The Potential of Utilizing Buildings' Foundations As Thermal Energy

energies

Article The Potential of Utilizing Buildings’ Foundations as Thermal Energy Storage (TES) Units from Solar Plate Collectors

Lazaros Aresti 1 , Paul Christodoulides 2,* , Gregoris P. Panayiotou 2 and Georgios Florides 2

1 Department of Electrical Engineering, Computer Engineering and Informatics, Cyprus University of Technology, P.O. Box 50329, Limassol 3603, Cyprus; [email protected] 2 Faculty of Engineering and Technology, Cyprus University of Technology, P.O. Box 50329, Limassol 3603, Cyprus; [email protected] (G.P.P.); georgios.ﬂ[email protected] (G.F.) * Correspondence: [email protected]

Received: 30 April 2020; Accepted: 23 May 2020; Published: 27 May 2020

Abstract: Underfloor heating systems provide comfort due to the natural heat flow distribution by a network of pipes, conventionally connected to a heat pump operating at low temperatures. To this extent, a renewable energy source could be an alternative solution. Acting as a case to investigate such systems, the Mediterranean island of Cyprus with a plethora of sunny days points to solar energy as the obvious solution. In this study, solar collector systems are recruited to supply the required heat for a typical Cypriot house, with the building’s foundation acting as a thermal energy system (TES) unit. The heat supply to the building can then be distributed with natural convection from the TES. The solar collectors and the building’s foundation system are studied with the aid of two software programs, namely TRNSYS and COMSOL Multiphysics. The former is used for the calculation of the heating and cooling load of the house as well as to estimate the energy provided by the flat plate solar collectors at specific conditions. The latter is then used to examine the TES unit with the heat gain/loss of the building. The obtained results, including analyses on the solar collectors’ area and the foundation thickness indicate that the suggested system would be able to sufficiently cover, partially or fully, the building’s heating load.

Keywords: thermal energy storage (TES); building foundation TES; solar collectors; building heating load; COMSOL Multiphysics; TRNSYS

1. Introduction Solar energy is a freely available source that can either be harvested for electricity or converted into thermal energy for heating purposes as well as for domestic hot water applications. The problem with thermal energy conversion is that the availability of solar energy is mismatched with demand. In winter thermal energy demand is high during the night and early morning, with the maximum solar energy is available around midday. In the summer solar energy availability is significantly high, but demand is minimum. Still, solar energy is used for domestic hot water (DHW) with the aid of solar collectors and thermosiphons, with a high percentage of application in sunny countries, both in summer and winter. A related noteworthy fact is that the maximum efficiency of a photovoltaic/thermal (PVT) system is at about 63% (glazed and water-based; it can be higher for other fluids) [1], solar flat plate collectors (FPC) at 80% (water-flow based) [2], while that of photovoltaics (PV) for electricity production at 28% (crystalline-cells based; can be higher for other cells) [3], depending of course on the climatic, operating and design parameters. It would be, therefore, natural and sensible to use solar energy for thermal

Energies 2020, 13, 2695; doi:10.3390/en13112695 www.mdpi.com/journal/energies Energies 2020, 13, 2695 2 of 14 purposes. This raises the issue of utilizing the solar energy in buildings by storing the thermal energy when available and releasing it on demand when needed. Thermal storage can be described by the storage concept and the storage mechanism [4]. The storage concept is divided into active and passive, where in an active method there is use of forced convection, and in a passive method gravitational forces are utilized to circulate the fluid in the system [5]. Storage mechanisms on the other hand can be sensible, latent and chemical systems. The chemical process uses a chemical reaction where the latent uses phase changing materials (PCMs). Although the chemical and the latent systems have proven to be the more advantageous mechanisms [6], the sensible mechanism is the most commonly used, whereby heat is stored as internal energy in the medium, which is either a liquid or a solid [5]. Water, rocks and soil are the most common materials, but also concrete has been tested for short term sensible storage [7]. Concrete, brick and gypsum have been studied as sensible thermal storage building materials, with conventional 20 cm concrete shown to be the most suitable one [8]. For the selection of the sensible heat storage medium (including concrete), Hariri and Ward [5,9] have provided principal characteristics limiting the choices. to the volumetric heat capacity, the system operation temperatures, the medium transfer properties (such as toxicity, corrosiveness, etc.), the stratification of the storage unit, the charging and discharging power, and the thermal loss control. Additionally, the choice of the storage medium lies on the available space, where at residential applications there is usually a shortage. Sensible TES systems, in order to avoid the heat losses when fully charged, are fully insulated and often placed underground. Ucar and Inalli [10] have performed a comparison between the cylindrical tanks placed uninsulated underground and insulated and uninsulated above ground, with results indicating that the best performance is provided by the underground placed storage tank. Methodologies for performance evaluation and analysis of sensible TES systems (always including concrete) have been presented by several authors [11–14], where the exergy analysis seems to have a higher potential among other methods [14]. The idea of incorporating PCM materials in building materials in order to achieve higher thermal properties for thermal energy storage has been proposed in the literature. The feasibility of such a system is extensively reviewed in [15,16]. Although promising as a thermal storage unit, PCM exhibits weaknesses when it is used as a building foundation, with regard to cost (higher), strength (lower) and fire resistance (lower) compared to reinforced concrete. Good examples of solar energy storage systems have been presented in [17,18], where the energy collected by the solar collectors has been stored in a pebble bed. The systems have been applied to a building (large scale, 3320 m2) in China and the results obtained can be considered as satisfactory, with the temperature of the pebbles increasing during the day (charging stage) and dropping during the night (discharging stage). It should be noted that the pebble bed was preheated days before the heating season. In the current study the building’s foundation, constructed with reinforced concrete, is used and studied as passive heating with the aid of solar collectors. The study is an extension of the previously proposed system, by Aresti el al. [19], where a preliminary investigation of passive floor heating from solar collectors was performed. There are several components/elements that act as thermal energy storage (TES) or seasonal TES (STES) units, such as, water tanks, aquifers, ponds, rock or gavel beds, ground, oil or salt solutions and solid blocks from different materials (concrete). Conventionally, sensible heat storage is used, and water is assumed to be the storage of choice for years to come [6]. A review on sensible heat storage materials can be found in [20], where it is stated that low cost, availability and good mechanical and thermal properties are among the advantages of using concrete as a TES unit. The authors also noted that the compressive strength decreases by about 20% when the temperature is at 400 ◦C. Thermal properties and costs for different sensible TES solid materials have been compared in [21]. The authors present concrete as the lowest cost material in terms of USD per kg and USD per kWhT (namely, (0.05 USD/kg and 1 USD/kWhT). Also, a large quantity Energies 2020, 13, 2695 3 of 14

(300) of engineering materials have been compared for sensible heat storage in [22], with the authors suggesting that for long term storage the materials require to have high heat capacity per unit volume and thermal conductivity above 1 W m 1 K 1. Concrete is included in the selection of materials, the · − · − authors identified as with high energy density. Concrete can be considered as a heterogeneous material with several factors such as water cement, voids and aggregation, affecting its thermal properties [23,24]. Different types of concrete with their thermal properties have been presented in [24]. The thermal properties and their accurate estimation are, of course, of great importance as they contribute to the energy calculations and energy rating of a building. For shallow foundations, such as residential buildings in Cyprus, concrete is reinforced with horizontal two-way steel bars. The reinforced concrete compared to standard concrete has a greater thermal conductivity and density, due to the enhancement with steel, but lower specific heat at constant pressure. Such and similar cases have been studied in [25], where the thermal conductivity presented varies between 1.4–1.6 W m 1 K 1. · − · − Similar to a shell and tube type heat exchangers [26], for solid media storage, concrete would act as the shell, where the fluid passes through the tubes. A piping network is placed inside the storage medium and is used as an exchanger for heat transfer to and from the building’s foundation. Dwellings in Cyprus have foundations made of reinforced concrete, which is hence an obvious choice for TES, with the only additional cost being that of piping and labor. Although the reinforced concrete foundation is already available, the piping installation can only be installed in new dwellings and this system cannot be retrofitted. Since the concrete foundation block provides structural support and strength to the dwellings, it cannot be examined only as a TES unit, but as a hybrid system as well. Such effects on the concrete properties as due to temperature changes have been presented in [27]. The authors have reported a reduction in the concrete’s strength with an increase of temperature, where cracks become visible with temperatures above 600 ◦C and a complete decomposition occurs at temperatures of 1200 ◦C. The use of high temperature concrete modules as TES unit of sensible heat in solar power plans has been examined in [28] and guidelines have been presented. The authors have specified a maximum operating temperature of 300 ◦C. As stated in [29], the first effects on the concrete will occur just above 100 ◦C due to the evaporation of the moisture. Hence, in order to achieve this concrete hybrid TES structure, it is important to maintain concrete foundation at a temperature lower than 90–100 ◦C, in order to avoid evaporation and preserve its integrity and strength. As mentioned above such a system is examined here for a typical house in the moderate climate in Cyprus. The study is performed numerically, with the computational modeling—described in Section2—being conducted in two stages. In the first stage the TRNSYS software is used to simulate a south facing building with solar collectors at an optimal slope. The house load is computed in hourly steps for a typical Cypriot house at fixed room temperature, with the solar collectors exposed at the climate conditions of the capital Nicosia, in central Cyprus. Then, in Section3, the results from the TRNSYS software are incorporated into the COMSOL Multiphysics software for the further examination of using the building’s concrete foundation as a sensible TES unit.

2. Computational Modeling The model design and the simulation process were carried out in two stages. The first stage was performed in the TRNSYS software (Thermal Energy System Specialists, LLC, Madison, WI, USA) through two different configurations, while the second stage was performed in the COMSOL Multiphysics software [30] for a more detailed analysis of the system. In order to examine the concept under investigation, a typical house in Cyprus was chosen as a building of study. The characteristics of the residential buildings in Cyprus have been presented in a statistical format in [31]. Out of a sample of 500 residential buildings, the authors have found that the large majority of the houses are between 51–200 m2, with a mean area of 173 m2 corresponding to 57 m2/occupant. It has also been calculated that the primary energy per total area is between Energies 2020, 13, 2695 4 of 14

Energies 2020, 13, x FOR PEER REVIEW 4 of 14 51–150 kWh/m2. In addition, statistics composed by the Cyprus Ministry of Finance Statistical NoteService also [32] that, have according shown that into thea Renewables construction industryGlobal status the constructed report [33] dwellings, Cyprus are is mainly the world houses leader in solar(in 2016,water 1709 collector houses per versus capita 746, apartmentswith 541 kW were thermal completed). per 1000 inhabitants. As previously reported Note also that, according to a Renewables Global status report [33], Cyprus is the world leader in in [34], solar water heating systems were installed in more than 93% of houses in 2006. solar water collector per capita, with 541 kW thermal per 1000 inhabitants. As previously reported in [34], solar water heating systems were installed in more than 93% of houses in 2006. 2.1. Typical Residential Building loads 2.1. Typical Residential Building Loads The selected residential building modeled here had a rectangular shape with the elongated side facing southThe and selected an area residential of 120 buildingm2. It consisted modeled of here a single had a floor rectangular with minimum shape with insulation the elongated onside the outer 2 wall facingand some south insulation and an area on of the 120 mroof.. It consisted of a single ﬂoor with minimum insulation on the outer wallThe andwalls some considered insulation during on the roof.the modeling consisted of the following layers: 0.025 m of plaster The walls considered during the modeling consisted of the following layers: 0.025 m of plaster and 0.2 m hollow clay brick. The roof consists of a layer of 0.025 m plaster, 0.15 m reinforced concrete and 0.2 m hollow clay brick. The roof consists of a layer of 0.025 m plaster, 0.15 m reinforced concrete with with1% iron 1% iron and and 0.05 0.05 m mof ofextruded extruded polystyrene polystyrene insulation.insulation For. For the the estimation estimation of the of buildingthe building loads loads the floorthe ﬂoor was was considered considered to tohave have no no heat heat losses toto the the ground. ground. The The model model considered considered was builtwas onbuilt on TRNSYSTRNSYS (see (seeFigure Figure 1),1 ),with with following following types used used for for the the computations: computations:

i. Typei. Type 109— 109—TMY2TMY2 (weather (weather data data processing processing model),model), ii. Typeii. Type 33— 33—Psychrometrics,Psychrometrics, iii. Typeiii. Type 69— 69—EEffectiveffective sky sky temperature temperature for for long-wavelong-wave radiation radiation exchange, exchange, iv. Typeiv. Type 2—ON/OFF 2—ON/OFF Differential Differential controller, controller, v. v.Type Type 65— 65—OnlineOnline graphical graphical plotter, plotter, vi. vi.TypeType 25— 25—Printer—TRNSYS—suppliedPrinter—TRNSYS—supplied units printedprinted to to output output file, file, vii. vii.TypeType 56— 56—Multi-ZoneMulti-Zone Building. Building.

Figure 1. TRNSYS house load model set up. Figure 1. TRNSYS house load model set up. TRNSYS [35] is a user interface friendly software used to simulate transient systems and their performanceTRNSYS [35] with is a the user aid ofinterface integrated friendly types/modules. software Manual used to types simulate can be createdtransient with systems one need and to their performanceuse FORTRAN with orthe C ++aidlanguages. of integrated The usertypes is/ responsiblemodules. Manual to ﬁll the types parameters can be of created the types with based one on need to usethe FORTRAN manufacturers or C++ speciﬁcations languages. and The organize user theis responsible system’s input to and fill output.the parameters TRNSYS is of usually the types used based on thefor manufacture energy systemsrs specifications but can have a widerand organize application, the with system fast producing’s input and analytical output. results. TRNSYS is usually The building was set to steady temperatures of 21 C and 27 C for heating and cooling, respectively. used for energy systems but can have a wider application,◦ with◦ fast producing analytical results. The building’s energy demand in terms of heating and cooling loads were calculated with one-hour Thetime stepsbuilding for one was year. set Additionally, to steady thetemperatures energy collected of 21 through °C and the solar27 °C collector for heating system wasand also cooling, respectively.calculated The during building the same’s energy time and demand under the in same terms conditions. of heating and cooling loads were calculated with one-hour time steps for one year. Additionally, the energy collected through the solar collector system was also calculated during the same time and under the same conditions.

2.2. Solar Collectors Model In order to store solar energy, a system, similar to domestic hot water (DHW), was modeled and simulated using TRNSYS. The building was considered to have an 8 m2 flat plate solar collector, facing south with a slope of 45° on the building’s roof. Specifically, the flat place solar collectors are used to transfer heat, with the aid of a pump, into a TES unit; the building’s foundation in this case. The model is a simulation on TRNSYS (Figure 2), with the use of various types. TMY2 (type 15) was used for producing the meteorological data required (i.e., incident solar radiation, diffuse solar

Energies 2020, 13, 2695 5 of 14

2.2. Solar Collectors Model Energies 2020, 13, x FOR PEER REVIEW 5 of 14 In order to store solar energy, a system, similar to domestic hot water (DHW), was modeled and simulated using TRNSYS. The building was considered to have an 8 m2 flat plate solar collector, facing radiation), while the solar collectors (type 1b) was used to model the thermal performance of a flat- south with a slope of 45◦ on the building’s roof. Specifically, the flat place solar collectors are used to plate solartransfer collector heat, withs. The the thermal aid of a pump, performance into a TES unit; of the the building’scollector foundation array wa ins determined this case. The model by the number of modulesis a simulationin series onand TRNSYS the characteristics (Figure2), with the of useeach of variousmodule. types. The TMY2 coefficients (type 15) wasof the used function for were suppliedproducing by an ASHRAE the meteorological or equivalent data required test. (i.e., The incident circulation solar radiation, pump di ff(tuseype solar 114 radiation),) was a while constant fluid the solar collectors (type 1b) was used to model the thermal performance of a flat-plate solar collectors. mass flowThe rat thermale pump performance. The equation of the collector type wa arrays used was determinedto act as the by thecontroller number of of modules the pump in series setting it ON or OFF accordingand the characteristics to the season of each module.(only winter The coe ffirequirescients of the heating) function wereand suppliedthe difference by an ASHRAE between outlet temperatureor equivalent of the test.solar The collector circulation and pump the (type temperature 114) was a constant stored fluid in the mass TES flow u ratenit. pump. A more The advanced controllerequation was also type wasexamine used tod act using as the controllerthe microprocessor of the pump setting controller it ON or OFF (type according 40) to to thereplace season the signal (only winter requires heating) and the difference between outlet temperature of the solar collector equation type. The introduced controller applies a time delay of one hour when setting the pump and the temperature stored in the TES unit. A more advanced controller was also examined using OFF. Comparedthe microprocessor to the simple controller controller (type 40) tousi replaceng the the signal signal equationequation type. type The the introduced advance controllerd controller does not seemapplies to have a time a significant delay of one hourchange when on setting the theoutcome pump OFF. compared Compared to to theON simple-OFF controller, when using using 1h delay. When a thefurther signal delay equation of type two the hours advanced or controllermore is doesapplied not seem, the to TES have aaverage significant temperature change on the decreases, outcome compared to ON-OFF, when using 1 h delay. When a further delay of two hours or more is something not desirable for the application. Hence, as the choice is between the 1h delay and the no- applied, the TES average temperature decreases, something not desirable for the application. Hence, delay ONas-OFF, the choice for simplicity is between the the 1 hlatter delay controller and the no-delay was used ON-OFF, in the for simplicityforward thesimulations latter controller. It should also be notedwas that used the in TES the forwardunit had simulations. a low overall It should heat also transfer be noted coe thatfficient the TES in unit relation had a low to the overall surrounding environment.heat transfer This was coeffi explicitlycient in relation calculated to the surrounding in COMSOL environment. with the This build was explicitly-in equations calculated and in was found COMSOL with the build-in equations and was found to correspond to an overall 0.6 W m 2 K 1, to be to correspond to an overall 0.6 W∙m−2 ∙K−1, to be applied in TRNSYS. · − · − applied in TRNSYS.

Figure 2. TRNSYS solar collectors model set-up. Figure 2. TRNSYS solar collectors model set-up. The simulations were carried out for an entire year with hourly time steps for the weather conditions of Nicosia, Cyprus. During the simulation process the useful energy produced by the solar The collectorssimulation in thes formwere of c hotarried water out together for an with entire the temperature year with of thehourly water, time the ﬂowrate steps andfor thethe weather conditionspump of Nicosia signal were, Cyprus. recorded During in order tothe be simulation used in the COMSOL process Multiphysics the useful stageenergy of this produced work. by the solar collectors in the form of hot water together with the temperature of the water, the flowrate and the 2.3. Building’s Foundation pump signal were recorded in order to be used in the COMSOL Multiphysics stage of this work. Next, to examine the thermal distribution in the building’s foundation, the COMSOL Multiphysics 2.3. Buildingsoftware’s Foundation was used. COMSOL Multiphysics is a multi-physics software with user friendly interface, where the geometry, construction of the mesh and prost-processing procedures are in the same Next, to examine the thermal distribution in the building’s foundation, the COMSOL Multiphysics software was used. COMSOL Multiphysics is a multi-physics software with user friendly interface, where the geometry, construction of the mesh and prost-processing procedures are in the same environment. Equations were selected according to the physical parameters involved; note that equations can also be added and edited manually. A 3D computer aided design (CAD) model of the building’s foundation was constructed using COMSOL Multiphysics’s integrated part builder. The model consisted of two domains; the concrete and the pipes. To convert the model into a heat exchanger, a piping system was introduced following a “snake” configuration, as shown in Figure 3. Higher mesh density is concentrated on the pipes and the top surface with a growth factor of 1.4.

Energies 2020, 13, 2695 6 of 14 environment. Equations were selected according to the physical parameters involved; note that equations can also be added and edited manually. A 3D computer aided design (CAD) model of the building’s foundation was constructed using COMSOL Multiphysics’s integrated part builder. The model consisted of two domains; the concrete and the pipes. To convert the model into a heat exchanger, a piping system was introduced following a “snake” conﬁguration, as shown in Figure3. Higher mesh density is concentrated on the pipes and the Energies 2020, 13, x FOR PEER REVIEW 6 of 14 top surface with a growth factor of 1.4.

Figure 3. Geometry of the buildings foundation showing the piping system, units in m.

GroundFigure thermal 3. Geometry and other of the characteristics buildings foundation and reinforced showing the concrete piping characteristicssystem, units in werem. based on previous publications [25,36,37]. The building’s foundation dimensions were 12 10 1 m Ground thermal and other characteristics and reinforced concrete characteristics were× based× on (length width depth), insulated in all directions except the top surface. Initial conditions were previous× publications× [25,36,37]. The building’s foundation dimensions were 12 × 10 × 1 m (length × taken after a year’s run from TRNSYS, assuming that the reinforced concrete was pre-heated. width × depth), insulated in all directions except the top surface. Initial conditions were taken after a The convection diffusion equation was used under the Heat Transfer in Solids module, with year’s run from TRNSYS, assuming that the reinforced concrete was pre-heated. a modification for 1D for the piping system using the Heat Transfer in Pipes module of COMSOL The convection diffusion equation was used under the Heat Transfer in Solids module, with a Multiphysics. The heat distribution over time was described by the general heat transfer equation modification for 1D for the piping system using the Heat Transfer in Pipes module of COMSOL based on the energy balance. Thus, the three-dimensional conservation of the transient heat equation Multiphysics. The heat distribution over time was described by the general heat transfer equation for an incompressible fluid used is as follows: based on the energy balance. Thus, the three-dimensional conservation of the transient heat equation for an incompressible fluid used is as follows:∂T ρCp + q = Q (1) ∂t∂T ∇· ρCp + ∇∙q = Q (1) ∂t 3 where T is the temperature (K), t is time (s), ρ is the density of the foundation material (kg m− ), Cp is 1 1 · −3 thewhere specific T is heat the capacitytemperature of the (K), foundation t is time (s), material ρ is the at density constant of pressure the foundation (J kg K material), (kg∙m ), C · − · − is theQ isspecific the heat heat source capacity (W m of3 the) and foundation q (W m 2 )material is given at by constant the Fourier’s pressure law of(J∙kg heat−1∙K conduction−1), Q is the that heat · − · − describessource (W the∙m relationship−3) and q (W between∙m−2) is thegiven heat by flux the vector Fourier’s field law and of the heat temperature conduction gradient. that describes the relationshipBased on between Equation the (1), heat the equationflux vector applied field and in the the pipe temperature network, gradient. where heat is directly supplied from theBased solar on thermal Equation collectors, (1), the equation is described applied as follows: in the pipe network, where heat is directly supplied from the solar thermal collectors, is described as follows: ∂T 1 ρA 2 ρACp +∂TρACpuet tT = t q(Ak tT) +1 fDρA u u (2) ∂t ·∇ ∇ · ∇ 2 d ρACp +ρACue∙∇T= ∇∙qAk∇T+ f h|u|| | u (2) ∂t 2 d 2 1 where A is the area of the pipe (m ), uet2 is the tangential velocity (m s− ), dh is− the1 inner pipe diameter where A is the area of the pipe (m ), ue is the tangential velocity· (m∙s ), d is the inner pipe (m), fD is the Darcy friction factor. The input temperature to the system and the flowrate are provided diameter (m), f is the Darcy friction factor. The input temperature to the system and the flowrate byare TRNSYS provided and by are TRNSYS time depended. and are time The depended. pipe was chosen The pipe to bewas a DN25chosen HDPE to be pipe.a DN25 HDPE pipe. TheThe data data from from the the TRNSYS TRNSYS were were read read in in COMSOL COMSOL Multiphysics Multiphysics as as interpolations interpolations and and the the house house loadload was was applied applied as as a boundarya boundary condition condition on on the the top top surface surface of theof the model model (where (where the the screed screed and and tile oftile of the house usually sits). The boundary is applied only on condition when the top surface temperature is higher than the house set temperature of 21 °C:

Q =Qt|__° (3) The described model allows for the heat distribution in the ground from the supplied heat, provided by the solar collectors, where at the same time heat is lost with natural convection to the building, which is assumed as the top surface of the reinforced concrete foundation. In order to achieve a heat exchange between the building and the concrete foundation, a heat flux boundary condition was applied with the building’s heating demand taken from TRNSYS. The COMSOL Multiphysics simulation runs were set for the whole month of January.

Energies 2020, 13, 2695 7 of 14 the house usually sits). The boundary is applied only on condition when the top surface temperature is higher than the house set temperature of 21 ◦C:

Qb = Qhouseload(t) (3) top_surface_temperature>21 ◦C

The described model allows for the heat distribution in the ground from the supplied heat, provided by the solar collectors, where at the same time heat is lost with natural convection to the building, which is assumed as the top surface of the reinforced concrete foundation. In order to achieve a heat exchange between the building and the concrete foundation, a heat ﬂux boundary condition was applied with the building’s heating demand taken from TRNSYS. The COMSOLEnergies 2020 Multiphysics, 13, x FOR PEER simulation REVIEW runs were set for the whole month of January. 7 of 14

3.3. ResultsResults andand DiscussionDiscussion SeveralSeveral simulations simulations were were performed performed in in order order to to obtain obtain the the optimum optimum scenarios scenarios and and the the best best conversionconversion of of computationalcomputational timetime andand memory.memory. Firstly,Firstly, the the investigating investigatingresidential residential building buildingheating heating andand coolingcooling loadsloads werewere simulatedsimulated inin TRNSYSTRNSYS andand presentedpresented inin FigureFigure4 ,4 where, where the the negative negative values values representrepresent the the cooling cooling load load and and the the positive positive values values the the heating heating load. load. TheThe simulatedsimulated yearlyyearly heatingheating andand coolingcooling loadsloads asas shownshown inin TableTable1 1are are 5600 5600 and and 23,160 23,160 kWh kWh respectively. respectively. Similar Similar loads loads (for (for a a similar similar case)case) havehave beenbeen obtainedobtainedin in [[38]38],, wherewhere thethe presentedpresented measuredmeasured heatingheating andand coolingcooling loadsloads werewere 6200 6200 andand 20,60020,600kWh kWh respectively.respectively. HeatingHeating andand coolingcooling loadsloads areare notnot onlyonly weatherweather dependent,dependent,but but mostlymostly casecase (size,(size, materials,materials, buildingbuilding orientation,orientation, residenceresidence activity,activity, etc.)etc.) andand locationlocation (coastal(coastal regionsregions oror higherhigher altitude,altitude, citycity oror village)village) sensitivesensitive too. too. BalancedBalanced loadsloads couldcould alsoalso bebe observedobserved inin didifferentﬀerentcases cases inin Cyprus Cyprus [ 39[39]] but but with with the the heating heating load load in in similar similar cases cases not not varying varying dramatically. dramatically.

0.10

0.05

) 2

0.00

-0.05 Load, Q Q (kW/m Load, -0.10

-0.15 0 30 60 90 120 150 180 210 240 270 300 330 360 time (days)

FigureFigure 4. 4Heating. Heating (positive (positive values) values) and cooling and cooling (negative (negative values) values) load, Q, ofload, the investigatedQ, of the investigat residentialed buildingresidential for buildi a year.ng for a year.

The house required heating and cooling loads are presented in Table1, where it can be observed Table 1. Heating and Cooling loads per month. that the most demanding month is January (with ambient temperatures below 18 ◦C, see Figure 6), which at theHeating same time (kWh exhibits) Cooling the lowest (kWh) solar irradiation45 Tilted Surface to heating 45 demand Tilted Surface ratio. TheSurface calculation Irradiation of Month (Heating per (Cooling per Area Irradiation for 8 m2 Irradiation for 16 m2 to Heating Demand the loads wasArea based kWh/m on2) set temperatureskWh/m2) of 21 ◦C and(kWh 27) ◦C, for winter(kWh and) summer indoor(for 8 m thermal2) Ratio comfortJanuary respectively.1780 (15) Moreover, when0 the season changes3510 from winter to6910 spring, the required2.0 load of theFebruary house to keep1450 a (12) constant temperature0 drops close to3390 1/ 3 of the previous6780 month. The ‘best’2.3 month is NovemberMarch exhibiting520 (4) at the same−30 time (0.25)a low heating4260 energy demand and8520 a high irradiation8.2 on the April 0 −890 (7) - - - tiltedMay surface (with0 the highest ratio).−1880 (16) This very month- could be considered- for a preheating- period. TheJune current study0 examines the possibility−4500 (38) to take advantage- of the higher- irradiation and store- it on theJuly reinforced concrete0 foundation−6070 to (51) provide a partial- heating though natural- convection. - August 0 −5440 (45) - - - Septembe 0 −3460 (29) - - - r October 0 −830 (7) - - - Novembe 220 (2) −60 (0.5) 3800 7460 17.3 r December 1630 (14) 0 3420 6650 2.1 Total 5600 (37) −23,160 (201) - - -

The house required heating and cooling loads are presented in Table 1, where it can be observed that the most demanding month is January (with ambient temperatures below 18 °C, see Figure 6), which at the same time exhibits the lowest solar irradiation to heating demand ratio. The calculation of the loads was based on set temperatures of 21 °C and 27 °C, for winter and summer indoor thermal comfort respectively. Moreover, when the season changes from winter to spring, the required load of the house to keep a constant temperature drops close to 1/3 of the previous month. The ‘best’ month is November exhibiting at the same time a low heating energy demand and a high irradiation on the

Energies 2020, 13, 2695 8 of 14

Table 1. Heating and Cooling loads per month.

45 Tilted 45 Tilted Surface Irradiation Heating (kWh) Cooling (kWh) ◦ ◦ Surface Surface to Heating Month (Heating per (Cooling per Irradiation for Irradiation for Demand (for 8 m2) Area kWh/m2) Area kWh/m2) 8 m2 (kWh) 16 m2 (kWh) Ratio January 1780 (15) 0 3510 6910 2.0 February 1450 (12) 0 3390 6780 2.3 March 520 (4) 30 (0.25) 4260 8520 8.2 − April 0 890 (7) - - - − May 0 1880 (16) - - - − EnergiesJune 2020, 13, x FOR PEER 0 REVIEW 4500 (38) - - - 8 of 15 − July 0 6070 (51) - - - − August 0 5440 (45) - - - constant temperature drops close− to 1/3 of the previous month. The ‘best’ month is November September 0 3460 (29) - - - exhibiting at the same time a low heating− energy demand and a high irradiation on the tilted surface October 0 830 (7) - - - − (withNovember the highest ratio).220 (2) This very month60 (0.5) could be considered 3800 for a preheating 7460 period. The 17.3 current − studyDecember examines the1630 possibility (14) to take 0 advantage of 3420 the higher irradiation 6650 and store 2.1 it on the Total 5600 (37) 23,160 (201) - - - reinforced concrete foundation to− provide a partial heating though natural convection. Following on, the solar collector system was examined using TRNSYS. Since heating is required only fromFollowing November on, the to solarMarch collector (as shown system in Error! was examined Reference using source TRNSYS. not found. Since) the heating pump is requiredshould beonly switched from November off during to the March summer (as shown months in Tablein order1) the to pump avoid shouldexcessive be switchedheating of o fftheduring rooms. the Additionally,summer months a comparative in order to parameter avoid excessive between heating the TES of unit the temperature rooms. Additionally, and the solar a comparative collectors’ outputparameter temperature between theis used TES unitto control temperature the single and the‐speed solar pump collectors’ signal output and allow temperature the flow. is used This to parametercontrol the reduces single-speed the risk pump to lower signal the and temperature allow the flow. of the This TES parameter on a cloudy reduces day. When the risk the to signal lower theis settemperature to 1, flow is of circulated the TES on through a cloudy the day. pump When and the the signal solar collectors’ is set to 1, temperature flow is circulated is plotted through along the withpump ambient and the temperature solar collectors’ and signal temperature for the is whole plotted of alongJanuary. with Error! ambient Reference temperature source and not signal found. for alsothe illustrates whole of January. that the Tablesolar 1irradiation also illustrates is always that thehigher solar that irradiation the heating is always load required. higher that This, the though, heating doesload not required. mean that This, the though, load could does be not covered mean that completely the load by could the becollectors covered since completely during by the the day, collectors when solarsince radiation during the is present, day, when the solarheating radiation demand is is present, lower compared the heating to demand the night is demand. lower compared to the nightUpon demand. circulating the energy from the solar thermal collectors through the network pipes installedUpon in the circulating TES unit, the it energywas noticed from thethat, solar for thermala whole collectorsyear, the average through concrete the network temperature pipes installed did notin rise the TESabove unit, 35 °C, it was neither noticed did that,the temperature for a whole year,at inlet the (from average the solar concrete collectors) temperature above did45 °C not (see rise Error! Reference source not found., days 0–365). Therefore, the strength of the concrete foundation above 35 ◦C, neither did the temperature at inlet (from the solar collectors) above 45 ◦C (see Figure5, isdays not affected 0–365). as Therefore, the temperature the strength is maintained of the concrete well below foundation 100 °C is(see not the aff discussionected as the in temperature the previous is section). maintained well below 100 ◦C (see the discussion in the previous section).

50.00

40.00

30.00

20.00

10.00 Temperature, T (°C) 0.00 0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 time, t (days)

Collectors_Temp TES_average_Temp

Figure 5. Yearly results of solar collectors’ (with an 8 m2 area) outlet temperature and the rise of TES 2 Figureunit average 5. Yearly temperature; results of solar the collectors’ orange dashed (with an line 8 indicatesm area) outlet the simulation temperature start and time the and rise theof TES blue unitdashed average line thetemperature; end time. the orange dashed line indicates the simulation start time and the blue dashed line the end time.

Additionally, one can observe that in order to reach a concrete average temperature of 25–30 °C a preheating period of 20–30 days is required (days 0–30), although the exact number of days cannot be predicted due to the randomness of the weather (related to cloud coverage, temperature, etc.). After the summer period, with the beginning of the heating seasons (November), due to the fact that higher temperatures are directed from the solar collectors to the TES unit, the reinforced concrete seems to maintain a close to steady average temperature of 25–35 °C, as shown in Error! Reference source not found.. This is favorable to the system since, for example, January, which is the weakest month in terms of thermal energy directed from the collectors and level of heating demand, could begin with higher initial TES temperatures, thus providing the required partial heating load to the building. Indeed, the TES system was preheated for a whole month (October) before the heating season (November), a fact that enabled the TES to reach the desired temperatures at the right time, as shown in Error! Reference source not found.. One can see that the TES acquired nearly steady temperatures

Energies 2020, 13, 2695 9 of 14

Additionally, one can observe that in order to reach a concrete average temperature of 25–30 ◦C a preheating period of 20–30 days is required (days 0–30), although the exact number of days cannot be predicted due to the randomness of the weather (related to cloud coverage, temperature, etc.). After the summer period, with the beginning of the heating seasons (November), due to the fact that higher temperatures are directed from the solar collectors to the TES unit, the reinforced concrete seems to maintain a close to steady average temperature of 25–35 ◦C, as shown in Figure5. This is favorable to the system since, for example, January, which is the weakest month in terms of thermal energy directed from the collectors and level of heating demand, could begin with higher initial TES temperatures, thus providing the required partial heating load to the building. EnergiesIndeed, 2020, 13 the, x FOR TES PEER system REVIEW was preheated for a whole month (October) before the heating season9 of 15 (November), a fact that enabled the TES to reach the desired temperatures at the right time, as shown inaround Figure 305. One°C in can the see month that the under TES investigation acquired nearly (January, steady temperaturesdays 366–396). around On the 30 other◦C in hand, the month high undertemperature investigation over 40 (January, °C on the days building’s 366–396). flooring, On the are other not hand, recommended high temperature due to the over discomfort 40 ◦C on thethis building’swould cause ﬂooring, to the areresidents. not recommended due to the discomfort this would cause to the residents. FigureError!6 Referenceshows the solarsource collectors’ not found. (of area shows 8 m the2) temperature solar collectors’ acquired (of forarea the 8 monthm2) temperature of January underacquired investigation, for the month which of January remain under below investigation, 40 ◦C. Also shown which are remain the ambient below 40 temperatures °C. Also shown (below are 18the◦ C)ambient and the temperatures discrete value (below of 1 of18 the °C) pump and the signal discrete throughout value of the 1 of month, the pump with signal the exception throughout of daythe month, 21, where with the the pump exception signal of remained day 21, where at 0 owing the pump to the signal collectors’ remained temperatures at 0 owing being to the lower collectors’ than thetemperatures TES temperatures being lower (due than to heavy the TES cloudiness). temperatures (due to heavy cloudiness).

50.00 5

45.00

40.00 4

35.00 (°C)

30.00 3

25.00 Signal 20.00 2

Temperature, 15.00

10.00 1

5.00

0.00 0 024681012141618202224262830 time, t (days) Ambient Temperature Collectors Temperature signal

Figure 6. Solar collectors’ temperature plotted with ambient temperature and pump signal for January withFigure 8 m 6.2 Solarsolar collectors’ collectors. temperature plotted with ambient temperature and pump signal for January with 8 m2 solar collectors. In order to examine the temperature distribution in the reinforced concrete foundation (TES unit) in moreIn detail,order to COMSOL examine Multiphysics the temperature was thendistribution employed. in the The reinforced model was concrete initially foundation run with a solar(TES collectors’unit) in more area detail, of 6.4 mCOMSOL2 to investigate Multiphysics whether was the then energy employed. was enough The tomodel cover was the initially house full run heating with a load.solar Figurecollectors’7 shows area that, of 6.4 for m January,2 to investigate the selected whether solar the collectors’ energy areawas ofenough 6.4 m to2 could cover not the be house enough full toheating cover theload. full Error! heating Reference load required source by not the found. building shows and, that, as afor result, January, the TES the selected unit cannot solar support collectors’ the building’sarea of 6.4 maintaining m2 could not the be desired enough set to temperature cover the full of heating 21 ◦C. Larger load required solar collectors’ by the building areas were and, tested as a inresult, order the for TES this issueunit cannot to be addressed. support the The building’s outcome maintaining of this test is shownthe desired in Figure set temperature7, where one of can 21 see °C. thatLarger areas solar of overcollectors’ 12 m2 areasseem were to be enough.tested in order for this issue to be addressed. The outcome of this test is shown in Error! Reference source not found., where one can see that areas of over 12 m2 seem to be enough.

Energies 2020, 13, x FOR PEER REVIEW 10 of 15

Energies 2020, 13, 2695 10 of 14 Energies 352020, 13, x FOR PEER REVIEW 10 of 15 33 (°C) 3531 T

3329 (°C) 31 T 27

2925 27

Temperature, 23 2521

Temperature, 23 012345678910111213141516171819202122232425262728293031 21 time, t (days) 012345678910111213141516171819202122232425262728293031 6.4m2 8m2 time,12m2 t (days) 16m2 20m2

Figure 7. Average TES6.4m2 temperature 8m2with different12m2 solar collectors’16m2 area 20m2with full heating load exchanged (parabolic model) for January. Figure 7. Average TES temperature with diﬀerent solar collectors’ area with full heating load exchanged Figure 7. Average TES temperature with different solar collectors’ area with full heating load It(parabolic is expected model) that for January. for high‐enough areas of solar collectors applied (yielding a higher exchanged (parabolic model) for January. temperature and load delivered in the TES unit), not only would the system respond to the heating It is expected that for high-enough areas of solar collectors applied (yielding a higher temperature load, but additionally the TES unit average temperature would increase and energy could be saved andIt load is deliveredexpected inthat the for TES high unit),‐enough not only areas would of solar the system collectors respond applied to the (yielding heating a load, higher but for the days where a higher heating load were needed. temperatureadditionally and the TES load unit delivered average in temperature the TES unit), would not increaseonly would and the energy system could respond be saved to the for theheating days The main purpose of this system is to be economic and to cover part of the heating load of the load,where but a higher additionally heating the load TES were unit needed. average temperature would increase and energy could be saved building. Now, installing more and more solar collectors would increase the system’s initial cost, or for theThe days main where purpose a higher of this heating system load is were to be needed. economic and to cover part of the heating load of the cause other practical complications related to solar collectors and the building. It can always be building.The main Now, purpose installing of morethis system and more is to solar be economic collectors and would to cover increase part theof the system’s heating initial load cost, of the or argued whether it is advantageous to reduce the initial cost of the system and cover partial of the building.cause other Now, practical installing complications more and related more solar to solar collectors collectors would and theincrease building. the system’s It can always initial be cost, argued or load with conventional means (albeit environmentally non‐friendly), or whether a system would be causewhether other it is practical advantageous complications to reduce related the initial to solar cost ofcollectors the system and and the cover building. partial It ofcan the always load with be designed to eliminate the need of any additional HVAC units. Hence, increasing the solar collectors’ arguedconventional whether means it is (albeitadvantageous environmentally to reduce non-friendly), the initial cost or whetherof the system a system and would cover bepartial designed of the to area cannot always be the ideal solution, and other solutions may be desired. This though requires loadeliminate with conventional the need of any means additional (albeit HVACenvironmentally units. Hence, non increasing‐friendly), theor whether solar collectors’ a system area would cannot be an economic analysis, which is beyond the scope of the current study. designedalways be to the eliminate ideal solution, the need and of any other additional solutions HVAC may be units. desired. Hence, This increasing though requires the solar an collectors’ economic To this end, one can compare the additional energy the system would require for covering the areaanalysis, cannot which always is beyond be the theideal scope solution, of the and current other study. solutions may be desired. This though requires building’s heating load (as presented in Figure 4). In order to calculate the additional heat, the an economicTo this end,analysis, one canwhich compare is beyond the additionalthe scope of energy the current the system study. would require for covering the provided heat from the TES unit was subtracted from the building’s heating load for various building’sTo this heating end, one load can (as compare presented the in additional Figure4). In energy order tothe calculate system thewould additional require heat, for covering the provided the scenarios of the solar collectors’ area, with the result shown in Error! Reference source not found.. It building’sheat from theheating TES unitload was (as subtractedpresented in from Figure the building’s 4). In order heating to calculate load for the various additional scenarios heat, of the the can be seen therein that the additional heat ranges from around 700 kWh to 10 kWh for ascending providedsolar collectors’ heat from area, the with TES the resultunit was shown subtracted in Figure from8. It can the be building’s seen therein heating that the load additional for various heat collector’s areas between 6 m2 and 20 m2. scenariosranges from of the around solar 700collectors’ kWh to area, 10 kWh with for the ascending result shown collector’s in Error! areas Reference between source 6 m2 and not 20found. m2. . It can be seen therein that the additional heat ranges from around 700 kWh to 10 kWh for ascending collector’s areas 1000between 6 m2 and 20 m2.

800 1000 600 (kWh)

Q 800 400 600 Heat, (kWh)

Q 200 400 0 Heat, 200 6 8 10 12 14 16 18 20 0 Solar Collectorsʹ Area, A (m2) 6 8 10 12 14 16 18 20 Figure 8. Additional load, Q, required to cover the building’s heating load with diﬀerent solar collectors’ Figure 8. Additional load, Q, required to cover the building’s heating load with different solar areas for January (percentage errorSolar bars Collectors shown,ʹ Area, based A on(m2 10%).) collectors’ areas for January (percentage error bars shown, based on 10%). FigureFigure 98. showsAdditional the available load, Q, required TES stored to energycover the computed building’s on heating the basis load of with the didifferentﬀerence solar between the building’scollectors’ areas set temperature for January (percentage (21 ◦C) and error the bars TES shown, average based temperature. on 10%). It can clearly be seen that

Energies 2020, 13, x FOR PEER REVIEW 11 of 15

Figure 9 shows the available TES stored energy computed on the basis of the difference between the building’s set temperature (21 °C) and the TES average temperature. It can clearly be seen that for all times and all collector areas (even the lowest of 6.4 m2), the available energy never goes below 200 kWh. However, as the heat transfer to the building is solely done by natural convection through the TES unit’s top surface, heat is transferred at a low rate. Such an issue could be overcome with methods deployed in ground heat exchangers, and by increasing the TES thermal conductivity using additives [40]. Note here that ground TES units are used with various materials (such as rock bed, concrete, EnergiesEnergiessand,2020 and2020, 13 , more13, 2695, x FOR [41]) PEER and REVIEW are always well insulated in order to maintain their energy and supply11 of11 it15 of 14 when needed. We consider such a scenario for the TES under investigation, where the reinforced concreteFigure could 9 shows be totally the available insulated, TES and stored hence energy it could computed be used ason a the sheer basis TES. of theThe difference stored energy between can forthethen all building’s times be transferred and set all temperature collector into the house areas (21 by (even °C) controlled and the the lowest TESforced ofaverage convection. 6.4 m 2temperature.), the This available scenario It can energy is clearly presented never be seen goesthrough that below 200forError! kWh. all timesReference However, and all source ascollector the heatnot areas transferfound. (even, where to the the lowest buildingthe TESof 6.4 isaverage m solely2), the donetemperatureavailable by natural energy is observed convectionnever goes to below throughkeep the200increasing TES kWh. unit’s However, with top time surface, as for the inlet heat and transfer is transferred outlet to fluid the building attemperatures. a low is rate. solely Such done a an behaviorby issue natural could raises convection be the overcome concern through to with methodsthecontrol TES deployed theunit’s energy top in surface,stored ground so heat that heat is the exchangers, transferred TES (concrete) at and a low bytemperature increasing rate. Such stays thean issue TESwell thermal belowcould 100be conductivity overcome °C, in order with usingto additivesmethodsmaintain [ 40deployed the]. buildings in ground foundation heat exchangers, integrity. and by increasing the TES thermal conductivity using additives [40]. Note here1000 that ground TES units are used with various materials (such as rock bed, concrete, sand, and more900 [41]) and are always well insulated in order to maintain their energy and supply it when needed.800 We consider such a scenario for the TES under investigation, where the reinforced concrete could700 be totally insulated, and hence it could be used as a sheer TES. The stored energy can then be transferred600 into the house by controlled forced convection. This scenario is presented through (kWh) 500 Error! ReferenceQ source not found., where the TES average temperature is observed to keep

increasing with400 time for inlet and outlet fluid temperatures. Such a behavior raises the concern to 300 control theHeat, energy stored so that the TES (concrete) temperature stays well below 100 °C, in order to maintain the200 buildings foundation integrity. 100 0 1000 0 1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728293031 900 Time, t (days) 800 700 600 6.4m2 8m2 12m2 16m2 20m2 (kWh) 500 Q FigureFigure 9. Available9. Available heat storedheat stored for diﬀ erentfor different solar collector solar areas collector based areas on TES based average on temperatures TES average 400 temperaturesfor January. for January. 300 Heat, 200 Note here40 that ground TES units are used with various materials (such as rock bed, concrete, sand, and more100 [41]) and are always well insulated in order to maintain their energy and supply it 35 0 when needed.T(°C) We consider such a scenario for the TES under investigation, where the reinforced 0 1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728293031 concrete could30 be totally insulated, and hence it could be used as a sheer TES. The stored energy Time, t (days) can then be25 transferred into the house by controlled forced convection. This scenario is presented through Figure 10, where the TES average temperature is observed to keep increasing with time for 6.4m2 8m2 12m2 16m2 20m2 inlet and outlet20 ﬂuid temperatures. Such a behavior raises the concern to control the energy stored

Temperature, so thatFigure the TES15 9. (concrete)Available temperatureheat stored for stays different well below solar 100 collector◦C, in orderareas tobased maintain on TES the average buildings foundationtemperatures integrity. for0 1 January. 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728293031 Time, t (days) 40 Figure 10. Inlet and outlet temperature, with average concrete foundation temperature for January 35

for T(°C) a totally insulated TES system. Blue line: outlet temperature, green line: inlet temperature, and

black30 line TES average temperature

Finally,25 a ‘sensitivity’ analysis was also performed on the thickness of the foundation by using a 20% difference20 from the standard thickness of 1 m, i.e., between 0.8–1.2 m (1.0  0.2 m), and also a

Temperature, 15 0 1 2 3 4 5 6 7 8 9 10111213141516171819202122232425262728293031 Time, t (days) FigureFigure 10. 10.Inlet Inlet and and outlet outlet temperature, temperature, with with average average concrete concrete foundation foundation temperature temperature forfor JanuaryJanuary for a totallyfor a totally insulated insulated TES system.TES system. Blue Blue line: line: outlet outlet temperature, temperature, green green line: line: inlet inlet temperature, temperature, and and black lineblack TES line average TES average temperature temperature

Finally,Finally, a a ‘sensitivity’ ‘sensitivity’ analysis analysis waswas alsoalso performed on on the the thickness thickness of of the the foundation foundation by byusing using a a 20%20% di differencefference from from the the standardstandard thickness thickness of of 1 1 m, m, i.e., i.e., between between 0.8–1.2 0.8–1.2 m (1.0 m (1.0  0.2 m),0.2 and m), andalso alsoa ± ± a 33% on flowrate, i.e., between 10–20 L/min (15 5 L/min). As the length and width dimensions ± ± of the foundation remain fixed at 12 m 10 m, the corresponding foundation volumes are between × Energies 2020, 13, x FOR PEER REVIEW 12 of 15

33% on flowrate, i.e., between 10–20 L/min (15  5 L/min). As the length and width dimensions of the foundation remain fixed at 12 × m × 10 m, the corresponding foundation volumes are between 96–144 m3. The analysis is done through comparisons of (a) the average available stored energy and (b) the additional heat required, for different TES thicknesses/volumes and flowrate. The obtained results indicate an insignificant effect of the thickness on the system, yielding a maximum difference of only 65 kWh of average stored heat and 80 kWh of additional heat required between the two extreme foundation thickness cases (0.8 and 1.2 m) for the month of January (see linear regression results in Error! Reference source not found.). The flowrate analysis indicates an insignificant effect with only 90 kWh of average stored heat and 250 kWh of additional heat required between the lowest and the highest compared flowrates. By increasing the flowrate, the solar collectors’ temperature decreases but, on the other hand, the supplied heat increases. The opposite occurs when decreasing the flowrate, where lower heat is provided into the TES but with higher temperature.

Energies4. Conclusions2020, 13, 2695 12 of 14 The potential of storing thermal energy in a building’s reinforced concrete foundation acting as a TES unit has been numerically examined. A typical Cypriot house was selected to obtain heating 3 96–144and m cooling. The loads analysis by simulation is done through on the TRNSYS comparisons software. of (a) The the concept average of seasonal available TES stored was energyused for and (b) theshort additional time TES, heat where required, the TES forunit di (thefferent building’s TES thicknesses foundation/ involumes this case) and would flowrate. provide The a partial obtained resultsheating indicate load, an to insignificant the residential eff building,ect of the with thickness the use on of thenatural system, convection yielding (similar a maximum to floor diheating).fference of only 65 kWhIn addition of average to TRNSYS stored heata second and software 80 kWh ofhas additional been deployed, heat requirednamely COMSOL between Multiphysics, the two extreme foundationwith TRNSYS thickness providing cases (0.8yearly and results 1.2 m) and for COMSOL the month further of January investigating (see linearone particular regression month, results in FigureJanuary, 11 ).with The the flowrate most demanding analysis ‘worst indicates‐case’ an scenario. insignificant Once the eff building’sect with only heating 90 load kWh had of averagebeen storedcalculated, heat and the 250 solar kWh collectors of additional (simulated heat required with TRNSYS), between were the lowesttested andfor different the highest areas. compared The flowrates.simulations By increasing have shown the that flowrate, an area of the 8 m solar2 was collectors’ enough to cover temperature two thirds decreases of the chosen but, building’s on the other 2 hand,heating the supplied load, and heat if the increases. area of Thethe collectors opposite occurswas increased when decreasingto 20 m , the the buildings flowrate, load where could lower potentially be covered by the investigated hybrid system. heat is provided into the TES but with higher temperature.

800 800

600 600 Stored (kWh)

Heat,

400 400 Q

200 (kWh) 200 Heat, Average 0 0 90 100 110 120 130 140 150 Additional 90 100 110 120 130 140 150 3 3 TES volume, V (m ) TES volume, V (m )

(a) Average energy stored for different TES (b) Total additional heat required for different volumes TES volumes

800 800 Q 600 600 Stored (kWh)

400 Heat, 400

Q 200 200 (kW) 0 Heat, 0 Average 8 10121416182022 8 10121416182022 Additional Flowrate (l/min) Flowrate (l/min)

Figure 11. ‘Sensitivity’ analysis (a,b) on the foundation volume (for thicknesses between 0.8 m (volume = 96 m2) and 1.2 m (volume = 144 m2), and (c,d) on ﬂowrate (for ﬂowrates between 10 and 20 L per minute) (percentage error bars shown, based on 10%).

4. Conclusions The potential of storing thermal energy in a building’s reinforced concrete foundation acting as a TES unit has been numerically examined. A typical Cypriot house was selected to obtain heating and cooling loads by simulation on the TRNSYS software. The concept of seasonal TES was used for short time TES, where the TES unit (the building’s foundation in this case) would provide a partial heating load, to the residential building, with the use of natural convection (similar to ﬂoor heating). In addition to TRNSYS a second software has been deployed, namely COMSOL Multiphysics, with TRNSYS providing yearly results and COMSOL further investigating one particular month, January, with the most demanding ‘worst-case’ scenario. Once the building’s heating load had been calculated, the solar collectors (simulated with TRNSYS), were tested for diﬀerent areas. The simulations have shown that an area of 8 m2 was enough to cover two thirds of the chosen building’s heating load, and if the area of the collectors was increased to 20 m2, the buildings load could potentially be covered by the investigated hybrid system. Energies 2020, 13, 2695 13 of 14

The presented system could offer an alternative possibility if used as a totally insulated TES unit. Moreover, an analysis on the thickness of the building’s foundation (TES unit) and the flowrate has shown an insignificant effect for 20% and 33% variations respectively. ± ± The encouraging conclusion is that the use of a building’s foundation reinforced concrete for a potential partial or full cover of the building’s heating load is realistic. A future development could be achieved though research and experimentation on more parameters and aspects before eventual implementation, including possible economic and lifecycle analyses.

Author Contributions: Conceptualization L.A., G.F.; Software L.A., G.P.P.; Methodology, L.A., P.C., G.F.; Writing—original draft preparation, L.A.; Writing-review and editing, L.A., P.C. and G.F.; 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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