a month.lasted for In more Figure than1, a heata month. map were recorded, and among these, a few are very massive events that lasted for more than a month. showingIn Figure the1, a locations heat map where showing power the outageslocations events where have power occurred outages across events the have globe occurred is indicated across [15 the]. Inglobe Figure is indicated 1, a heat [15].map showing the locations where power outages events have occurred across the globe is indicated [15].

Figure 1. Heat map highlighting locations where power outages occurred across the globe [[15].15]. Figure 1. Heat map highlighting locations where power outages occurred across the globe [15]. From the heat map shown in Figure1 1,, itit isis understood understood thatthat thethe USA.’sUSA.’s EG EG experiences experiences thesethese From the heat map shown in Figure 1, it is understood that the USA.’s EG experiences these power outages most often. A recent recent study, on signif signiﬁcanticant power outages across the USA, also reported power outages most often. A recent study, on significant power outages across the USA, also reported the data on power outages events and the aaffectedﬀected populationpopulation [[16].16]. A list of power outage events due the data on power outages events and the affected population [16]. A list of power outage events due to numerous resilienceresilience issuesissues acrossacross thethe USAUSA areare shownshown inin FigureFigure2 2[17 [17].]. to numerous resilience issues across the USA are shown in Figure 2 [17].

Figure 2. The number of power outage events in each state of the USA [17]. FigureFigure 2. 2. TheThe number number of of power power outage outage events events in in each each state state of of the the USA USA [17]. [17]. From Figure 2, it can be seen that the number of power outage events occurred are different for eachFrom state inFigure the USA 2, it canThe be least seen number that the of number power outageof power events outage occurred events areoccurred eight inare Rhode different Island, for eachand thestate maximum in the USA number The least of power number outage of power events outage occurred events are occurred537 in California. are eight As in perRhode the Island,United andStates the Department maximum numberof Energy of (US power DoE) outage statistics, events during occurred one suchare 537 major in California. power outage, As per at leastthe United 50,000 States Department of Energy (US DoE) statistics, during one such major power outage, at least 50,000

Energies 2020, 13, 4193 4 of 27

From Figure2, it can be seen that the number of power outage events occurred are di fferent for each state in the USA The least number of power outage events occurred are eight in Rhode Island, and the maximum number of power outage events occurred are 537 in California. As per the United States Department of Energy (US DoE) statistics, during one such major power outage, at least 50,000 customers have been impacted, and approximately 300 MW unplanned firm load loss is experienced [17]. Moreover, these power outages will have a significant impact on society, residential, and industrial operations. The effect would depend upon the frequency of power outage occurrence at a particular location. From Figure2, it is clear there is a significant impact in most states of the USA, and hence there is scope for researching on improving the resilience of PSS, so that, the REUs are ensured with adequate power supply. In order to ensure and improve the resilience of PSS, many solutions have been proposed in the literature. The most suggested and preferred solution in the literature is the use of a backup power facility, either as a storage or generation option [18]. For residential houses, microgrids are mostly recommended. For instance, a microgrid is modeled for powering 100% of electrical loads using a renewable-based power system. It is suggested that the RE-based power system is only capable of powering the houses based on the nature of resilient issues and also depends on the intensity of the power outage [19]. On the other hand, diesel generator (DG)-based studies are also presented by a few researchers, and they suggest that continuous power supply is possible during power outages [20]. Later with the advancement seen in DERs, the use of battery energy storage (BES) systems has become more popular. Few studies have shown any evidence on ensuring resilience; if the PSS configuration has the combination of BES with RE or DG based backup facilities or any RE-based hybrid configuration [21,22]. In a study conducted for the USA, for providing improved power resilience, Anderson et al. suggested the use of hybrid renewable energy-based microgrid composed of solar photovoltaics (PV), DG, and BES [22]. In addition, few studies conducted for the USA were mainly concentrated methods to assess and enhance resilience. For instance, a probabilistic method is developed to assess the resilience of the PSS considering the disruptive event caused due to a hurricane [23]. In another study, the concept of survivability through microgrids is introduced for enhancing the power resilience of PSS. From the brief literature review, it is understood that there is a thrust to carry out research on power resilience, but so far, although many different PSS configurations have been proposed, none have been compared based on their feasibilities [24,25]. In addition, the studies related to the resilience of PSS, highlighting of the techno-economic and environmental indicators is very limited. Hence, different PSS configurations that are location-specific were proposed by considering a resilience framework embedding techno-economic and environmental indicators. These PSS configurations include; electric grid + battery energy storage, electric grid + battery energy storage + natural gas power generator, electric grid + battery energy storage + photovoltaics. The proposed resilience framework is based on being prepared, sustaining, recovery, and learning to adapt. Based on the proposed framework, realistic and meaning full indicators are explored from the techno-economic-environmental point of view. The main contributions of this manuscript are as follows:

A four-component resilience framework with techno-economic and environmental indicators to • understand the resilience of residential electricity user (REU) power supply system (PSS). A battery energy storage (BES) as a preparedness measure that is not considered in most of the • literature is considered here while modeling the proposed PSS configurations. The proposed three different REUs are modeled considering power outage duration as well as the • electric load conditions of the New York-based residential multi-story building as a case study. Evaluation of unmet and compensated electric loads for resilience comparison between the three • PSS configurations of an REU.

This paper has a total of six sections; Section2 presents the four-component resilience framework and the considered indicators; in Section3, the description of the proposed three PSS conﬁgurations for Energies 2020, 13, 4193 5 of 27

REUs along with modeling is given; in Section4, data collection, techno-economic and environmental modeling and simulation for the proposed three PSS conﬁguration is shown along with control strategies. In Section5, the results are presented, and a thorough discussion is made, and in Section6, conclusionsEnergies 2020, are13, xprovided. 5 of 26

2.2. Resilience Resilience Framework Framework ForFor understanding understanding the the resilience resilience of the of PSS the conﬁguration, PSS configuration, a well-structured a well-structured framework framework is necessary. is Thenecessary. proposed The framework proposed shouldframework ensure should that the ensure system that can the sustain, system recover, can sustain, and adapt recover, to theand power adapt outagesto the power or any outages other disruptive or any other events. disruptive In this events study,. theIn this proposed study, frameworkthe proposed has framework four components, has four namely;components, preparedness, namely; robustness, preparedness, recovery, robustness, and adaptation recovery, [13, 14and]. These adaptation four components [13,14]. These are clearly four presentedcomponents and are depicted clearly in presented Figure3. and depicted in Figure 3.

FigureFigure 3. 3.Four Four component component resilience resilience framework. framework.

FromFrom Figure Figure3, 3, it it is is understood understood that that the the highlighted highlighted depictions depictions of of the the disruptive disruptive events events (Level-1 (Level-1 andand Level-2) Level-2) and and the the variation variation in in PSS PSS functionality functionality as as per per the the disruptive disruptive events events matches matches the the resilience resilience definition.definition. EachEach componentcomponent ofof thethe proposedproposed resilienceresilience cyclecycle isis didifferentfferent based based on on its its nature, nature, and and the the fourfour components components are are briefly briefly explained explained below: below: Preparedness: This component suggests the preparedness level of the PSS configuration for power Preparedness: This component suggests the preparedness level of the PSS configuration for power outages. Here, an assumption is made that each REU is already prepared for power outages by outages. Here, an assumption is made that each REU is already prepared for power outages by employing backup energy storage. employing backup energy storage. • AsAs aa preparednesspreparedness measure,measure, batterybattery energyenergy storagestorage isis used.used. • Robustness:Robustness: ThisThis componentcomponent suggestssuggests thethe levellevel totowhich which the the PSS PSS configuration configuration can can sustain sustain and and willwill be be able able to to supply supply power power to to REUs REUs in in the the event event of of a a power power outage. outage. • For understanding this component, an indicator, i.e., an increase in unmet electric load, is For understanding this component, an indicator, i.e., an increase in unmet electric load, • considered. is considered. Recovery: This component suggests the level to which the PSS configuration was recovered and ableRecovery: to supplyThis power component back to the suggests REUs. the level to which the PSS configuration was recovered and able to supply power back to the REUs. • Here compensated load by the PSS configuration during the event of a power outage is Hereconsidered compensated as an indicator. load by the PSS configuration during the event of a power outage is considered • as an indicator. Adaptation: This component suggests the level of learning by the PSS configuration based on the experienced power outages. More or less, it will give information on how the preparedness levels are improved based on the learning from previous power outages incidents. • The adaptation step demands the renovation and modernization of the energy infrastructure. Here, based on the above four-component resilience cycle, and for each component, indicators were identified, which were further used as constraints during techno-economic and environmental optimization of PSS configurations. These indicators include the; unmet electric load, compensated loads, power supplied by the PSS configuration, the cost of energy, net present value, initial capital investments, and the emissions (carbon dioxide; sulfur dioxide; nitrogen oxides).

Energies 2020, 13, 4193 6 of 27

Adaptation: This component suggests the level of learning by the PSS conﬁguration based on the experienced power outages. More or less, it will give information on how the preparedness levels are improved based on the learning from previous power outages incidents.

The adaptation step demands the renovation and modernization of the energy infrastructure. • Here, based on the above four-component resilience cycle, and for each component, indicators were identified, which were further used as constraints during techno-economic and environmental optimization of PSS configurations. These indicators include the; unmet electric load, compensated loads, power supplied by the PSS configuration, the cost of energy, net present value, initial capital investments,Energies 2020, 13 and, x the emissions (carbon dioxide; sulfur dioxide; nitrogen oxides). 6 of 26

3. Power Supply System (PSS) Modelling for Residential Electricity Users (REUs) 3. Power Supply System (PSS) Modelling for Residential Electricity Users (REUs) TheThe PSSPSS is is a networka network of variousof various electrical electrical and electronicand electronic equipments equipments that are that deployed are deployed to generate, to transfer,generate, and transfer, distribute and electricaldistribute energy. electrical A typicalenergy example. A typical of example such PSS of is thesuch EG PSS upon is the which EG mostupon electricitywhich most users electricity depend. users Here, depend. a case of Here, REUs a case alone of is REUs considered. alone is In considered. the context In of the REUs, context the examples of REUs, ofthe the examples PSS would of the fall PSS broadly would under fall broadly two categories; under tw offo-grid categories; and on-grid. off-grid Here, and theon-grid. PSS configurations Here, the PSS thatconfigurations fall under on-grid that fall mode under is considered. on-grid mode This section is considered. briefly describes This section the considered briefly threedescribes different the PSSconsidered configurations three different for REUs PSS along configurations with the modeling. for REUs along with the modeling. 3.1. PSS Configurations for REUs 3.1. PSS Configurations for REUs As mentioned above, in this study, three different PSS configurations are chosen to serve the electric As mentioned above, in this study, three different PSS configurations are chosen to serve the load demand for REUs. The considered PSS configuration has both renewable and non-renewable electric load demand for REUs. The considered PSS configuration has both renewable and non- power generating sources, and each PSS configuration has an energy storage component. Among renewable power generating sources, and each PSS configuration has an energy storage component. these three, PSS configuration, one is a consumer, and the other two are prosumers. In Figure4, the Among these three, PSS configuration, one is a consumer, and the other two are prosumers. In Figure schematic view of studied PSS for REUs is shown, and in the below subsections, the configurations are 4, the schematic view of studied PSS for REUs is shown, and in the below subsections, the described briefly for individual REUs. configurations are described briefly for individual REUs.

FigureFigure 4.4. SchematicSchematic viewview ofof thethe proposedproposed threethree powerpower supplysupply systemsystem conﬁgurationsconfigurations forfor residentialresidential electricityelectricity usersusers [Note:[Note: EG—ElectricEG—Electric grid;grid; BES—BatteryBES—Battery energyenergy storage;storage; NGPG—NaturalNGPG—Natural gasgas powerpower generator;generator; PV—Photovoltaics].PV—Photovoltaics.]

The energy governance function is shown in Equation (1), and this describes the variation in PSS functionality (as shown in Figure 3) for an REU within the specified PSS configuration:

= (,) (1)

where, is the energy demanded by the REU in kWh; is the energy supplied by the PSS configuration in kWh; and the is the energy available or supplied by the battery energy storage in kWh.

3.1.1. REU with EG + BES based PSS Configuration In Figure 5, a PSS configuration for a REU under the consumer-only category is shown. In this PSS configuration, the REU will only consume electricity that is coming from the electric grid for operating the residential electrical loads (which means there is no on-site energy generation facility). The main equipment in this configuration are the electrical loads, power converter, BES, and EG. These components are connected in the PSS network of an REU, considering both the alternating current (AC) bus and direct current (DC) bus. The AC load bus was linked to electrical loads and EG,

Energies 2020, 13, 4193 7 of 27

The energy governance function is shown in Equation (1), and this describes the variation in PSS functionality (as shown in Figure3) for an REU within the speciﬁed PSS conﬁguration:

EREU = f (EPSS, EBES) (1) where, EREU is the energy demanded by the REU in kWh; EPSS is the energy supplied by the PSS conﬁguration in kWh; and the EBES is the energy available or supplied by the battery energy storage in kWh.

3.1.1. REU with EG + BES Based PSS Configuration In Figure5, a PSS configuration for a REU under the consumer-only category is shown. In this PSS configuration, the REU will only consume electricity that is coming from the electric grid for operating the residential electrical loads (which means there is no on-site energy generation facility). The main equipment in this configuration are the electrical loads, power converter, BES, and EG. These components are connected in the PSS network of an REU, considering both the alternating Energies 2020, 13, x 7 of 26 current (AC) bus and direct current (DC) bus. The AC load bus was linked to electrical loads and EG, whilewhile the the DC DC load load bus bus was was linked linked to to BES. BES. Both Both the th ACe AC and and DC DC load load buses buses were were connected connected through through the powerthe power converter. converter. Here, Here, the the power power converter converter is mainlyis mainly used used to to regulate regulate and and convert convert the the DC DC power power outputsoutputs to to AC AC and and vice vice versa. versa. From From the the resilience resilience point point of view, of view, the main the aimmain of eachaim of REU each is toREU prepare is to forprepare power for outages. power outages. In this PSS In this configuration, PSS configuration, the REU the is REU already is already prepared prepared for power for power outages. outages. As a preparednessAs a preparedness measure, measure, a backup a backup power facilitypower employingfacility employing a BES within a BES the within electricity the electricity supply network supply ofnetwork an REU of is an taken. REU is taken.

FigureFigure 5.5.Schematic Schematic viewview ofof EGEG+ + BESBES basedbased powerpower supplysupply systemsystem conﬁgurationconfiguration[Note: [Note: EG—ElectricEG—Electric grid;grid; BES—BatteryBES—Battery energyenergy storage; storage; DC—Direct DC—Direct current; current; AC—Alternating AC—Alternating current.] current.]

3.1.2.3.1.2. REUREU withwith EGEG ++ NGPG + BES based Based PSS PSS Configuration Configuration InIn FigureFigure6 ,6, a a PSS PSS configuration configuration for for REU REU under under the the fossil fossil fuel-based fuel-based prosumer prosumer category category is is shown. shown. InIn thisthis PSSPSS configuration,configuration, thethe REUREU willwill buybuy thethe electricityelectricity fromfrom thethe EG.EG. AtAt thethe samesame time,time, excessexcess electricityelectricity producedproduced atat thethe facilityfacility willwill bebe soldsold toto thethe EG.EG. TheThe conditionsconditions forfor buyingbuying andand sellingselling willwill dependdepend onon thethe REUREU andand thethe deployeddeployed controlcontrol strategy.strategy. Here,Here, thethe EGEG isis givengiven thethe primaryprimary prioritypriority forfor servingserving thethe residentialresidential electricalelectrical loadsloads andand thethe BESBES facility.facility. InIn thethe eventevent ofof powerpower outageoutage condition,condition, thethe electricalelectrical loadsloads areare operatedoperated throughthrough BES.BES. IfIf stillstill thethe EGEG isis notnot repaired,repaired, thenthen thethebackup backup powerpower generationgeneration facilityfacility willwill comecome into into connection connection and and serves serves the the residential residential electrical electrical loads. loads.

Figure 6. Schematic view of EG + NGPG + BES based power supply system configuration [Note: EG— Electric grid; BES—Battery energy storage; NGPG—Natural gas power generator; DC—Direct current; AC—Alternating current].

Energies 2020, 13, x 7 of 26 while the DC load bus was linked to BES. Both the AC and DC load buses were connected through the power converter. Here, the power converter is mainly used to regulate and convert the DC power outputs to AC and vice versa. From the resilience point of view, the main aim of each REU is to prepare for power outages. In this PSS configuration, the REU is already prepared for power outages. As a preparedness measure, a backup power facility employing a BES within the electricity supply network of an REU is taken.

Figure 5. Schematic view of EG + BES based power supply system configuration [Note: EG—Electric grid; BES—Battery energy storage; DC—Direct current; AC—Alternating current.]

3.1.2. REU with EG + NGPG + BES based PSS Configuration In Figure 6, a PSS configuration for REU under the fossil fuel-based prosumer category is shown. In this PSS configuration, the REU will buy the electricity from the EG. At the same time, excess electricity produced at the facility will be sold to the EG. The conditions for buying and selling will depend on the REU and the deployed control strategy. Here, the EG is given the primary priority for serving the residential electrical loads and the BES facility. In the event of power outage condition, Energiesthe electrical2020, 13 ,loads 4193 are operated through BES. If still the EG is not repaired, then the backup power8 of 27 generation facility will come into connection and serves the residential electrical loads.

Figure 6. SchematicSchematic view view of of EG EG + NGPG+ NGPG + BES+ BES based based power power supply supply system system configuration conﬁguration [Note: [Note: EG— EG—ElectricElectric grid; grid;BES—Battery BES—Battery energy energy storage; storage; NG NGPG—NaturalPG—Natural gas gas power power generator; generator; DC—DirectDC—Direct current; AC—AlternatingAC—Alternating current].current].

The main equipment in this conﬁguration are the electrical loads, power converter, BES, natural gas power generator (NGPG), and EG. These components are connected to the PSS network of an REU, considering both the AC bus and the DC bus. The AC load bus was linked to electrical loads, NGPG, and the EG, while the DC load bus was linked to BES. Both the AC and DC load buses were connected through the power converter, which is mainly used to regulate and convert the DC power outputs to AC and vice versa. From the resilience point of view, the main aim of each REU is to prepare for power outages. In this particular PSS conﬁguration also, the REU is already prepared for power outages. As a preparedness measure, a backup power facility employing a BES and NGPG within the electricity supply network of an REU is taken.

3.1.3. REU with EG + PV + BES Based PSS Configuration In Figure7, a PSS configuration for REU under the renewable-based prosumer category is shown. In this PSS configuration, the REU will buy the electricity from the EG. At the same time, excess electricity produced at the facility will be sold to the EG. The conditions for buying and selling will depend on the deployed control strategy. Here, the electrical loads are served by taking power from the PV system and the EG. In the event of a power outage condition, the electrical loads are operated entirely on the PV system. If there is variation in the output power produced by the PV system, then the loads are served by taking power from the BES. If still the EG is not repaired, then the backup power generation facility will come into connection and serves the residential electrical loads. The main equipment in this configuration are the electrical loads, power converter, BES, PV, and EG. These components are connected to the PSS network of an REU, considering both the AC bus and the DC bus. The AC load bus was linked to electrical loads and the EG, while the DC load bus was linked to BES directly and to the PV system through maximum power point tracking (MPPT) enabled DC-DC converter [26]. Both the AC and DC load buses were connected through the power converter, which is mainly used to regulate and convert the DC power outputs to AC and vice versa. From the resilience point of view, the main aim of each REU is to prepare for power outages. In this particular PSS configuration also, the REU is already prepared for power outages. As a preparedness measure, a backup power facility employing a BES and PV system within the electricity supply network of an REU is taken. Energies 2020, 13, x 8 of 26

The main equipment in this configuration are the electrical loads, power converter, BES, natural gas power generator (NGPG), and EG. These components are connected to the PSS network of an REU, considering both the AC bus and the DC bus. The AC load bus was linked to electrical loads, NGPG, and the EG, while the DC load bus was linked to BES. Both the AC and DC load buses were connected through the power converter, which is mainly used to regulate and convert the DC power outputs to AC and vice versa. From the resilience point of view, the main aim of each REU is to prepare for power outages. In this particular PSS configuration also, the REU is already prepared for power outages. As a preparedness measure, a backup power facility employing a BES and NGPG within the electricity supply network of an REU is taken.

3.1.3. REU with EG + PV + BES based PSS Configuration In Figure 7, a PSS configuration for REU under the renewable-based prosumer category is shown. In this PSS configuration, the REU will buy the electricity from the EG. At the same time, excess electricity produced at the facility will be sold to the EG. The conditions for buying and selling will depend on the deployed control strategy. Here, the electrical loads are served by taking power from the PV system and the EG. In the event of a power outage condition, the electrical loads are operated entirely on the PV system. If there is variation in the output power produced by the PV system, then the loads are served by taking power from the BES. If still the EG is not repaired, then the backup power generation facility will come into connection and serves the residential electrical loads. The main equipment in this configuration are the electrical loads, power converter, BES, PV, and EG. These components are connected to the PSS network of an REU, considering both the AC bus and the DC bus. The AC load bus was linked to electrical loads and the EG, while the DC load bus was linked to BES directly and to the PV system through maximum power point tracking (MPPT) enabled DC-DC converter [26]. Both the AC and DC load buses were connected through the power converter, which is mainly used to regulate and convert the DC power outputs to AC and vice versa. From the resilience point of view, the main aim of each REU is to prepare for power outages. In this particular PSS configuration also, the REU is already prepared for power outages. As a preparedness

Energiesmeasure,2020 ,a13 backup, 4193 power facility employing a BES and PV system within the electricity supply9 of 27 network of an REU is taken.

Energies 2020, 13, x 9 of 26 FigureFigure 7. SchematicSchematic view view of EG of + EGPV + BESPV based+ BES power based supply power system supply configuration system configuration [Note: EG— [Note:Electric EG—Electricgrid; BES—Battery grid; energy BES—Battery storage; energy PV—Photovoltaics; storage; PV—Photovoltaics; DC—Direct current; DC—Direct AC—Alternating current; 3.2.1. Electrical Loads and Electric Grid (EG) AC—Alternatingcurrent; MPPT—Maximum current; MPPT—Maximum power point tracking]. power point tracking]. The electrical loads that consume energy are the most common type of equipment that we see 3.2.3.2. REUsREUs PSSPSS EquipmentEquipment ModellingModelling in REUs. The most seen electrical loads are broadly categorized based on their nature that may be resistive, inductive,InIn thethe above above and discussed capacitive.discussed three threeThe PSStypical PSS configurations, configurations, examples of theyresidential they are are di ff differentelectricalerent types typesloads of equipmentofare equipment lights, fans, related related to computers,powerto power cooking generation, generation, facilities, power power heaters, conversion, conversion, pumps, and air energyand cond energyitioners, storage. storage. etc. Overall, In thisOverall, instudy, three in we configurations,three have configurations, considered the main the the realisticequipmentmain equipmentelectrical are the loads are electrical the of electricala residential loads andloads EG,building and NGPG, EG, locatedNG PV,PG, power inPV, a converter,powerhumid converter, subtropical and BES. and Thisecosystem BES. section This is brieflysection considered.dealtbriefly with The dealt thedata with equipment related the equipment to modeling, electrical modeling, andloads the ar and detailse obtained the aredetails given from are below. thegiven Open below. EI Database for load profiles [27]. The detailed load profile and its discussion is given in Section 4. The3.2.1. EG Electrical that is chosen Loads in and this Electric study Grid is the (EG) national power grid. In general, the national power grid is a combinationalThe electrical loadsnetwork that that consume unites energy the power are the producers most common and consumers. type of equipment Here both that the we see powerin producers REUs. The and most consumers seen electrical are interconnected, loads are broadly and categorizedmostly the produced based on power their nature comes that from may be both theresistive, renewables inductive, and non-renewables. and capacitive. The In most typical co examplesuntries, REUs of residential are connected electrical to the loads national are lights, EG, fans, and theycomputers, assumed cooking to have facilities,a continuous heaters, supply pumps, of energy. air conditioners, Although the etc. EG In suffers this study, from we the have blackouts considered that leadthe to realistic power electricaloutages at loads the residential of a residential level, still building the residential located in buildings a humid are subtropical connected ecosystem to the is EG. Inconsidered. this study, three The data different related PSS to configurations electrical loads are are considered, obtained from and thein all Open the three, EI Database the EGfor is load used. profiles [27]. The detailed load profile and its discussion is given in Section4. The EG that is chosen in this study is the national power grid. In general, the national power grid 3.2.2. Naturalis a combinational Gas Power networkGenerator that (NGPG) unites the power producers and consumers. Here both the power Aproducers fossil fuel-powered and consumers generator are interconnected, that uses natural and gas mostly as a fuel the producedfor power power generation comes is fromused bothas the one ofrenewables the backup and facilities non-renewables. in one of the In consider most countries,ed PSS REUsconfigurations. are connected The toelectrical the national output EG, from and they the NGPGassumed is a function to have a of continuous the fuel burned, supply and of energy. it can be Although estimated the using EG su Equationffers from (2) the [28]: blackouts that lead to power outages at the residential level, still the residential buildings are connected to the EG. In this Ꞃ ×𝑚 ×𝐻𝑉 study, three different PSS configurations𝑃 = are considered, and in all the three, the EG is used.(2) 3.6 3.2.2. Natural Gas Power Generator (NGPG) where, 𝑃 is the electrical power output in kW; Ꞃ is the efficiency of the NGPG in %; 𝑚 is the massA flow fossil rate fuel-powered of the natural generator gas th thatat is uses burned natural for gas power as a fuelgeneration for power in generationkg/h; 𝐻𝑉 is usedis the as one heatingof value the backup of the facilitiesnatural gas in onein MJ/kg. of the considered PSS configurations. The electrical output from the NGPG is a function of the fuel burned, and it can be estimated using Equation (2) [28]:

Ꞃ m HV P = NGPG × NG × NG (2) NGPG 3.6

The masswhere, flowPNGPG rate ofis the the natural electrical gas power in m3 output is given in by kW; usingNGPG Equationis the (3), eﬃ ciencyand it is of the the multiplication NGPG in %; m NG is of naturalthe mass gas fuel ﬂow density rate of theto the natural amount gas thatof fu isel burned consumed. for power The natural generation gas infuel kg /consumptionh; HVNG is the can heating also bevalue estimated of the using natural Equation gas in MJ (4):/kg.

𝑚 =𝜌 ×𝐹𝐶 (3)

(4) 𝐹𝐶 =(𝐹 ×𝑃)+(𝐹 ×𝑃)

3 where, 𝜌 is the density of the natural gas fuel (kg/m ); 𝐹𝐶 is natural gas fuel consumption in 3 m /h; 𝑃 is the rated capacity of the natural gas power generator in kW; and 𝐹 and 𝐹 are the natural gas fuel curve intercept and slope in m3/h/kW.

The supplied energy by the natural gas power generator represented using 𝐸 is given by multiplying the electrical power output with the operating time, t in h, see Equation (5).

𝐸 =𝑃 ×𝑡 (5)

3.2.3. Photovoltaics (PV) In this study, the solar PV system is also used as one of the backup power generation facilities in one of the PSS configurations. In the solar PV system, the PV modules and other energy conversion

Energies 2020, 13, x 9 of 26

3.2.1. Electrical Loads and Electric Grid (EG) The electrical loads that consume energy are the most common type of equipment that we see in REUs. The most seen electrical loads are broadly categorized based on their nature that may be resistive, inductive, and capacitive. The typical examples of residential electrical loads are lights, fans, computers, cooking facilities, heaters, pumps, air conditioners, etc. In this study, we have considered the realistic electricalEnergies loads2020 ,of13 ,a 4193 residential building located in a humid subtropical ecosystem is 10 of 27 considered. The data related to electrical loads are obtained from the Open EI Database for load profiles [27]. The detailed load profile and its discussion is given in Section 4. 3 The EG that is chosenThe massin this ﬂow study rate ofis the naturalnational gas power in m grid.is given In bygeneral, using Equationthe national (3), andpower it is the multiplication grid is a combinationalof natural network gas fuel that density unites to the the power amount producers of fuel consumed. and consumers. The natural Here gas both fuel the consumption can also power producers andbe estimated consumers using are Equationinterconnected, (4): and mostly the produced power comes from m = ρ FC (3) both the renewables and non-renewables. In most countries,NG REUs areNG ×connectedNG to the national EG, and they assumed to have a continuous supply of energy. Although the EG suffers from the blackouts FCNG = Fo PrNGPG + (F1 PNGPG) (4) that lead to power outages at the residential level, still the residential× buildings× are connected to the 3 3 EG. In this study, where,three differentρNG is the PSS density configurations of the natural are gasconsidered, fuel (kg/m and); FC inNG allis the natural three, gas the fuel EG consumption is in m /h; used. PrNGPG is the rated capacity of the natural gas power generator in kW; and Fo and F1 are the natural gas fuel curve intercept and slope in m3/h/kW. 3.2.2. Natural Gas PowerThe Generator supplied (NGPG) energy by the natural gas power generator represented using ENGPG is given by multiplying the electrical power output with the operating time, t in h, see Equation (5). A fossil fuel-powered generator that uses natural gas as a fuel for power generation is used as one of the backup facilities in one of the considered PSS configurations.Xn The electrical output from the NGPG is a function of the fuel burned, and it can be estimatedE = usingP Equationt (2) [28]: (5) NGPG NGPG × t=1 Ꞃ ×𝑚 ×𝐻𝑉 𝑃 = (2) 3.2.3. Photovoltaics (PV) 3.6 𝑃 Ꞃ 𝑚 where, is the electricalIn this study,power the output solar in PV kW; system is also is the used efficiency as one ofof thethe backup NGPG powerin %; generation facilities in is the mass flow rateone of the PSSnatural conﬁgurations. gas that is burned In the solar for power PV system, generation the PV in modules kg/h; 𝐻𝑉 and otheris the energy conversion heating value of thedevices natural help gas inin convertingMJ/kg. the incident solar irradiance into useful electricity. The electrical output from the PV system is given by using Equation (6) [24,29]:

Ꞃ PPV = PV PC APV GPV (1 + γ(TPV Tref)) (6) × × × × −

where, PPV is power produced from the PV system in kW; PV is the power conversion efficiency PV 3 The mass flow ratemodule of the natural in %; gasPC inis m the is e ffigivenciency by ofusing the Equation power converter (3), and it in is %; theA PVmultiplicationis the areaof the PV array in 2 2 of natural gas fuelm density; GPV isto the the solar amount irradiance of fuel incidentconsumed. on theThe plane natural of thegas PVfuel array consumption in kW/m can; γ is the temperature also be estimated usingco-effi Equationcient of the(4): PV module; TPV is the PV module temperature in ◦C; and Tref is the reference temperature in ◦C. 𝑚 =𝜌 ×𝐹𝐶 (3) The PV module temperature is one of the most crucial factors that need to be considered while modeling any sought of PV application. In the literature, mostly the nominal operating(4) cell temperature 𝐹𝐶 =(𝐹 ×𝑃)+(𝐹 ×𝑃) (NOCT) model is used, which is widely accepted and can be more appropriate for open rack-mounting where, 𝜌 is the density of the natural gas fuel (kg/m3); 𝐹𝐶 is natural gas fuel consumption in installations [29]. But in this study, the considered REUs have opted for rooftop PV. So accordingly for 3 m /h; 𝑃 is the rated capacity of the natural gas power generator in kW; and 𝐹 and 𝐹 are the the REU with rooftop, the TPV is calculated by using the arbitrary temperature model, as shown in natural gas fuel curve intercept and slope in m3/h/kW. Equation (7) [30]: The supplied energy by the natural gas power generator represented 0.32 using 𝐸 is given by TPV = Tamb + mc GPV (7) multiplying the electrical power output with the operating time, t in h,8.91 see+ Equation2Ws (5). where, Tamb is the ambient temperature in ◦C; mc is the mounting co-efficient; and Ws is the wind speed in m/s. 𝐸 =𝑃 ×𝑡 (5) The supplied energy by the photovoltaic power generation system is represented by multiplying the electrical power output with the operating time, t in h, see in Equation (8): 3.2.3. Photovoltaics (PV) Xn In this study, the solar PV system is also used as one ofE the= backupP powet r generation facilities (8) PV PV × in one of the PSS configurations. In the solar PV system, the PV modulest=1 and other energy conversion

3.2.4. Power Converter In this study, in all the three PSS conﬁguration, as a preparedness measure, we have used the BES. In general, the battery stores electrical energy in the form of chemical energy, but during the charging and discharging conditions, it only operates with the direct current electricity, but the REUs use alternating current electricity, hence, to facilitate this conversion of power from DC to AC and vice versa, a power converter is used. Here, the power converter is crucial to facilitate a continuous ﬂow of Energies 2020, 13, x 9 of 26

3.2.2. Natural Gas Power Generator (NGPG) A fossil fuel-powered generator that uses natural gas as a fuel for power generation is used as one of the backup facilities in one of the considered PSS configurations. The electrical output from the NGPG is a function of the fuel burned, and it can be estimated using Equation (2) [28]:

Ꞃ ×𝑚 ×𝐻𝑉 𝑃 = (2) 3.6 Energies 2020, 13, 4193 11 of 27 where, 𝑃 is the electrical power output in kW; Ꞃ is the efficiency of the NGPG in %; 𝑚 is the mass flow rate of the natural gas that is burned for power generation in kg/h; 𝐻𝑉 is the heatingenergy value between of the the natural PSS and gas the in REUs.MJ/kg. The power ﬂow conditions for the power converter, acting both as inverter and rectiﬁer are modeled using Equations (9) and (10) [29,31]:

Ꞃ PPC_Inv = PC_Inv PDC (9) × P = P (10) PC_Rec PC_Rec × AC The mass flow rate of the natural gas in m3 is given by using Equation (3), and it is the multiplication where P is the power converter output in kW when it is acting as an inverter; P is the power of naturalPC_Inv gas fuel density to the amount of fuel consumed. The natural gas fuel consumptionPC_Rec can alsoconverter be estimated output using in kW Equation when it is(4): acting as a rectifier; PC_Inv and PC_Rec are the efficiencies of the power converter in inverter and rectifier modes, respectively; PDC and PAC are the input feeds in kW for the power converter in inverter and rectifier𝑚 =𝜌 modes, ×𝐹𝐶 respectively. (3) (4) 3.2.5. Battery Energy Storage (BES)𝐹𝐶 =(𝐹 ×𝑃)+(𝐹 ×𝑃)

3 where,As 𝜌 a preparedness is the density measure, of the natural a backup gasenergy fuel (kg/m storage); 𝐹𝐶 facility is natural using BES gas is fuel considered consumption in all thein 3 mthree/h; 𝑃 PSS configurations. is the rated capacity As per theof the resilience natural conditions, gas power it generator is always in better kW; to and be prepared𝐹 and 𝐹 for are power the naturaloutages, gas and fuel in curve the event intercept of a power and slope outage, in m the3/h/kW. BES will serve the REUs. In order to facilitate this, we allowThe the supplied batteries energy to charge by whilethe natural the REU gas ispower connected generator to PSS represented configuration. using 𝐸 is given by multiplyingIn this the study, electrical in all three power configurations, output with the the operating BES will be time, charged, t in h, which see Equation means whenever (5). there is excess electricity in PSS, the batteries will store the excess portion and provide to the load when power

outages or any sought of disruption occurs.𝐸 =𝑃 ×𝑡 (5) In BES, the conditions for charging and discharging are mostly dependent on power generation availability and REUs consumption patterns in all the three PSS conﬁgurations. These conditions are 3.2.3.modeled Photovoltaics using the (PV) Equations (11) and (12). Equation (11) represents the state of charge condition, and Equation (12) represents the depth of the discharge condition. In both conditions, a self-discharge rate In this study, the solar PV system is also used as one of the backup power generation facilities is considered [29,32,33]: in one of the PSS configurations. In the solar PV system, the PV modules and other energy conversion Xn Xn h i E = (E (t 1) (1 S )) + (E E ) (11) BES BES − × − dr PSS − REU × PC_Inv × BES t=1 t=1

Xn Xn h i E = (E (t 1) (1 S )) + E E (12) BES BES − × − dr REU × PC_Inv − PSS t=1 t=1

where EBES, EPSS, and EREU are the energy stored in a battery in kWh, energy generated by the PSS conﬁguration in kWh, and energy required by the REU in kWh, respectively; Sdr represents the self-discharging rate of the battery in %; BES is the eﬃciency of the BES system in %; and t is the time represented in h.

4. Data Inputs and Simulation of Proposed PSS Configurations This section provides information regarding the data used for performing the resilience simulations and the techno-economic and environmental assessments. In addition, the simulation procedure used in PSS configurations is briefly presented.

4.1. Data Inputs

4.1.1. Electrical Load Profile In this study, the resilience assessment of three different PSS configurations, and their techno-economic and environmental feasibility is carried out. As a case study, a multi-floor residential building located in New York, (NY, USA), that experiences humid subtropical climates is considered. The most critical data for the REUs is the electrical load’s energy consumption pattern, as shown in Figure8. The data on energy consumption patterns are obtained from the Open EI Database for load profiles [27]. Energies 2020, 13, x 11 of 26

=(−1) × (1−)+( −)×Ꞃ_ × Ꞃ (11)

=(−1) × (1−) + (( × Ꞃ_)−) (12)

where , , and are the energy stored in a battery in kWh, energy generated by the PSS configuration in kWh, and energy required by the REU in kWh, respectively; represents the self-

discharging rate of the battery in %; Ꞃ is the efficiency of the BES system in %; and t is the time represented in h.

4. Data Inputs and Simulation of Proposed PSS Configurations This section provides information regarding the data used for performing the resilience simulations and the techno-economic and environmental assessments. In addition, the simulation procedure used in PSS configurations is briefly presented.

4.1. Data Inputs

4.1.1. Electrical Load Profile In this study, the resilience assessment of three different PSS configurations, and their techno- economic and environmental feasibility is carried out. As a case study, a multi-floor residential building located in New York, (NY, USA), that experiences humid subtropical climates is considered. The most critical data for the REUs is the electrical load’s energy consumption pattern, as shown in EnergiesFigure2020 8. The, 13, 4193data on energy consumption patterns are obtained from the Open EI Database for12 load of 27 profiles [27].

FigureFigure 8.8. ElectricityElectricity load proﬁleprofile ofof aa multi stair residential building located in New York, USAUSA [Note: REU—ResidentialREU—Residential electricityelectricity users;users; USA—UnitedUSA—United StatesStates ofof America).America).

FromFrom FigureFigure8 8,, it it is is observed observed that that the the load load proﬁle profile is is time-dependent, time-dependent, with with a a maximum maximum variation variation observedobserved inin the the months months (from (from May May to to October). October). The The maximum maximum load load consumption consumption was was recorded recorded in the in monththe month of July, of makingJuly, making it the monthit the month of peak of energy peak demand,energy demand, which is which around is 59.04 around kW. 59.04 The observedkW. The averageobserved load average is 25.4 load kW, andis 25.4 the kW, average and dailythe averag energye daily required energy by the required REU is 610.5by the kWh. REU Theis 610.5 estimated kWh. annualThe estimated AC primary annual load AC is primary 222,834 kWhload /isy. 222,834 kWh/y.

4.1.2.4.1.2. ElectricElectric GridGrid PowerPower OutageOutage andand TariTariffﬀ Data Data EvenEven inin thethe daysdays ofof modernmodern electricelectric gridgrid infrastructure,infrastructure, uncertaintyuncertainty inin thethe powerpower supplysupply isis moremore oftenoften seen,seen, especiallyespecially inin thethe studiedstudied location,location, andand isis mainlymainly duedue toto powerpower outages.outages. TheThe reasonsreasons forfor power outages might be any of the following: extreme weather events, asset failure, natural disasters, power surges, acute accidents, and even the operational errors by the workforce. Here the studied location is New York (NY, USA). Based on the recent data article by Mukherjee et al. 2018 [16]. It is understood that the considered location has frequent power outages, and at times, power outages were last from a few minutes to weeks. Here, for carrying out the simulation study and understanding the resilience of PSS, the power outages that occurred in the year 2016 were considered [16]. In 2016, alone, three major power outage events occurred in New York, and the details are shown in Table1.

Table 1. Data on power outages in New York, USA.

Start Time Restoration Time Outage Duration Reason Notation 7:49 AM on 7 May 9:02 AM on 7 May 1.21 h Intentional attack Outage-1 9:29 PM on 26 May 12:40 AM on 27 May 3.18 h System disruption Outage-2 7:30 AM on 31 May 7:27 AM on 13 June 311.95 h Fuel supply issue Outage-3 Note: USA—United States of America; h—hours.

In the USA, the electricity tariﬀ rates vary from state to state, but when compared to other states, the REUs in New York pay 44% more for electricity than the average USA residential price [34,35]. In New York State, the average electricity tariﬀ rate for REUs is 17.42 cents/kWh, which is roughly 0.1742 US$/kWh [34,35]. Energies 2020, 13, 4193 13 of 27

4.1.3. Power Supply Systems Equipment Cost In this study, three different PSS configurations are simulated for understanding resilience. The proposed PSS configurations contain EG, NGPG, BES, and PV as the primary assets or the equipment. The cost details, along with the technical specifications each equipment used in PSS configuration, are given Tables2 and3 respectively.

Table 2. Cost details of the most important equipments used in power supply systems [24,29,36,37].

Power Natural Gas Cost Parameters Photovoltaics Battery Converter Power Generator Capital cost 3100 $/kW 137.5 $/kW 156 $/kWh 500 $/kW Replacement cost 3100 $/kW 137.5 $/kW 156 $/kWh 500 $/kW Operation & maintenance cost 310 $/y 13.7 $/y 15.6 $/y 0.03 $/op. h Note: kW—kilo watt; y—year; kWh—kilo watt hour; op. h—Operational hours; $—United States Dollars.

Table 3. Technical speciﬁcations of the power supply systems equipment.

Parameter Values with Units Photovoltaics (PV) Peak power 1 kW Temperature coefficient 0.3%/ C − ◦ Nominal operating temperature 47 ◦C Efficiency at the standard test condition 21% Lifetime 25 y Battery Energy Storage (BES) Type Lithium-ion Nominal capacity 16.7 kWh Nominal voltage 12 V The initial state of charge 100% Minimum state of charge 20% Self-discharge rate (including the safety circuit) 5%/day Lifetime 15 y Natural Gas Power Generator (NGPG) Input fuel Natural gas Capacity 65 kW Efficiency 95% Lifetime 15,000 h Power Converter (PC) Capacity 60 kW Efficiency 95% Lifetime 15 y

Note: kW-kilo watt; %—Percentage; ◦C—Degree centigrade; y—year; kWh-kilo watt hour; V—Volts; h—hours.

4.1.4. Data Inputs on Natural Gas Fuel and Emission Factors In the considered three PSS configurations, in only one configuration, the NGPG is used. In this PSS configuration, natural gas is burnt to produce electricity in the event of grid failure. While modeling Energies 2020, 13, x 13 of 26

Natural Gas Power Generator (NGPG) Input fuel Natural gas Capacity 65 kW Efficiency 95% Lifetime 15,000 h Power Converter (PC) Capacity 60 kW Efficiency 95% Lifetime 15 y Note: kW-kilo watt; %—Percentage; °C—Degree centigrade; y—year; kWh-kilo watt hour; V—Volts; h—hours.

4.1.4. Data Inputs on Natural Gas Fuel and Emission Factors Energies 2020, 13, 4193 14 of 27 In the considered three PSS configurations, in only one configuration, the NGPG is used. In this PSS configuration, natural gas is burnt to produce electricity in the event of grid failure. While modelingthe study, the the study, data related the data to related natural to gas natural prices gas speciﬁc prices to specific New York to New are considered.York are considered. The variation The variationtrend in naturaltrend in gas natural prices gas over prices the lastover 10 the years last in10 New years York in New is shown York inis shown Figure9 in. Figure 9.

FigureFigure 9. 9. TrendsTrends in in natural natural gas gas price price variation. variation.

InIn New New York, York, natural natural gas gas can can be be purchased purchased for for an an average average price price of of 12.68$/thousand 12.68$/thousand cu. cu. ft ft [38]. [38]. ThisThis value value is is converted converted in in $/cu.m $/cu.m and and is is around around 0.44779 0.44779 %/cu.m. %/cu.m. While While modelling, modelling, the the data data related related to to naturalnatural gas gas fuel fuel properties properties are are consid considered,ered, and and they they are are given in Table 4 4..

TableTable 4. 4. PropertiesProperties of of the the natural natural gas gas fuel fuel us useded in in a a natural natural gas gas power power generator. generator.

ParametersParameters ValueValue with Unitswith Units LowerLower heating heating value value 45 MJ 45/kg MJ/kg DensityDensity 0.79 kg0.79/m3 kg/m3 CarbonCarbon content content 67% 67% SulfurSulfur content content 0% 0% Note:Note: MJ-mega MJ-mega joules; joules; kg—kilo kg—kilo gram; gram; m 33—Cubic meter; meter; %—Percentage. %—Percentage.

ForFor thethe environmentalenvironmental assessment assessment of theof PSSthe conﬁgurations,PSS configurations, the emission the emission factors data factors is considered. data is considered.The considered The emissionconsidered factors emission are basedfactors on are life ba cyclesed on assessments. life cycle assessments. As the REUs As are the located REUs andare locatedconnected and toconnected the national to the EG national in New EG York, in New the EGYork, emission the EG factorsemission for factors CO2, SOfor2 ,CO and2, SO NOx2, and are NOxconsidered are considered as 287 g /askWh, 287 g/kWh, 0.36 g/kWh, 0.36 g/kWh, and 0.20 and g/kWh, 0.20 g/kWh, respectively respectively [39]. From [39]. theFrom literature, the literature, it was itidentiﬁed was identified that a crystallinethat a crystalline PV technology PV technology based solar based module solar emitsmodule 55 emits g CO2 55/kWh, g CO 0.382/kWh, g SO 20.38/kWh, g SOand2/kWh, 0.2 g NOand2 0.2/kWh g NO [402]./kWh The [40]. BES wasThe BES considered was considered to emit 338 to emit kg CO 3382/kWh kg CO based2/kWh on based its life on cycle, its life in addition to CO2, the batteries SO2 emissions were 2.23 g/kWh [41]. The emissions related to NGPG are modeled within the simulation tool.

4.2. Simulation of PSS Configurations Here, the hybrid optimization model for electric renewables (HOMER) simulation tool used to model different PSS configurations. This tool allows us to model any of the conceptual PSS configurations, along with multiple constraints. The database of this tool provides us with numerous inbuilt power conversion devices and DERs. The simulation is carried out for the steps presented in the form of a flow chart depicted in Figure 10. The simulation has proceeded for three different PSS configurations that are discussed in the previous section. While performing the simulations, in the first step, the pre-assessment of collected residential electrical load profiles, resources such as solar irradiance and natural gas fuel, Energies 2020, 13, x 14 of 26 cycle, in addition to CO2, the batteries SO2 emissions were 2.23 g/kWh [41]. The emissions related to NGPG are modeled within the simulation tool.

4.2. Simulation of PSS Configurations Here, the hybrid optimization model for electric renewables (HOMER) simulation tool used to model different PSS configurations. This tool allows us to model any of the conceptual PSS configurations, along with multiple constraints. The database of this tool provides us with numerous inbuilt power conversion devices and DERs. The simulation is carried out for the steps presented in the form of a flow chart depicted in Figure Energies10. The2020 simulation, 13, 4193 has proceeded for three different PSS configurations that are discussed 15in ofthe 27 previous section. While performing the simulations, in the first step, the pre-assessment of collected residential electrical load profiles, resources such as solar irradiance and natural gas fuel, and system and system design is done. Here, the selection of meteorological data for the study location using the design is done. Here, the selection of meteorological data for the study location using the pre-built pre-built data sets is made for evaluating PV performance. After preparing the input datasets ready, data sets is made for evaluating PV performance. After preparing the input datasets ready, the PSS the PSS conﬁguration models are built by selecting the appropriate electric power components from configuration models are built by selecting the appropriate electric power components from the the HOMER library. In a similar way, all three PSS conﬁguration models were built. HOMER library. In a similar way, all three PSS configuration models were built.

Figure 10. 10. FlowchartFlowchart for simulation for simulation modeling modeling of power of supply power system supply configurations. system configurations. [Notes: EG- [Notes:Electric grid; EG-Electric BES-Battery grid; energy BES-Battery storage; NGPG—N energy storage;atural gas NGPG—Natural power generator; gas PV—Photovoltaics.]. power generator; PV—Photovoltaics.]. In the second step, the resilience and techno-economic and environmental analysis is carried out. Here,In while the second modeling step, each the resiliencecomponent and of techno-economicPSS configuration, and their environmental technical and analysis cost parameters is carried and out. Here,optimum while sizing modeling search each spaces component are enabled of PSS along configuration, with the theirspecific technical constraints and cost to achieve parameters techno- and optimumeconomic sizingand environmentally search spaces are feasible enabled configuration along with the and, specific at the constraints same time, to achieve that ensures techno-economic resilience. andAt this environmentally stage of the simulation, feasible configuration the power outage and, atscenarios the same are time, applied that for ensures each PSS resilience. configuration, At this stageand the of indicators the simulation, considered the power for resilience outage scenarios are quanti arefied. applied In addition, for each the PSS sensitivity configuration, analysis and is also the indicatorscarried out considered by considering for resilience the sensitive are quantified.parameters Inin addition,order to achieve the sensitivity a lower analysisnet present is also cost carried (NPC) outand bythe considering cost of energy the sensitive (CoE). Here, parameters in each in orderPSS, the to achieveapplicable a lower sensitive net present parameters cost (NPC) like solar and theirradiance, cost of energydiscount (CoE). rates, Here, and natural in each gas PSS, prices the applicable are considered. sensitive parameters like solar irradiance, discount rates, and natural gas prices are considered. The cost parameters are based on the following approach: Among them, the NPC that represents the overall costs of the PSS configuration considered capital, operation, and maintenance (O & M), replacement cost, etc. for its lifetime is calculated as per Equation (13) [29,31]:

TC NPC = n (13) i (1+ i0 F ) × 1+−F n 1 + i0 F − (1 1+−F ) where TC, n, i, i’, and F represent the total cost, number of years the annual real interest rate, real interest rate, nominal interest rate, and annual inﬂation rate, respectively. Energies 2020, 13, x 15 of 26

The cost parameters are based on the following approach: Among them, the NPC that represents the overall costs of the PSS configuration considered capital, operation, and maintenance (O & M), replacement cost, etc. for its lifetime is calculated as per Equation (13) [29,31]: TC NPC = −F ×(1+ ) 1+F (13) −F (1 + ) Energies 2020, 13, 4193 1+F 16 of 27 where TC, n, i, i’, and F represent the total cost, number of years the annual real interest rate, real interestCost rate, ofenergy nominal (CoE) interest is calculated rate, and asannual given inflation in Equation rate, (14) respectively. [29,31]: Cost of energy (CoE) is calculated as given in Equation (14) [29,31]: TC CoE = TC (14) CoE =Lprim,AC + Lprim,DC (14) , +, where Lprim,AC, and Lprim,DC are the AC primary load and the DC primary load, respectively. where Lprim,AC, and Lprim,DC are the AC primary load and the DC primary load, respectively. In the third step, the obtained results for three PSS configurations were analyzed and In the third step, the obtained results for three PSS configurations were analyzed and compared compared based on the selected indicators for resilience and techno-economic and life cycle based based on the selected indicators for resilience and techno-economic and life cycle based environmental feasibility. environmental feasibility. 5. Results and Discussion 5. Results and Discussion The results of three PSS configurations under grid outages are presented briefly along with the The results of three PSS configurations under grid outages are presented briefly along with the resilience assessment. In addition, the three PSS configurations are compared based on techno-economic resilience assessment. In addition, the three PSS configurations are compared based on techno- and life cycle based environmental indicators. Grid outage modeling is the common result of the three economic and life cycle based environmental indicators. Grid outage modeling is the common result PSS configurations. Based on the power outage data, the disruption in the power supply to the REUs of the three PSS configurations. Based on the power outage data, the disruption in the power supply is evaluated and presented in Figure 11. to the REUs is evaluated and presented in Figure 11.

Figure 11. Electricity purchased from grid highlighting on the power outage duration for the year. Figure 11. Electricity purchased from grid highlighting on the power outage duration for the year. From Figure 11, it is understood that a total of three power outages occurred, and the duration of From Figure 11, it is understood that a total of three power outages occurred, and the duration the outages lasted from 1.21 h to 311.95 h. The first power outage was an intentional attack and is of the outages lasted from 1.21 h to 311.95 h. The first power outage was an intentional attack and is shorter, and within 1.21 h, the grid restoration has taken place. The second power outage is due to shorter, and within 1.21 h, the grid restoration has taken place. The second power outage is due to system operational error, and however, the grid restoration happened within 3.18 h. These two power system operational error, and however, the grid restoration happened within 3.18 h. These two power outages were shorter compared to the third incident that took almost 311.95 h to restore. The third outages were shorter compared to the third incident that took almost 311.95 h to restore. The third power outage was the major one, and it happened due to the shortage of fuel resources on the grid power outage was the major one, and it happened due to the shortage of fuel resources on the grid side. Due to the power outages, the unmet electric load at the REUs is increased in each power outage side. Due to the power outages, the unmet electric load at the REUs is increased in each power outage incident, see in Figure 12. incident, see in Figure 12. From Figure 12, it is understood that the unmet electric load will increase if the EG is not restored within the specified time. The cumulative unmet electric load for the duration of power outage events is 31.211 kW (outage-1), 98.2 kW (outage-2), and 7676.08 kW (outage-3), respectively. If no backup power facility is available (either energy storage or generation), the REUs have to face the emergency, and this clearly shows that the preparedness towards grid outages from the REUs side is nil. As a result, REUs will have to experience deficit energy that is around 7805.49 kWh/y. In this case, the PSS configurations are not resilient. The indicators for resilience and the techno-economic and environmental feasibility of PSS configuration without any backup energy storage and generation are presented in Table5. Energies 2020, 13, 4193 17 of 27 Energies 2020, 13, x 16 of 26

Figure 12. The increasing trends of unmet electric load proﬁle vs. electric grid restoration time: Figure 12. The increasing trends of unmet electric load profile vs. electric grid restoration time: (a) (a) outage-1 (intentional attack); (b). outage-2 (system operational error); (c). outage-3 (fuel outage-1 (intentional attack); (b). outage-2 (system operational error); (c). outage-3 (fuel supply issue). supply issue).

FromTable 5.FigureTechno-economic 12, it is understood and environmental that the unmet indicators elec oftric a powerload will supply increase system if without the EG anyis not backup restored withinenergy the specified storage or time. generation The cumulative facility. unmet electric load for the duration of power outage events is 31.211 kW (outage-1), 98.2 kW (outage-2), and 7676.08 kW (outage-3), respectively. If no backup power facility is available (eitherParameters energy storage or generation), Valuethe REUs with have Units to face the emergency, and this clearly shows thatInitial the capital preparedness cost towards grid 0 (theoutages user doesfrom not the invest) REUs side is nil. As a result, REUs will have to experienceOperation costdeficit energy that is around 37,458.057805.49 kWh/y. $/y In this case, the PSS Cost of energy 0.1742 $/kWh configurations are not resilient. The indicators for resilience and the techno-economic and Total load demand 222,834 kWh/y environmental feasibilityLoad of PSS consumption configuration without any backup 215,029 energy kWh/ ystorage and generation are presented inPurchase Table 5. value of electricity from grid 37,458.05 $/y Grid sales 0 (there is no provision) Table 5. Techno-economicUnmet and electric environmental load indicators of a power 7805 supply kWh/y system without any backup energy storage orCO generation2 emissions facility. 61,713,323.00 g/y SO2 emissions 77,410.44 g/y NOParametersx emissions Value 43,005.80 with g /Unitsy Note:Initial $—United capital States cost dollar; y—year; kWh—kilo 0 (the watt user hour; does g—grams. not invest) Operation cost 37,458.05 $/y However, when it comesCost of to energy a practical situation, the REUs will 0.1742 be $/kWh prepared for a certain level Total load demand 222,834 kWh/y of emergencies, and aligns with the assumption made under the preparedness component of the Load consumption 215,029 kWh/y proposed resilience framework. Here, the REUs are already prepared for power outages, and as a Purchase value of electricity from grid 37,458.05 $/y result, in each PSS configuration,Grid sales BES is considered. Hence, by 0 (there considering is no provision) the grid uncertainty, the proposed three PSS configurationsUnmet electric for load REUs were studied to understand 7805 kWh/y which configuration is more resilient. In the below subsections,CO2 emissions the results of proposed PSS configurations 61,713,323.00 g/y are briefly presented. SO2 emissions 77,410.44 g/y 5.1. REU with EG + BES BasedNOx PSSemissions Configuration 43,005.80 g/y In EG + BESNote: based $—United PSS configuration, States dollar; the y—year; REUs loadkWh—kilo is directly watt hour; connected g—grams. to the EG. Since REUs are connected to the grid, the power supply will obviously be disrupted in the event of a power outage. However, when it comes to a practical situation, the REUs will be prepared for a certain level of But as per the proposed resilience framework, preparedness measure is already considered. So, the emergencies, and aligns with the assumption made under the preparedness component of the REUs are already prepared for power outage emergencies with backup power facilities using a BES. proposed resilience framework. Here, the REUs are already prepared for power outages, and as a result, in each PSS configuration, BES is considered. Hence, by considering the grid uncertainty, the

Energies 2020, 13, 4193 18 of 27

In general, the REUs will not have sufficient information to decide the battery storage capacity specific to unmet electric load during power outage events. Here, to have an understanding of the feasibility of EG + BES configuration, irrespective of the REUs ability to afford the BES, we modeled the battery capacity considering the maximum possible power outage duration as battery autonomy, i.e., roughly 13.18 days. Simulation is carried out with the power outage events during the usual conditions, where the REUs draw power from the EG, and at the same, the batteries are fully charged. Here, when the power outage occurred, the battery starts discharging for meeting the REUs electric load requirements. As batteries are charged to 100%, they can power the REUs for the complete blackout duration and thus meet the required load. The PSS configuration with EG + BES was able to supply power and can be one of the resilient solutions. The indicators for resilience and the techno-economic and environmental feasibility of PSS configuration are presented in Table6. However, when it comes to the practical situation, an individual REU may not be able to afford a BES with 13.18 days of battery autonomy.

Table 6. Techno-economic and environmental indicators of EG+BES based power supply system.

Parameters Value with Units Initial capital cost $2,028,188.00 Operation cost 37,458.05 $/y Cost of energy 0.1742 $/kWh Net present cost $4,045,528.28 Total load demand 222,834 kWh/y Load consumption 215,029 kWh/y Purchase value of electricity from grid 37,458.05 $/y Grid sales 0 (there is no provision) Unmet electric load 0 kWh/y CO2 emissions 64,351,413.00 g/y SO2 emissions 94,815.59 g/y NOx emissions 43,005.80 g/y Note: EG—Electric grid; BES—Battery energy storage; $—United States dollar; y—year; kWh—kilo watt hour; g—grams.

5.2. REU with EG + NGPG + BES Based PSS Configuration In EG + NGPG + BES based PSS configuration, the REUs electric load is connected to the NGPG and a BES. These two back up options will meet the electric loads in the case of emergencies. This configuration is considered under the prosumer category, but the choice of operation of the NGPG and power selling to the grid is more dependent on the REUs. The simulation is carried out, assuming that the REUs will operate the NGPG only in emergency situations. As per the PSS configuration, a simulation model is developed in the HOMER tool. The model based on sensitivity analysis showed that the NGPG was able to generate the electricity for the gird outage durations. The compensated load by NGPG during the power outage events is presented in Figures 13 and 14. In Figure 15, the power produced by the NGPG and along with the power purchased from the EG, is shown. From Figure 15, it is clear that in the event of a grid outage, the NGPG able to restore the PSS thereby provides continuous power supply to REU. The indicators for resilience and the techno-economic and environmental feasibility of PSS configuration are presented in Table7. Energies 2020, 13, x 18 of 26

Energies 2020, 13, 4193 19 of 27 Energies 2020, 13, x 18 of 26 Energies 2020, 13, x 18 of 26

Figure 13. Time series graph showing the operation of the natural gas power generator.

Figure 13. Time series graph showing the operation of the natural gasgas powerpower generator.generator. Figure 13. Time series graph showing the operation of the natural gas power generator.

Figure 14. Power compensated by the natural gas power generator.

In Figure 15, the power produced by the NGPG and along with the power purchased from the EG, is shown. Figure 14. Power compensated by the natural gas power generator. Figure 14. Power compensated by the natural gas power generator. In Figure 15, the power produced by the NGPG and along with the power purchased from the In Figure 15, the power produced by the NGPG and along with the power purchased from the EG, is shown. EG, is shown.

Figure 15. ShareShare of power production in EG + NGPGNGPG + BESBES based based power power supply supply system system configuration conﬁguration [Note: EG-Electric grid; NGPG NGPG-Natural-Natural gas power generator; BES-Battery energy storage].

Figure 15. Share of power production in EG + NGPG + BES based power supply system configuration Figure 15. Share of power production in EG + NGPG + BES based power supply system configuration [Note: EG-Electric grid; NGPG-Natural gas power generator; BES-Battery energy storage]. [Note: EG-Electric grid; NGPG-Natural gas power generator; BES-Battery energy storage].

Energies 2020, 13, x 19 of 26

From Figure 15, it is clear that in the event of a grid outage, the NGPG able to restore the PSS thereby provides continuous power supply to REU. The indicators for resilience and the techno- Energies 2020, 13, 4193 20 of 27 economic and environmental feasibility of PSS configuration are presented in Table 7.

Table 7. 7. Techno-economicTechno-economic and and environmental environmental indicators indicators of EG of + EG NGPG+ NGPG + BES +basedBES power based supply power system.supply system. Parameters Value with Units Parameters Value with Units Initial capital cost $38,741.09 OperationInitial capital cost cost 39,857.95 $38,741.09 $/y Operation cost 39,857.95 $/y Cost of energy 0.1903 $/kWh Cost of energy 0.1903 $/kWh Net present cost $634,431.83 Net present cost $634,431.83 TotalTotal load loaddemand demand 222,834 222,834 kWh/y kWh/y LoadLoad consumption consumption from fromgrid grid 215,029 215,029 kWh/y kWh/y PurchasePurchase value value of electricity of electricity from from grid grid 38,817.68 38,817.68 $/y $/y GridGrid sales sales 00 (there (there is is provision provision but but used used as as standby standby option) UnmetUnmet electric electric load load 0 0 kWh/y kWh/y LoadLoad supplied supplied by a bynatural a natural gas generator gas generator 7805 7805 kWh/y kWh/y Fuel Fuelcost cost 1,157.41 1157.41 $/y $/y CO2 COemissions2 emissions 65,741,000.00 65,741,000.00 g/y g/y SO emissions 76,400.00 g/y SO2 emissions2 76,400.00 g/y NOx emissions 46,000.00 g/y NOx emissions 46,000.00 g/y Note:Note: EG—ElectricEG—Electric grid; grid; NGPG—Natural NGPG—Natural gas powergas powe generator;r generator; BES—Battery BES—Batte energyry storage; energy $—United storage; States $— dollar; y—year; kWh—kilo watt hour; g—grams. United States dollar; y—year; kWh—kilo watt hour; g—grams. 5.3. REU with EG + PV + BES Based PSS Conﬁguration 5.3. REU with EG + PV + BES based PSS Configuration 5.3.1. Weather Data for PV Modelling 5.3.1. Weather Data for PV Modelling For the studied residential location, throughout the year, there exists a signiﬁcant amount of solar For the studied residential location, throughout the year, there exists a significant amount of radiation potential but with varying capacities. The data on solar radiation potential is collected from solar radiation potential but with varying capacities. The data on solar radiation potential is collected the National Aeronautics and Space Administration (NASA) databases using the data access provision from the National Aeronautics and Space Administration (NASA) databases using the data access provided by the HOMER tool and presented in Figure 16. provision provided by the HOMER tool and presented in Figure 16.

FigureFigure 16. 16. SolarSolar radiation radiation potential potential available available in in the the location. location.

FromFrom Figure Figure 16,16 ,at at the the REUs, REUs, the the observed observed sola solarr radiation radiation power power potential potential varies varies from from0.02 kW/m0.02 kW2/day/m2 /today 0.48 to 0.48kW/m kW2/day,/m2/day, which which is sufficient is suﬃcient to topower power the the REUs. REUs. In In terms terms of of energy energy potential, potential, thethe dailydaily potentialpotential for for the the given given month month is varied is varied between between 1.67 to 5.671.67 kWhto 5.67/m2 /daykWh/m recording2/day maximumrecording maximumin June and in minimum June and in minimum December. in The December. annual average The annual daily solaraverage radiation daily issolar 3.80 kWhradiation/m2/day, is 3.80 and kWh/mthe observed2/day, clearnessand the observed sky index clearness is 0.4906, whichsky index is suitable is 0.4906, for solarwhich power is suitable generation. for solar Apart power from solar radiation, in the PV system modeling, we used wind speed and ambient temperatures to estimate the impact of module temperature on the total power produced. In the considered locations, the Energies 2020, 13, x 20 of 26

Energiesgeneration.2020, 13 Apart, 4193 from solar radiation, in the PV system modeling, we used wind speed and ambient21 of 27 temperatures to estimate the impact of module temperature on the total power produced. In the considered locations, the observed annual average wind speed for the last ten years was around 5.7 observed annual average wind speed for the last ten years was around 5.7 m/s. The wind speeds are m/s. The wind speeds are ranged between 4.60 to 6.68 m/s, with an average of 2.75 m/s. The annual ranged between 4.60 to 6.68 m/s, with an average of 2.75 m/s. The annual average temperature is average temperature is observed at 11.68°C. The monthly average daily temperatures were observed observed at 11.68 C. The monthly average daily temperatures were observed to vary between 0.99 to to vary between −◦0.99 to 23.89°C recording the minimum in January and maximum in July. − 23.89◦C recording the minimum in January and maximum in July.

5.3.2. Power Performance of EG + PVPV ++ BESBES based Based PSS PSS Configuration Configuration In EGEG + PV + BES-basedBES-based PSS PSS configuration, configuration, the the elec electrictric load load is connected to an on-site solar PV power plant as well as the EG. This configuration configuration is considered under the prosumer category, where the REUs sellsell thethe powerpower toto thethe grid.grid. The simulation is carried out byby consideringconsidering the locallocal weatherweather parameters, as discussed in Sectio Sectionn 5.3.1.5.3.1 andand powerpower outageoutage conditions,conditions, as shown in Table1 1.. InIn ourour simulation, thethe eeffectffect of of sensitive sensitive parameters parameters on on the th overalle overall power power generation generation is also is also observed. observed. From From the investigation,the investigation, it was it understood was understood that the that PV plant the wasPV ableplant to was meet able the electricalto meet load the requirementselectrical load of REUs,requirements and at theof REUs, same and time, at it the was same able time, to sell it was power able to to the sell EG. power The to power the EG. generation The power profile generation under EGprofile+ PV under+ BES-based EG + PV PSS + BES-based configuration PSS is configuration depicted in Figure is depicted 17. The in indicators Figure 17. for The resilience indicators and thefor techno-economicresilience and the andtechno-economic environmental and feasibility environmenta of PSSl configuration feasibility of PSS are presentedconfiguration in Table are 8presented. in Table 8.

Figure 17. ShareShare of power production EG + PVPV ++ BESBES ba basedsed power power supply supply system system configuration conﬁguration [Note: [Note: EG-Electric grid; PV-Photovoltaics; BES-BatteryBES-Battery energyenergy storage].storage].

Table 8.8.Techno-economic Techno-economic and and environmental environmental indicators indicators of EG of+ EGPV ++ PVBES + based BES powerbased power supply supply system consideringsystem considering only REUs only load. REUs load.

ParametersParameters ValueValue with Units with Units InitialInitial capital capital cost cost $83,078.76 $83,078.76 OperationOperation cost cost 47,236.07 47,236.07 $/y $/y Cost ofCost energy of energy 0.05560 0.05560 $/kWh $/kWh Net presentNet present cost cost $485,892.00 $485,892.00 Total loadTotal demand load demand 222,834 222,834 kWh/ ykWh/y Load consumptionLoad consumption from grid from grid 26,515.00 26,515.00 kWh/ ykWh/y PurchasePurchase value ofvalue electricity of electric fromity the from grid the grid 4618.91 4618.91 $/y $/y EnergyEnergy fed to gridfed to sales grid sales 471,079 471,079 kWh/ ykWh/y Grid salesGrid value sales value 349,540.62 349,540.62 $/y $/y Unmet electric load 0 kWh/y Unmet electric load 0 kWh/y CO2 emissions 18,407,350.00 g/y CO2 emissions 18,407,350.00 g/y SO2 emissions 84,146.62 g/y SO2 emissions 84,146.62 g/y NOx emissions 44,566.80 g/y NOx emissions 44,566.80 g/y Note: EG—Electric grid; PV—Photovoltaics; BES—Battery energy storage; $—United States dollar; y—year; kWh—kiloNote: EG—Electric watt hour; g-grams.grid; PV—Photovoltaics; BES—Battery energy storage; $—United States dollar; y—year; kWh—kilo watt hour; g-grams.

Energies 2020, 13, 4193 22 of 27

5.4. Comparison of PSS Configurations In this section, the investigated three PSS configurations were compared based on their resilience, and techno-economic and life cycle-based environmental feasibility indicators. The detailed comparison of the indicators is presented in Table9. In Table 10, the renewable-based prosumer category of the PSS configuration (i.e., EG + PV + BES) results are compared considering the two conditions that are with and without sales to EG.

Table 9. Comparison of resilience and techno-economic and life cycle based environmental feasibility indicators for three PSS conﬁgurations.

Parameters EG + BES EG + NGPG + BES EG + PV + BES Initial capital cost $2,028,188.00 $38,741.09 $83,078.76 Operation cost 37,458.05 $/y 39,857.95 $/y 47,236.07 $/y Cost of energy 0.1742 $/kWh 0.1903 $/kWh 0.05560 $/kWh Net present cost $4,045,528.28 $634,431.83 $485,892.90 Unmet electric load 0 kWh/y 0 kWh/y 0 kWh/y CO2 emissions 64,351,413.00 g/y 65,741,000.00 g/y 44,316,890.00 g/y SO2 emissions 94,815.59 g/y 76,400.00 g/y 263,161.24 g/y NOx emissions 43,005.80 g/y 46,000.00 g/y 138,859.60 g/y Note: EG—Electric grid; BES—Battery energy storage; NGPG—Natural gas power generator; PV—Photovoltaics; $—United States dollar; y—year; kWh—kilo watt hour; g—grams.

Table 10. Comparison of resilience and techno-economic and life cycle based environmental feasibility indicators for EG + PV + BES based power supply system conﬁguration with and without sales to EG.

EG + PV + BES Parameters With Sales to EG Without Sales to EG Initial capital cost $83,078.76 $83,078.76 Operation cost 47,236.07 $/y 47,236.07 $/y Cost of energy 0.05560 $/kWh 0.05560 $/kWh Net present cost $485,892.90 $485,892.90 Unmet electric load 0 kWh/y 0 kWh/y CO2 emissions 44,316,890.00 g/y 18,407,350.00 g/y SO2 emissions 263,161.24 g/y 84,146.62 g/y NOx emissions 138,859.60 g/y 44,566.80 g/y Note: EG—Electric grid; BES—Battery energy storage; PV—Photovoltaics; $—United States dollar; y—year; kWh—kilo watt hour; g—grams.

Based on the obtained comparative results shown in Tables9 and 10, it is understood that the three configurations (EG + BES; EG + NGPG + BES; EG + PV + BES) were observed to be resilient and were able to supply power during power outages. Here, the resilience was achieved with optimal planning of energy infrastructure under each PSS configuration. In addition, based on the nature of the PSS category, i.e., consumer or prosumer, we observed a significant effect on the techno-economic and environmental indicators. For example, the initial capital cost was observed to be very high for EG + BES configuration, and this is because of the battery capacity needed to meet the massive power outage that lasts for almost 316 h. The initial capital cost for EG + NGPG + BES-based PSS configuration is quite less, as it is only planned for emergency backup. If the NGPG is made to operate continuously considering the grid sales, the capital cost will increase; also, the operating and maintenance cost would be very high. The third PSS configuration, i.e., EG + PV + BES, can be operated in prosumer mode, and in this configuration, the initial cost would be a little high but with almost zero investment on fuel resources, which is better when compared to NGPG. In addition, the payback is also viable. PV, when operated considering the REUs load consumption alone, has lower amounts of emission than the operation with grid sales. Even from the CO2 emission point of view, the PSS with PV and BES seems to have lower emissions. In addition, the observed CoE is lower for the PSS with PV and BES. Energies 2020, 13, 4193 23 of 27

Overall, after a thorough quantiﬁcation of the techno-economic and life cycle based environmental indicators, the EG + PV + BES was able to perform well by satisfying the resilient condition and ensuring the power supply.

5.5. Discussion on Ensuring Resilience and Future Research Directions In this section, a brief discussion is provided on how the studied three PSS configurations ensured the power resilience considering the four-component resilience framework. Besides, based on the provided discussion, few unaddressed issues were identified, and accordingly, the future research directions are proposed. In this study, the proposed three PSS configurations were resilient enough in supplying power to the REUs in the grid outage events, but each configuration differed in their techno-economic and life cycle-based environmental indicators as discussed in Section 5.4. The resilience is investigated based on PSS functionality variation, which is the function of the available power and the electricity demand at the REUs. In the case of REU that is dependent only upon the EG (where there is no facility for on-site generation and storage), due to grid outage, there is a sudden fall in PSS functionality to zero. The fall to zero would happen immediately as there is no backup, thus low preparedness. The value of PSS functionality will be maintained as zero until the grid restoration has taken place, making the system not robust to grid outages. However, the recovery is observed to be 100%, but only after a dedicated amount of time, i.e., the time taken for grid restoration. The fourth component, i.e., adaptation, is more important. Based on the experiences, the REUs must learn and must adapt existing approaches that make the system more resilient. The adaptation is more related to improving the system efficiency and associated assets, which involves capital investment. With this, the PSS can adapt to previously experienced situations and is able to enhance preparedness to deal with the outage in the future. In the case of EG + BES-based PSS configuration, during the grid outage, the PSS functionality is observed to vary. Here the resilience of PSS configuration is dependent upon the size of BES. If the considered BES capacity can manage the unmet loads, there will not be any drop in PSS functionality, and it is always maintained at maximum until and unless there is another disturbance at BES. The size of BES would again influence capital investment. The four-components of resilience will be affected; for instance, preparedness can be achieved even with low capacities of BES, which needs a lower investment, whereas the PSS may not be robust enough if the grid restoration times are longer. The recovery component is again dependent upon the capacity of the BES and grid restoration times. The other two considered PSS configurations can generate on-site power generation. During the grid outages, they provide continuous power supply making the PSS configurations more resilient. In the case of EG + NGPG + BES, the PSS functionality is more dependent upon the NGPG operation time and natural gas fuel availability at the REUs, whereas, in the case of EG + PV + BES, the PSS functionality is more dependent upon the intermittent nature of the solar irradiance. In the considered location, these two resources are available, hence the impact on PSS functionality and resilience components is not much. Based on the above-provided discussion, a few future research directions can be proposed. These include: Investigation of the investments made in energy infrastructure and their impacts on improving • resilience can be considered as one of the research directions, as in our study. We observed the variations in economic indicators based on the proposed energy infrastructure. Resilience framework incorporating islanding and other grid disturbance detection approaches • could be considered as one possible future research direction. Tools that support grid restoration can be beneficial in optimal scheduling of on-site power • generation facilities at REUs side, and in fact, they could influence the robustness component of the resilience cycle. Modernization and re-design of PSS infrastructure by using advanced technologies like • Blockchain [42–44], Internet of Things [45], Energy Internet [46], Blockchain-based Internet Energies 2020, 13, 4193 24 of 27

of Things (B-IoT) [47], and Artificial Intelligence (AI) techniques such as Machine Learning (ML), and Deep Learning (DL) [48] could favor in tracking the power outage events and thereby allows us to have a data-driven solution. Hence, power sector digital transformation from the context of resilience can be one of the possible future research directions. Location and ecosystem specific studies can be further modeled to evaluate the feasibilities of PSS • configurations in the context of the proposed resilience framework.

6. Conclusions In this study, three different PSS configurations to meet the load demand of the REUs were proposed. The main objective of this study was to understand the resilience of PSS configurations during the power outages events. Besides, the resilient PSS configuration should be feasible from the techno-economic and life-cycle environmental emission point of view. For understanding this, a four-component resilience framework is proposed along with few indicators. As per the proposed framework, we observed that all three PSS configurations (that include the EG + BES; EG + NGPS + BES; and EG + PV + BES) were found to provide power during the power outages events. In all the three PSS, the resilience indicator, i.e., the unmet electric load, is observed to be zero. Even though there is an equal improvement in resilience, but in the context of techno-economic and lifecycle-based environmental indicators, each PSS differed. Based on this investigation, the following conclusions were drawn:

The configuration with the BES alone as a support option may not be feasible for longer power • outage scenarios. The configuration with fossil fuel-based PSS configuration (i.e., NGPG) would definitely be a • solution; however, it is not feasible from the perspective of the least cost of energy and lowest life-cycle emissions. A PV plus BES-based PSS configuration would be much more feasible under the prosumer only • category, as it allows energy trade between the REUs and EG. The EG + PV + BES based PSS configuration is observed to enhance the overall resilience, thereby • help in improving the energy accessibility to REUs.

Overall, it is understood that the PSS conﬁgurations can be resilient as prosumers to the grid outage conditions. At the same, the PSS can be feasible from the techno-economic and life cycle-based environmental perspective.

Author Contributions: Conceptualization, N.M.K., and S.S.C.; Data curation, N.M.K.; Formal analysis, N.M.K.; Funding acquisition, A.G. (towards APC), and S.S.C.; Methodology, N.M.K., and S.S.C.; Resources, S.S.C.; Software, N.M.K., and S.S.C.; Supervision, S.S.C.; Validation, N.M.K.; Visualization, N.M.K.; Writing—original draft, N.M.K., Writing—review and editing, N.M.K., A.G., and S.S.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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