Comparative Analysis of Small-Scale Organic Rankine Cycle Systems for Solar Energy Utilisation

Home , Organic Rankine cycle, Rankine cycle, Thermal efficiency

Article Comparative Analysis of Small-Scale Organic Rankine Cycle Systems for Solar Energy Utilisation

Ruiqi Wang, Long Jiang *, Zhiwei Ma, Abigail Gonzalez-Diaz, Yaodong Wang and Anthony Paul Roskilly

Sir Joseph Swan Centre for Energy Research, Newcastle University, Newcastle NE1 7RU, UK; [email protected] (R.W.); [email protected] (Z.M.); [email protected] (A.G.-D.); [email protected] (Y.W.); [email protected] (A.P.R.) * Correspondence: [email protected]

Received: 20 December 2018; Accepted: 26 February 2019; Published: 2 March 2019

Abstract: Small-scale organic Rankine cycle (ORC) systems driven by solar energy are compared in this paper, which aims to explore the potential of power generation for domestic utilisation. A solar thermal collector was used as the heat source for a hot water storage tank. Thermal performance was then evaluated in terms of both the conventional ORC and an ORC using thermal driven pump (TDP). It is established that the solar ORC using TDP has a superior performance to the conventional ORC under most working conditions. Results demonstrate that power output of the ORC using TDP ranges from 72 W to 82 W with the increase of evaporating temperature, which shows an improvement of up to 3.3% at a 100 ◦C evaporating temperature when compared with the power output of the conventional ORC. Energy and exergy efficiencies of the ORC using TDP increase from 11.3% to 12.6% and from 45.8% to 51.3% when the evaporating temperature increases from 75 ◦C to 100 ◦C. The efficiency of the ORC using TDP is improved by up to 3.27%. Additionally, the exergy destruction using TDP can be reduced in the evaporator and condenser. The highest exergy efficiency in the evaporator is 96.9%, an improvement of 62% in comparison with that of the conventional ORC, i.e., 59.9%. Thus, the small-scale solar ORC system using TDP is more promising for household application.

Keywords: organic Rankine cycle; thermal driven pump; small-scale; energy and exergy efﬁciency

1. Introduction Solar thermal technology is gathering the momentum to meet household demands for both heating and electricity in the UK [1]. Due to the relatively low annual yield of incident solar radiation, low-temperature heat source recovery technologies have become more significant, e.g., thermoelectric power and the organic Rankine cycle (ORC), when considering thermal efficiency and system compactness [2,3]. Compared with the poor efficiency of thermoelectric power at low solar radiation, the ORC could be a better candidate for distributed household applications, as it is able to operate efficiently and affordably at relatively low working temperatures [4,5]. Considerable progress has been made in the development of solar ORC systems for small-scale power generation [6,7]. With a range of solar collector designs, both concentrating and non-concentrating types had been featured for solar ORC systems [8,9]. An overall efficiency of 4.2% was obtained by using evacuated tube collectors and 3.2% using flat-plate collectors [10]. Other concentrating parabolic through collectors demonstrated higher overall efficiencies in the region of 7.5–12.1% [6,11]. The nanofluid was utilised in a solar driven ORC with parabolic trough collectors to achieve a higher system performance and the system efficiency was found to be 20.11% [12].

Energies 2019, 12, 829; doi:10.3390/en12050829 www.mdpi.com/journal/energies Energies 2019, 12, 829 2 of 22

An onsite experimental evaluation of a low temperature solar ORC system was presented for reverse osmosis desalination. However, considerably low efficiency was observed [13]. Combined solar heat and power system based on the ORC cycle have advantageous ability to use low-cost thermal energy storage to provide continuous operation under intermittent solar-irradiance conditions [14]. A number of investigations have proposed thermal energy storage solutions using solid-liquid phase change materials for solar ORC system [15,16]. It is noted that there are vast potentials to further improve thermal performance through the organic working fluid exploration and ORC system optimisation [17,18]. Considering organic working fluids, different applications have been investigated by various researchers [19]. The slope of the saturation curve for working fluids in a temperature vs. specific entropy diagram can be positive, negative and vertical, which are correspondingly named “wet”, “dry” and “isentropic” fluids [20]. Wet fluids (e.g., water) are usually required to be superheated, whereas many other organic fluids may be dry or isentropic, which do not need superheating. Another advantage of organic working fluid is that a turbine established for ORCs typically requires only a single-stage expander, which brings about a simpler and more economical system in terms of cost and maintenance [21]. Common pure working fluids, e.g., R123 [22], R245fa [23], R245ca [24] and n-pentane [25], have been comprehensively studied. The mixtures are also developed and attempted for ORCs [26,27]. Thermophysical properties of working fluids, e.g., fluid density and thermal conductivity, are analysed and summarised, and working fluids can then be selected in terms of the above properties, thermal stability, compatibility, environmental impacts, safety and cost [28]. Based on selection criteria, R245fa and R134a are often regarded as suitable working fluids for recovering a low-temperature heat source [29]. System optimisation can be realized by improving the thermal performance of each component. Among the four main components of the conventional ORC, the expander is the most investigated due to the fact that it is related to the work output [30]. The work output acts as a positive numerator when calculating thermal efficiency [31]. A variety of expanders have been assessed to attempt improving thermal performance, e.g., scroll expander [32,33], piston expander [34], rotary expander [35], screw expander [36,37] and radial turbine [38]. In general, the scroll expander is the most suitable expander for a small-scale ORC system [39]. Recently, several research efforts have aimed to accomplish better performance by using a free-piston expander [40]. Moreover, two heat exchangers serve as the evaporator and condenser for conventional ORC systems. Thus, heat transfer enhancement is conducive to overall system evaluation, which is greatly relevant to heat input as the denominator for calculating thermal efficiency [41]. Different evaporation and condensation processes have been investigated in terms of heat exchanger type, tube structure, refrigerant, etc. [42]. Nonetheless, another item in the equation for calculating thermal efficiency may be easily ignored. Pump work acts as a negative numerator, which consumes electricity in a real application. Pump work in various studies is not included when calculating the overall efficiency. One such case is if the scale of the ORC system is large, i.e., pump work only accounts for less than 5% of the electricity output. Thus, pump work will not significantly influence the efficiency, even if it is ignored [43]. The other case is if thermal efficiency has a negative effect on the overall performance. This part is selectively overlooked to present decent results. However, pump consumption becomes even larger for a small-scale ORC when the heat source temperature becomes lower [44]. A possible solution is to realize the ORC without using an electrical driven pump. The pump is generally used to pressurize the refrigerant from condensing pressure to evaporating pressure. The pressure difference is also achievable by other methods. One concept is to use the gravity of the working fluid, i.e., enough height difference between evaporator and condenser [45]. Although the height brings about a pressure difference, the required value becomes a barrier in several applications. An alternative idea is to drive the ORC by using a thermal-driven pump (TDP), which is a thermodynamic way to replace the electrical driven pump [46]. A thermosiphon, i.e., a heat pipe, is one type of thermal driven pump, and it was first proposed in the last century [47]. Due to its Energies 2019, 12, 829 3 of 22 unique characteristic, the power generation is too small. Another thermal driven pump is the pressure difference realized by heating and cooling at intervals [46]. An experimental rig was established to prove the feasibility of this TDP [48]. Our previous work also investigated its working performance by using a lab-scale prototype, but the unstable power output was not satisfactory [49]. Later, a modified experimental rig was built to improve the thermal performance. Results indicated that the efficiency was further improved, and 90% of the power generating process was stable [50]. Due to the characteristic of solar energy, the fluctuation could be accepted by using battery technology. Thus, a solar ORC using TDP may be desirable for domestic application, which is expected to have a higher thermal efficiency when compared with that of the conventional type. To the authors’ best knowledge, few research studies on the solar ORC with TDP are reported. Thus, this paper aims to evaluate the performance of a small-scale ORC with TDP when operating at a low solar irradiance level. To further illustrate its advantages and disadvantages, the performance is compared with that of conventional solar ORC in terms of power output, energy and exergy efficiencies. The overarching framework of this paper is elaborated as follows. Solar ORCs using and not using TDP are presented in Section2. Then, modelling and evaluation equations are defined in Section3. Thermal performances of the solar ORCs are compared in Section4, which is followed by conclusions in Section5.

2. System Description Figure1 indicates the schematic diagrams of combined solar heat and power systems. The solar collector provides the thermal energy for evaporating the working fluid. One water tank between the ORC system and solar collector serves as a heat exchanger, which could be replaced by a thermal energy store vessel where phase change materials can be considered as the storage media. Another water tank could be used to supply the domestic hot water for end users from the heat rejected by the condensation [14]. It will be a reduction in the fuel demand to heat the water. The electricity generation by ORC is set to be prioritised over water heating. A proportion of the solar collector fluid will be transferred to the domestic hot water tank only when the collector fluid is at a higher temperature than the hot water tank plus a pinch difference. When this temperature is higher than the critical temperature of working fluids, excess solar heat is rejected directly to the hot water tank. No heating is delivered to the hot water tank if its mean temperature is above the maximum hot water storage temperature [5]. Since the ORC is the main investigation target, the storage part is not analysed in the rest of this paper. As shown in Figure1a, the conventional ORC is mainly composed of two heat exchangers (evaporator and condenser), an expander and a working fluid pump. Its principle is the same as that of the Rankine cycle. Working fluid is pumped into a boiler, where it is evaporated and passes through an expander for power generation. Then, it goes through a condenser, where it is finally condensed. Due to energy consumption of the electrical driven pump, the basic ORC may enjoy a very low energy efficiency when the heating temperature is low and the rated power output is small. To solve this problem, the ORC part replaces the electrical driven pump with TDP, which is shown in the dashed area of Figure1b. Figure2 presents different operation modes of ORC using TDP. The ORC using TDP is mainly composed of two high-efficiency heat exchangers, i.e., heat exchanger 1 and heat exchanger 2, an expander, a generator and other auxiliary components [49]. The working processes consist of a pre-expansion process and a power generation process, which are briefly illustrated as follows: (a) Pre-expansion process. Heat exchanger 1 acts as a condenser while heat exchanger 2 works as an evaporator. Water valves V1, V3, V6 and V8 are open, and all other valves are closed. The evaporator that is full of the working fluid, i.e., heat exchanger 2, undertakes isochoric heating through hot water, and its pressure increases gradually until it becomes constant. Meanwhile, the working fluid in the condenser, i.e., heat exchanger 1, starts as saturated vapour at a high temperature and pressure. The condenser applies an isochoric cooling process. (b) Power generation process. When Energies 2019, 12, 829 4 of 22 Energies 2019, 12, x FOR PEER REVIEW 4 of 21

whole process that is termed as a generalised working cycle is shown in Figure 2a. Then heat the pressure of the evaporator becomes constant and the condenser achieves a cooler level, RV2 and exchanger 1 and 2 swap their roles as condenser and expander. A new cycle starts as indicated in RV4 are opened. Then, the working fluid with high temperature and pressure from the evaporator Figure 2b, which is similar to previous pre-expansion and power generation processes. A working flows into the expander and generates the power. The power is outputted until there is no pressure mode for the ORC using TDP could be according to Table 1. For further clarification of the whole difference between the evaporator and condenser. During this process, the high-pressure working process, different operating modes of water and refrigerant valves for the ORC using TDP are listed fluid in the evaporator is isobarically heated. The exhaust enters the condenser and is condensed into in Table 2. It is noted that heat exchangers 1 and 2 are always heated and cooled inversely. No power saturated liquid. When the generator does not produce power, RV2, RV4, V1, V3, V6 and V8 are closed. output is obtained in the pre-expansion process, which slightly reduces the average output. This whole process that is termed as a generalised working cycle is shown in Figure2a. Then heat In fact, the working fluid pump is replaced by the TDP, i.e., the switch of two heat exchangers exchanger 1 and 2 swap their roles as condenser and expander. A new cycle starts as indicated in as shown in the dashed area of Figure 2. Compared with the conventional ORC, the ORC using TDP Figuremay have2b, which a higher is similarenergy toefficiency previous since pre-expansion the pump is and eliminated. power generation Additionally, processes. it is worth A working noting modethat continuous for the ORC power using output TDP could could be be according obtained toin Tablethe power1. For furthergeneration clarification process, and of the thermal whole process,continuity different is influenced operating by modesthe pre-expansion of water and process. refrigerant To solve valves this for problem the ORC of usingcontinuity, TDP areit tends listed to in Tableuse two2. It or is more noted ORCs that heat with exchangers TDPs for power 1 and generation 2 are always alternatively. heated and However, cooled inversely. it is acceptable No power for outputthe solar is obtainedORC, which in the can pre-expansion utilise battery process,technology which for slightlyelectricity reduces storage. the average output.

(a)

(b)

FigureFigure 1.1. SchematicSchematic diagramdiagram of combined solar heat and and power power systems systems based based on on (a (a) )the the conventional conventional organicorganic RankineRankine cyclecycle (ORC);(ORC); andand ((bb)) thethe ORCORC using thermal driven pump. pump.

Energies 2019, 12, 829 5 of 22 Energies 2019, 12, x FOR PEER REVIEW 5 of 21

(a)

(b)

FigureFigure 2. 2.SchematicSchematic diagram diagram of of ORC ORC with thermal drivendriven pump pump ( a()a ﬁrst) first cycle; cycle; and and (b )(b next) next cycle. cycle.

Table 1. Working mode for the organic Rankine cycle (ORC) using thermal driven pump (TDP). Table 1. Working mode for the organic Rankine cycle (ORC) using thermal driven pump (TDP). Working Mode Pre-Expansion Power Generation Pre-Expansion Power Generation Power Power Working Mode Pre-Expansion Pre-Expansion Heat exchanger 1 Cooling CoolingGeneration HeatingGeneration Heating Heat exchanger 2 Heating Heating Cooling Cooling Heat exchanger 1 Cooling Cooling Heating Heating Power Unable Able Unable Able Heat exchanger 2 Heating Heating Cooling Cooling Power Unable Able Unable Able Table 2. Different operating modes of valves for the ORC using TDP.

TableValves 2. Different operating V1, V3,modes V6, V8 of valves V2, V4, for V5, the V7 ORC using RV1, RV3 TDP. RV2, RV4 Pre-expansion Open Close Close Close First cycle ValvesPower generation V1, V3, V6, Open V8 V2, V4, Close V5, V7 RV1, Close RV3 RV2, Open RV4 Pre-expansion Close Open Close Close Next cycle Pre-expansion Open Close Close Close First cycle Power generation Close Open Open Close Power generation Open Close Close Open Pre-expansion Close Open Close Close Next cycle Power generation Close Open Open Close

A T-s diagram of a solar ORC cycle using a typical working fluid, i.e., R245fa, is shown in Figure 3. The heat is obtained from a solar collector using water as a heat transfer fluid. A fixed superheated

Energies 2019, 12, 829 6 of 22

In fact, the working fluid pump is replaced by the TDP, i.e., the switch of two heat exchangers as shown in the dashed area of Figure2. Compared with the conventional ORC, the ORC using TDP may have a higher energy efficiency since the pump is eliminated. Additionally, it is worth noting that continuous power output could be obtained in the power generation process, and thermal continuity is influenced by the pre-expansion process. To solve this problem of continuity, it tends to use two or more ORCs with TDPs for power generation alternatively. However, it is acceptable for the solar ORC, which can utilise battery technology for electricity storage.

EnergiesA 2019T-s diagram, 12, x FOR ofPEER a solar REVIEW ORC cycle using a typical working fluid, i.e., R245fa, is shown in Figure6 of 321. The heat is obtained from a solar collector using water as a heat transfer fluid. A fixed superheated ◦ temperature differencedifference∆ ΔTTsuperheatedsuperheated ofof 5 5°CC is is assumed assumed between between the the he heatat source source fluid fluid inlet inlet and and working fluidfluid outlet. Thus, Thus, the the working working fluid fluid outlet outlet temperature temperature is evaluated is evaluated as T as1 = TT1sc,1= −T Δsc,1Tsuperheated− ∆Tsuperheated. In order. Into comprehensively order to comprehensively evaluate the evaluate performance, the performance, refrigerants refrigerants of R245fa, ofR134a R245fa, and R134aisobutane and are isobutane chosen areas the chosen working as the fluids working for solar fluids ORC for systems. solar ORC Thes systems.e refrigerants These refrigerantshave been verified have been as well-suited verified as well-suitedcandidates candidatesfor low-temperature for low-temperature ORC applicatio ORC applicationsns due to due their to their thermodynamic thermodynamic and and physical properties, low flammability,flammability, corrosivenesscorrosiveness andand environmentalenvironmental impactsimpacts [[5,25,200].]. TheThe thermodynamicthermodynamic data of these organic compounds are obtained from REFPROP 9 (National Institute of Standards and Technology, Gaithersburg,Gaithersburg, MD,MD, USA).USA).

180 600 R245fa 800 400 160 1000 200 1200 ρ = 1400 kg⋅m-3 100 140 50 id 20 C) r flu o 120 Sola 8 7 1 100 80

Temperature ( 60 6 40 2 5 4 20 3 0 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 ⋅ -1⋅ -1 Entropy (kJ kg K ) Figure 3. T-s diagram of a solar ORC using R245fa as the working ﬂuid.fluid.

3. Methodology 3.1. Aspen Model 3.1. Aspen Model The solar ORC model was built in Aspen Plus (v 8.8, Aspen Technology, Inc., Bedford, MA, USA), The solar ORC model was built in Aspen Plus (v 8.8, Aspen Technology, Inc., Bedford, MA, which includes the speciﬁcation of each block [51]. The Peng–Robinson method was used to calculate USA), which includes the specification of each block [51]. The Peng–Robinson method was used to the properties of all the components. Figure4 shows the schematic diagram of the solar ORC system in calculate the properties of all the components. Figure 4 shows the schematic diagram of the solar Aspen Plus. The relevant assumptions are as follows: (1) The ORC cycle and its components operate ORC system in Aspen Plus. The relevant assumptions are as follows: (1) The ORC cycle and its under steady-state conditions; (2) thermodynamic equilibrium happens at the inlet and outlet of each components operate under steady-state conditions; (2) thermodynamic equilibrium happens at the component; (3) the kinetic energy of heat transfer and working ﬂuid in solar ORC cycles are negligible; inlet and outlet of each component; (3) the kinetic energy of heat transfer and working fluid in solar (4) heat loss and pressure drops in the system can be overlooked; and (5) the electricity consumption of ORC cycles are negligible; (4) heat loss and pressure drops in the system can be overlooked; and (5) water valves and refrigerant valves is ignored. The input parameters in the model are given in Table3. the electricity consumption of water valves and refrigerant valves is ignored. The input parameters in the model are given in Table 3.

Figure 4. Schematic diagram of the solar ORC model in Aspen Plus.

Energies 2019, 12, x FOR PEER REVIEW 6 of 21 temperature difference ΔTsuperheated of 5 °C is assumed between the heat source fluid inlet and working fluid outlet. Thus, the working fluid outlet temperature is evaluated as T1 = Tsc,1 − ΔTsuperheated. In order to comprehensively evaluate the performance, refrigerants of R245fa, R134a and isobutane are chosen as the working fluids for solar ORC systems. These refrigerants have been verified as well-suited candidates for low-temperature ORC applications due to their thermodynamic and physical properties, low flammability, corrosiveness and environmental impacts [5,20]. The thermodynamic data of these organic compounds are obtained from REFPROP 9 (National Institute of Standards and Technology, Gaithersburg, MD, USA).

180 600 R245fa 800 400 160 1000 200 1200 ρ = 1400 kg⋅m-3 100 140 50 id 20 C) r flu o 120 Sola 8 7 1 100 80

Temperature ( 60 6 40 2 5 4 20 3 0 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 ⋅ -1⋅ -1 Entropy (kJ kg K ) Figure 3. T-s diagram of a solar ORC using R245fa as the working fluid.

3. Methodology

3.1. Aspen Model The solar ORC model was built in Aspen Plus (v 8.8, Aspen Technology, Inc., Bedford, MA, USA), which includes the specification of each block [51]. The Peng–Robinson method was used to calculate the properties of all the components. Figure 4 shows the schematic diagram of the solar ORC system in Aspen Plus. The relevant assumptions are as follows: (1) The ORC cycle and its components operate under steady-state conditions; (2) thermodynamic equilibrium happens at the inlet and outlet of each component; (3) the kinetic energy of heat transfer and working fluid in solar ORC cycles are negligible; (4) heat loss and pressure drops in the system can be overlooked; and (5) Energiesthe electricity2019, 12, 829consumption of water valves and refrigerant valves is ignored. The input parameters7 of 22 in the model are given in Table 3.

Figure 4. Schematic diagram of the solar ORC model inin AspenAspen Plus.Plus. Table 3. Input parameters of the solar ORC model in Aspen Plus.

Section Parameter Value Discharge pressure Evaporating pressure Fluid pump Efficiency 0.65 Regenerator Cold stream outlet temperature T6 Preheater Cold stream outlet vapour fraction 0 Evaporator Cold stream outlet vapour fraction 1 Superheater Cold stream outlet temperature T1 Discharge pressure Condensing pressure Expander Isentropic efficiency 0.75 Condenser Condensation temperature 20 ◦C Working fluid Mass flow rate 0.003 kg·s−1 Mass flow rate a 0.03 kg·s−1 Solar collector fluid b Solar hot water inlet temperature Tsc,1 Mass flow rate 0.1 kg·s−1 Cooling water Cooling temperature 10 ◦C a Values are taken from the [5]. b The solar hot water inlet temperature is adopted from the [52].

3.2. Evaluation of Solar Collector The proposed solar ORC systems utilise thermal energy absorbed by a solar collector. The useful energy output from the solar collector per m2 can be predicted as a function of local climatic conditions using the widely adopted model by Peters and Bales [1,53], which can be expressed as Equation (1):

. Q = F0(τα) K (θ)I + F0(τα) K I − c uI − c (T − T )−c (T − T )2 sol en θb sol,b en θd sol,d 6 sol 1 sc ext 2 sc ext (1) 4 dTsc −c3u(Tsc − Text) + c4 EL − σText − c5 dt where Isol may be the direct normal or global irradiance depending on the collector type, and c1 to 0 c6 are solar collector efficiency curve coefficients. F (τα)en is zero loss efficiency for global or total radiation at normal incidence. Kθ is the solar collector incident angle modifier. Furthermore, u is the wind speed in (parallel to) the collector plane and EL is the long wavelength radiation (outside solar spectrum) onto the collector plane. For simplicity, the efficiency of the solar collector ηsc is calculated as a function of the incident solar irradiance I, the incident angle modifier Kθ, the mean collector temperature Tsc and the ambient air temperature Text, which can be expressed as Equation (2) [1,54].

2 Tsc − Text (Tsc − Text) ηsc = η0Kθ − c1 − c2 (2) Isol Isol Energies 2019, 12, 829 8 of 22

where Kθ = (Kθ,b Isol,b + Isol,d)/(Isol,b + Isol,d). The possible types of solar collectors that can be used in ORC systems are not reviewed in the present study since the main purpose of this paper is to compare the performance of ORCs using and not using TDP. Thus the type of solar collector is adopted from the reference [1] and the outlet temperature of solar collector array is chosen from reference [52]. The TVP SOLAR HT-Power, a high efﬁciency evacuated ﬂat-plate collector is selected, and the detailed parameters of the solar collector are listed in Table4.

Table 4. Solar collector model coefﬁcients [1].

Coefﬁcient Value

η0 0.82 c1 0.399 c2 0.0067 Kθ,(θ = 50◦) 0.91

The energy balance for the collector can be evaluated as Equation (3):

dT . . . M c sc = Q + m c T − m c T (3) sc p,w dt sol sc p,w sc,in sc p,w sc,out . where Msc, msc and cp,w are mass, mass flow rate and specific heat capacity of the solar collector fluid, respectively. Tsc,in and Tsc,out are the inlet and outlet temperature of the solar collector.

3.3. Performance Evaluation of ORC Systems The performance analysis of solar ORC systems is based on the ﬁrst and second thermodynamic laws. The general energy balance of the conventional ORC can be expressed as Equations (4) and (5):

. . ∑ min = ∑ mout (4)

. . . . Q + W = ∑ mouthout − ∑ minhin (5) . . where Q and W are the net heat flow rate and work inputs, h is specific enthalpy of the stream of the . . system, and min and mout are inlet and outlet mass flow rates. The energy balance of working fluids in the solar heating process can be evaluated by Equations (6)–(9): . . . Qpreheat = mwf(h7 − h6) = mw(hsc,3 − hsc,4) (6) . . . Qeva = mwf(h8 − h7) = mw(hsc,2 − hsc,3) (7) . . . Qsuperheat = mwf(h1 − h8) = mw(hsc,1 − hsc,2) (8) . . . . Qh = Qpreheat + Qeva + Qsuperheat (9) . . . where Qpreheat, Qeva and Qsuperheat are the thermal energy obtained by working fluids in the preheating, evaporation and superheating process, respectively. The subscripts of specific enthalpy h are based on the state points, as shown in Figure3. The heat balances in the condenser and regenerator can be expressed by Equations (10) and (11):

. . . Qcond = mwf(h3 − h4) = mc(hc,2 − hc,1) (10)

. . . Qregen = mwf(h2 − h3) = mwf(h6 − h5) (11) Energies 2019, 12, 829 9 of 22

Energy conservation in the expander and ﬂuid pump is deﬁned by Equations (12) and (13):

. . Wexp = mwf(h1 − h2) (12)

. . Wpu = mwf(h5 − h4) (13) . Thus, the net power Wnet and cycle thermal efﬁciency of the conventional ORC ηORC are presented by Equations (14) and (15): . . . Wnet = Wexp − Wpu (14) . Wnet ηORC = . (15) Qh The exergy analysis of the conventional ORC can be evaluated by Equations (16)–(20):

. . . . . E + W = ∑ Eout − ∑ Ein + I (16)

e = h − ho − T0(s − s0) (17) . . E = me (18) . T . E = (1 − 0 )Q (19) h ∑ T h . Wnet ηex,ORC = . (20) Eh . . . where E and I are the exergy rate and irreversibility rate, e is the specific flow exergy, Eh and ηexergy are the heat exergy and exergy efficiency of the conventional ORC, respectively, and T is the temperature of the heat input. The total heat input of the ORC using TDP is composed of two parts. One is the heat for the evaporation of the refrigerant. The other is the sensible heat for warming up the refrigerant. These are found by Equations (21) and (22):

. . 1 . Qh, TDP = Qwf + mwfcwf∆Twf (21) tcycle

. 1 . Z tpower . Qwf = [mwf(h6 − h4) + mwf(h1 − h6)dt] (22) tcycle 0 where tcycle is cycle time, which is composed of both pre-expansion and power generation processes. The average power output can be found by Equation (23):

. 1 Z cycle . 1 Z cycle . Wave = Winsdt = mwfηs hv − hexp,out (23) tcycle 0 tcycle 0

. where Wins is the instantaneous power output; ηs is the isentropic efﬁciency; hexp,out is the outlet enthalpy of the expander. The energy efﬁciency of the ORC using TDP can be determined according to Equation (24):

. Wave ηTDP = . (24) Qh,TDP Energies 2019, 12, 829 10 of 22

The heat exergy of the ORC with TDP can be calculated by Equation (25):

. . T0 Eh,TDP = Qh,TDP × (1 − ) (25) Th,ave where Th,ave is the average temperature of the heat input. The exergy efﬁciency of the ORC with TDP can be expressed as Equation (26):

. Wave ηex,TDP = . (26) Eh,TDP

3.4. Validation of ORC Model To validate the ORC model used in this paper, the results in this work are compared with the reference data in Ka¸ska’swork [55] which are shown in Table5. It is worth noting that the relative errors show a good agreement between the reference and present work. Thus, the ORC model in the present study is reliable to be used in the calculations.

Table 5. Comparison of results between the reference and present work.

◦ −1 ◦ ◦ Parameter T1 ( C) mwf (kg·s ) T4 ( C) T5 ( C) Wnet (kW) Qh (kW) ηORC (%) Ka¸ska,Ö [55] 92.9 11.06 34.9 35.4 262.2 2479 10.58 Present study 93.3 11.06 34.87 35.41 244.6 2313 10.57 Error (%) 0.43 0 0.09 0.03 6.71 6.69 0.09

4. Results and Discussions

4.1. Dynamic Output Characteristics Figure5 shows a range of daily solar collector outlet temperatures which are adopted from the reference [52]. The climatic data is made up of annually average temperatures for each hour of the day. Solar hot water with different temperatures ranging from 34 ◦C to 122.6 ◦C is adopted to drive the ORC system, and the evaporating temperature is fixed at 80 ◦C. Figure6 indicates the cumulative power output of the conventional solar ORC based on daily solar collector outlet temperatures in Figure5. Results demonstrate that the cumulative power output of the solar ORC has the same trend in terms of three working fluids. The power generation process starts only if the outlet temperature of solar collector exceeds the evaporating temperature. The cumulative power output from the ORC increases considerably while the collector outlet temperature is higher than 80 ◦C. It can be noted that the ORC using isobutane shows a better performance than those using R245fa and R134a, and it has the highest cumulative work of 52 MJ. The maximum cumulative work can reach 31 MJ and 26 MJ by using R245fa and R134a, respectively. Figure7 shows a comparison of power output using three working fluids under a range of collector outlet temperatures. It can be observed that the net power output increases with the increase of outlet temperature. This is mainly because when the solar hot water outlet temperature increases, the expander inlet temperature will accordingly increase, which leads to a higher power output. The optimum outlet temperature is found to be 122.6 ◦C of which the maximum net power output can be obtained. It is indicated that the fluid isobutane still performs well when compared with the other working fluids, and it has the highest power output of 149 W. For working fluids R245fa and R134a, the maximum power outputs could reach 87 W and 77 W, respectively. Energies 2019, 12, x FOR PEER REVIEW 10 of 21 Energies 2019, 12, x FOR PEER REVIEW 10 of 21 Figure 5 shows a range of daily solar collector outlet temperatures which are adopted from the referenceFigure [52]. 5 showsThe climatic a range data of daily is made solar up collector of annually outlet average temperatures temperatures which arefor adoptedeach hour from of the the day.reference Solar hot[52]. water The climatic with different data is temperaturesmade up of a rangnnuallying fromaverage 34 °Ctemperatures to 122.6 °C foris adopted each hour to driveof the theday. ORC Solar system, hot water and thewith evaporating different temperatures temperature rangis fixeding atfrom 80 °C. 34 Figure°C to 122.6 6 indicates °C is adopted the cumulative to drive powerthe ORC output system, of theand conventional the evaporating solar temperature ORC based is onfixed daily at 80 solar °C. Figurecollector 6 indicates outlet temperatures the cumulative in Figurepower 5. output Results of demonstrate the conventional that the solar cumulative ORC based power on outputdaily solar of the collector solar ORC outlet has temperaturesthe same trend in inFigure terms 5. of Results three working demonstrate fluids. that The the power cumulative generati poweron process output starts of the only solar if ORC the outlet has the temperature same trend ofin solar terms collector of three exceeds working the fluids. evaporating The power temperat generatiure.on The process cumulative starts onlypower if theoutput outlet from temperature the ORC increasesof solar collector considerably exceeds while the the evaporating collector outlet temperat temperure.ature The cumulativeis higher than power 80 °C. output It can frombe noted the ORCthat theincreases ORC using considerably isobutane while shows the a collector better performance outlet temper thanature those is higher using than R245fa 80 °C.and It R134a, can be andnoted it hasthat thethe highest ORC using cumulative isobutane work shows of 52 a MJ. better The performance maximum cumulative than those work using can R245fa reach and 31 MJR134a, and and26 MJ it hasby usingthe highest R245fa cumulative and R134a, work respectively. of 52 MJ. The maximum cumulative work can reach 31 MJ and 26 MJ by Energies 2019, 12, 829 11 of 22 using R245fa and R134a, respectively. 150 150 C) ° 100 C) ° 100

50 Temperature (

50 Twf,eva Temperature (

T Tsc,outwf,eva T 0 sc,out 6 8 10 12 14 16 18 0 6 8 10Time (hour) 12 14 16 18

Time (hour) Figure 5. Daily solar collector outlet temperature vs. time [52]. Figure 5. Daily solar collector outlet temperature vs. time [52]. Figure 5. Daily solar collector outlet temperature vs. time [52]. 60 60 R245fa 50 R134a R245fa 50 Isobutane R134a 40 Isobutane 40 30 30 20

Cumulative work (MJ) work Cumulative 20 10 Cumulative work (MJ) work Cumulative 10 Energies 2019, 12, x FOR PEER REVIEW0 11 of 21 6 8 10 12 14 16 18 0 working fluids, and it has the highest6 power 8 output 10Time of(hour) 12 149 W. 14 For working 16 18 fluids R245fa and R134a,

the maximum power outputs could reach 87 W Timeand 77 (hour) W, respectively. Figure 6. Cumulative power output based on daily solar collector outlet temperatures. Figure 6. Cumulative power output based on daily solar collector outlet temperatures. Figure 6. Cumulative160 power output based on daily solar collector outlet temperatures. Figure 7 shows a comparison of power output using three working fluids under a range of collectorFigure outlet 7 showstemperatures. a comparison It140 can be of observed power output that the using net power three output working increases fluids withunder the a increaserange of ofcollector outlet temperature. outlet temperatures. This is mainly It can be because observed when that the the solar net power hot water output outlet increases temperature with the increases, increase theof outletexpander temperature. inlet temperature This is 120mainly will accordingly because when increase, the solar which hot leadsR245fa water to outlet a higher temperature power output. increases, The R134a optimumthe expander outlet inlet temperature temperature is found will accordingly to be 122.6 °Cincrease, of which which the maximumleads to a higher net power power output output. can The be Isobutane obtained.optimum It outlet is indicated temperature that the 100is found fluid toisobutane be 122.6 still°C of performs which the well maximum when compared net power with output the canother be obtained. It is indicated that the fluid isobutane still performs well when compared with the other

Power output (W) 80

90 100 110 120 ° Temperature ( C) Figure 7.7. Net power output as a function of thethe solar collectorcollector outletoutlet temperature.temperature. 4.2. Parametric Analysis 4.2 Parametric Analysis In this section, a parametric analysis was undertaken to compare the performance between In this section, a parametric analysis was undertaken to compare the performance between solar solar ORCs using and not using TDP. Following variables are considered for power outputs of the ORCs using and not using TDP. Following variables are considered for power outputs of the systems: systems: (1) evaporating temperature, Tevap; (2) condensation temperature, Tcond; (3) ORC working (1) evaporating temperature, Tevap; (2) condensation temperature, Tcond; (3) ORC working fluid flow

rate, 𝑚 ; (4) fluid flow rate of solar collector, 𝑚 ; (5) isentropic efficiency of the expander; (6) efficiency of heat exchanger. A single parameter was adjusted while the others were held constant in the parametric analysis. The solar collector outlet temperature is selected for the optimum power output in the simulation, i.e., 122.6 °C. Considering the solar ORC using TDP, the periods for the pre- expansion process and power generation under the condition of different parameters were selected based on the results of our previous experimental work [50]. R134 is selected as a typical working fluid which aims to show the minimum influence by using TDP. Figure 8 reveals the power outputs of the solar ORC using and not using TDP when the evaporating temperature varies from 75 °C to 100 °C. It is worth noting that the power output of the solar ORC using TDP ranges from 72 W to 82 W, which is improved by up to 3.3% at a 100 °C evaporating temperature when compared with that of the conventional ORC. It is demonstrated that the power output for conventional ORC system climbs slightly from 90 °C to 95 °C and then fall rapidly after 95 °C. This is mainly due to the fact that with an increase of the evaporating temperature, the working fluid requires more heat for evaporation, which results in a decrease in the superheating capacity of the working fluid when constant heat is transferred by the solar collector. Then, the decrease in the expander inlet pressure leads to the decline in power output. Additionally, the power output of the solar ORC with TDP is higher than that of the conventional ORC when the evaporating temperature is higher than 90 °C. This is mainly because the working fluid pump consumes more electricity when the evaporating temperature increases, which exceeds the difference between the power output of the ORC using TDP and that of the conventional ORC. Thus, increased power output is achieved using the ORC with TDP in comparison with that using the conventional ORC.

Energies 2019, 12, 829 12 of 22

. . fluid flow rate, mwf; (4) fluid flow rate of solar collector, msc; (5) isentropic efficiency of the expander; (6) efficiency of heat exchanger. A single parameter was adjusted while the others were held constant in the parametric analysis. The solar collector outlet temperature is selected for the optimum power output in the simulation, i.e., 122.6 ◦C. Considering the solar ORC using TDP, the periods for the pre-expansion process and power generation under the condition of different parameters were selected based on the results of our previous experimental work [50]. R134 is selected as a typical working fluid which aims to show the minimum influence by using TDP. Figure8 reveals the power outputs of the solar ORC using and not using TDP when the evaporating temperature varies from 75 ◦C to 100 ◦C. It is worth noting that the power output of the solar ORC using TDP ranges from 72 W to 82 W, which is improved by up to 3.3% at a 100 ◦C evaporating temperature when compared with that of the conventional ORC. It is demonstrated that the power output for conventional ORC system climbs slightly from 90 ◦C to 95 ◦C and then fall rapidly after 95 ◦C. This is mainly due to the fact that with an increase of the evaporating temperature, the working fluid requires more heat for evaporation, which results in a decrease in the superheating capacity of the working fluid when constant heat is transferred by the solar collector. Then, the decrease in the expander inlet pressure leads to the decline in power output. Additionally, the power output of the solar ORC with TDP is higher than that of the conventional ORC when the evaporating temperature is higher than 90 ◦C. This is mainly because the working fluid pump consumes more electricity when the evaporating temperature increases, which exceeds the difference between the power output of the ORC using TDP and that of the conventional ORC. Thus, increased power output is achieved using theEnergies ORC 2019 with, 12, TDPx FOR in PEER comparison REVIEW with that using the conventional ORC. 12 of 21

85 R134a

) Conventional ORC 80

ORC with TDP 75 Power output (W

70 75 80 85 90 95 100 ° Evaporating temperature ( C) FigureFigure 8.8. PowerPower outputsoutputs ofof twotwo solarsolar ORCsORCs vs.vs. differentdifferent evaporatingevaporating temperatures.temperatures.

FigureFigure9 indicates9 indicates that that the powerthe power outputs outputs of both solarof both ORCs solar when ORCs the working when ﬂuidthe working condensation fluid ◦ ◦ temperaturecondensation is temperature increased from is increased 15 C to from 35 C. 15 With °C to the 35 increasing°C. With the of condensationincreasing of temperature,condensation thetemperature, expander outletthe expander pressure outlet increases, pressure thus increase the decreasess, thus of the 30.5% decreases are found of 30.5% in power are found outputs in forpower the ORCoutputs with for TDP the andORC 33.6% with forTDP the and conventional 33.6% for the ORC. conventional The working ORC. ﬂuid The can working be fully fluid cooled can down be fully to saturatedcooled down liquid to forsaturated more power liquid output for more when power the temperatureoutput when of the cooling temperature water is of lower. cooling The water power is outputslower. The of solar power ORCs outputs using of andsolar not ORCs using using TDP and range not from using 61.8 TDP W range to 88.9 from W and 61.8 58.8 W to W 88.9 to 88.5 W and W, respectively.58.8 W to 88.5 The W, differencerespectively. between The difference two ORCs be becomestween two larger ORCs with becomes the increment larger with of condensationthe increment temperatureof condensation since temperature conventional since ORC conventional generates less ORC work generates at higher less condensation work at higher temperature. condensation temperature.

90 R134a ) 80 ORC with TDP

70 Conventional ORC Power output (W

15 20 25 30 35 ° Condensation temperature ( C) Figure 9. Power outputs of two solar ORCs vs. different condensation temperatures.

Figure 10 illustrates the power outputs of two ORCs with the increase in solar collector fluid mass flow rate. Given that the flow rate of solar collector fluid varies from 0.01 kg·s−1 to 0.2 kg·s−1, the work outputs of ORCs using and not using TDP tend to be steady at solar collector mass flow rate above 0.05 kg·s−1. It can be noted that the rapid rise of power output exists before flow rate of 0.05 kg·s−1. This is mainly because lower solar fluid flow rate which has a higher outlet temperature will lead to more superheating capacity of the working fluid. The increase of the expander inlet pressure leads to considerable increment in power output. It is also observed that the power output of ORC with TDP approaches that of conventional ORC with the increase of solar collector fluid flow rate. Because flow rate of solar collector fluid has little influence on pump work, thus the difference of

Energies 2019, 12, x FOR PEER REVIEW 12 of 21

85 R134a

) Conventional ORC 80

ORC with TDP 75 Power output (W

70 75 80 85 90 95 100 ° Evaporating temperature ( C) Figure 8. Power outputs of two solar ORCs vs. different evaporating temperatures.

Figure 9 indicates that the power outputs of both solar ORCs when the working fluid condensation temperature is increased from 15 °C to 35 °C. With the increasing of condensation temperature, the expander outlet pressure increases, thus the decreases of 30.5% are found in power outputs for the ORC with TDP and 33.6% for the conventional ORC. The working fluid can be fully cooled down to saturated liquid for more power output when the temperature of cooling water is lower. The power outputs of solar ORCs using and not using TDP range from 61.8 W to 88.9 W and 58.8 W to 88.5 W, respectively. The difference between two ORCs becomes larger with the increment

Energiesof condensation2019, 12, 829 temperature since conventional ORC generates less work at higher condensation13 of 22 temperature.

90 R134a ) 80 ORC with TDP

70 Conventional ORC Power output (W

15 20 25 30 35 ° Condensation temperature ( C) Figure 9. Power outputs of two solar ORCs vs. different condensation temperatures.

Figure 1010 illustrates illustrates the the power power outputs outputs of of two two ORCs OR withCs with the increasethe increase in solar in solar collector collector fluid massfluid · −1 · −1 flowmass rate. flow Given rate. Given that the that flow the rate flow of rate solar of collector solar collector fluid varies fluid from varies 0.01 from kg 0.01s to kg·s 0.2−1 kg to s0.2 ,kg·s the− work1, the outputswork outputs of ORCs of ORCs using using and not and using not TDPusing tend TDP to tend be steady to be steady at solar at collector solar collector mass flow mass rate flow above rate · −1 · −1 0.05above kg 0.05s .kg·s It can−1. It be can noted be noted that the that rapid the riserapid of rise power of powe outputr output exists beforeexists flowbefore rate flow of 0.05rate kgof 0.05s . Thiskg·s− is1. This mainly is mainly because because lower solarlower fluid solar flow fluid rate flow which rate haswhich a higher has a outlethigher temperature outlet temperature will lead will to morelead to superheating more superheating capacity capacity of theworking of the working fluid. Thefluid. increase The increase of the expanderof the expander inlet pressure inlet pressure leads toleadsEnergies considerable to 2019 considerable, 12, x FOR increment PEER increment REVIEW in power in power output. output. It is also It is observed also observed that the that power the power output output of ORC of13 ORCwithof 21 TDPwith approachesTDP approaches that of that conventional of conventional ORC with ORC the with increase the increase of solar of collector solar collector fluid flow fluid rate. flow Because rate. flowBecausepower rate output flow of solar rate between collector of solar two fluid collector solar has little ORCsfluid influence hascould littl onebe influence pumpnegligible work, on whenpump thus thethe work, difference collector thus the offluid powerdifference flow output rate of betweeninfinitely two increases. solar ORCs could be negligible when the collector fluid flow rate infinitely increases.

100 R134a 90

70 ORC with TDP 60

Power output (W) Power output Conventional ORC 50

40 0.00 0.05 0.10 0.15 0.20 ⋅ -1 Solar collector fluid mass flow rate (kg s ) Figure 10. Power outputs of two solar ORCs vs. various mass ﬂowflow rates of solar collector ﬂuid.fluid.

The work outputs of two solar ORCs in terms of various ORC working fluidfluid mass flowflow rates are also observed in Figure 1111.. Similar to Figure 1010,, thethe power output of convention ORC tends to be flatflat −1 −1 whenwhen refrigerantrefrigerant flowflow raterate variesvaries fromfrom 0.001 0.001 kg kg·s·s −1 toto 0.020.02 kgkg·s·s −1. .The The higher higher flow flow rate reduces thethe degree ofof superheatingsuperheating capacity required to operateoperate thethe expander.expander. In contrast,contrast, the powerpower outputoutput ofof ORC withwith TDPTDP keeps keeps increasing increasing with with the the increment increment of flow of flow rate. rate. Additionally, Additionally, the difference the difference of power of outputpower output between between two solar two ORCs solar escalates ORCs escalates as the working as the working fluid mass fluid flow mass rate flow increases. rate increases. It is mainly It is becausemainly because the larger the flow larger rate resultsflow rate in higherresults pump in higher work pump of conventional work of conventional ORC, which enlargesORC, which their differenceenlarges their of power difference outputs. of power outputs.

180 R134a 160

) 140 ORC with TDP

120

100 Conventional ORC 80 Power output (W 60

40 0.000 0.005 0.010 0.015 0.020 ⋅ -1 ORC working fluid mass flow rate (kg s ) Figure 11. Power outputs of two solar ORCs vs. various working fluid mass flow rates.

In reality, the isentropic efficiencies of the working fluid pump and expander of small-scale ORC systems may be even lower than those of the simulation in this paper, i.e., 0.65 and 0.75, due to the limited selection and inappropriate design of ORC systems. Therefore, the relatively higher power output can be obtained from the ORC with TDP, which is quite competitive with the conventional ORC in household applications. With the decline of pump efficiency, the conventional ORC has a higher pump work which results in a lower power output. In contrast, the power output of ORC with TDP keeps constant at various pump efficiencies. It is also worth noting that the isentropic efficiency of the expander varies dramatically with regard to different external partial loads in real applications.

Energies 2019, 12, x FOR PEER REVIEW 13 of 21 power output between two solar ORCs could be negligible when the collector fluid flow rate infinitely increases.

100 R134a 90

70 ORC with TDP 60

Power output (W) Power output Conventional ORC 50

40 0.00 0.05 0.10 0.15 0.20 ⋅ -1 Solar collector fluid mass flow rate (kg s ) Figure 10. Power outputs of two solar ORCs vs. various mass flow rates of solar collector fluid.

The work outputs of two solar ORCs in terms of various ORC working fluid mass flow rates are also observed in Figure 11. Similar to Figure 10, the power output of convention ORC tends to be flat when refrigerant flow rate varies from 0.001 kg·s−1 to 0.02 kg·s−1. The higher flow rate reduces the degree of superheating capacity required to operate the expander. In contrast, the power output of ORC with TDP keeps increasing with the increment of flow rate. Additionally, the difference of power output between two solar ORCs escalates as the working fluid mass flow rate increases. It is mainly because the larger flow rate results in higher pump work of conventional ORC, which Energies 2019, 12, 829 14 of 22 enlarges their difference of power outputs.

180 R134a 160

) 140 ORC with TDP

120

100 Conventional ORC 80 Power output (W 60

40 0.000 0.005 0.010 0.015 0.020 ⋅ -1 ORC working fluid mass flow rate (kg s ) Figure 11. Power outputs of two solar ORCs vs. va variousrious working fluid ﬂuid mass ﬂowflow rates.

In reality, thethe isentropic efficienciesefficiencies ofof thethe working fluidfluid pump and expander of small-scale ORC systems may be even lower than those of the simulati simulationon in this paper, i.e., 0.65 and 0.75, due to the limited selection and inappropriate design of ORC systems. Therefore, the relatively higher power output can be obtained from the ORC with TDP, whichwhich isis quitequite competitivecompetitive with the conventional ORC in household applications. With With the the decline decline of of pump efficiency, efficiency, the conventional ORC has a higher pump pump work work which which results results in in a lower a lower power power ou output.tput. In contrast, In contrast, the power the power output output of ORC of ORCwith EnergieswithTDP keeps TDP 2019, keeps12constant, x FOR constant PEER at various REVIEW at various pump efficiencies. pump efficiencies. It is alsoIt worth is also noting worth that noting the isentropic that the isentropicefficiency14 of 21 ofefficiency the expander of the varies expander dramatically varies dramatically with regard withto different regard external to different partial external loads partialin real applications. loads in real Figureapplications. 12 shows Figure the generated 12 shows power the generated outputs powerof two solar outputs ORCs of twowhen solar the ORCsisentropic when efficiency the isentropic varies fromefficiency 0 to varies0.75. fromIt is 0demonstrated to 0.75. It is demonstrated that the ORC that with the ORCTDP withhas TDPa superior has a superiorpower output power outputwhen comparedwhen compared with that with of that the of conventional the conventional ORC. ORC. With With the the increment increment of of isentropic isentropic efficiency, efficiency, the difference of genera generatedted power of two solar ORCs decreases.decreases. The isentropic efficiency efficiency of expander selected inin ourour previousprevious experimental experimental work work ranges ranges from from 0.42 0.42 to 0.6to 0.6 in termsin terms of various of various partial partial loads loads [50]. [Under50]. Under this scenario,this scenario, the power the power outputs outputs of two of solartwo solar ORCs ORCs range range from from 40 W 40 to W 62 to W 62 and W from and from46 W 46to 65W W,to 65 respectively. W, respectively In ideal. In ideal operation operation conditions, conditions, the isentropicthe isentropic efficiency efficiency of this of this expander expander can canvary vary from from 0.4 to0.4 0.75. to 0.75. When When the isentropicthe isentropic efficiency efficiency is 0.75, is 0.75, the maximum the maximum power power output output of two ofsolar two solarORCs ORCs are 80.6 are W 80.6 and W 81.8 and W, 81.8 respectively, W, respectively which, which could indicatecould indicate the minimum the minimum influence influence using TDP using as TDPmentioned as mentioned above. above.

80 R134a

Conventional ORC 40 ORC with TDP

20 Power output (W) output Power

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Isentropic efficiency FigureFigure 12. 12. PowerPower output outputss of of two two solar solar ORCs ORCs vs. vs. different different isentropic isentropic efficiencies efﬁciencies of expander expander..

Figure 13 also shows the results of power outputs when the heat exchangers efficiency varies. It can be observed that two solar ORCs have similar increasing trends with the increment of heat exchanger efficiency. ORC using TDP still performs better than conventional ORC. In the most cases, the heat exchanger efficiency is above 0.9. Thus, the power outputs of ORCs using and not using TDP at this interval range from 80.7 W to 81.8 W, and 79.4 W to 80.6 W, respectively.

82 R134a 80

78 ORC with TDP

72 Conventional ORC Power output (W) output Power 70

68 0.5 0.6 0.7 0.8 0.9 1.0 Heat exchanger efficiency

Figure 13. Power output of two solar ORCs vs. heat exchangers efficiencies.

Energies 2019, 12, x FOR PEER REVIEW 14 of 21

Figure 12 shows the generated power outputs of two solar ORCs when the isentropic efficiency varies from 0 to 0.75. It is demonstrated that the ORC with TDP has a superior power output when compared with that of the conventional ORC. With the increment of isentropic efficiency, the difference of generated power of two solar ORCs decreases. The isentropic efficiency of expander selected in our previous experimental work ranges from 0.42 to 0.6 in terms of various partial loads [50]. Under this scenario, the power outputs of two solar ORCs range from 40 W to 62 W and from 46 W to 65 W, respectively. In ideal operation conditions, the isentropic efficiency of this expander can vary from 0.4 to 0.75. When the isentropic efficiency is 0.75, the maximum power output of two solar ORCs are 80.6 W and 81.8 W, respectively, which could indicate the minimum influence using TDP as mentioned above.

80 R134a

Conventional ORC 40 ORC with TDP

20 Power output (W) output Power

0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Isentropic efficiency Energies 2019, 12, 829 15 of 22 Figure 12. Power outputs of two solar ORCs vs. different isentropic efficiencies of expander.

Figure 13 alsoalso shows shows the the results results of of power power output outputss when when the the heat heat exchangers exchangers efficiency efficiency varies. varies. It Itcan can be be observed observed that that two two solar solar ORCs ORCs have have similar similar increasing increasing trends trends with with the the increment of heatheat exchanger efficiency.efficiency. ORC using TDP still performs better than conventional ORC. In the most cases, cases, the heat exchanger efficiencyefficiency isis aboveabove 0.9.0.9. Thus,Thus, thethe power outputsoutputs of ORCs using and not using TDP at this interval range from 80.7 WW toto 81.881.8 W,W, andand 79.479.4 WW toto 80.680.6 W,W, respectively.respectively.

82 R134a 80

78 ORC with TDP

72 Conventional ORC Power output (W) output Power 70

68 0.5 0.6 0.7 0.8 0.9 1.0 Heat exchanger efficiency

Figure 13. Power output of two solar ORCs vs. heat exchangers eff efﬁciencies.iciencies. 4.3. Thermal Performance of Solar ORCs throughout the Day

This section aims to investigate the thermal performance of solar ORCs using and not using TDP in terms of various evaporating temperatures. Similar with Section 4.2, the solar collector outlet temperature is set as 122.6 ◦C for the maximum power output. Figure 14 indicates the energy and exergy efficiencies of the solar ORC using TDP when compared with those of the conventional ORC, with R134a as the working fluid. Figure 14a demonstrates that the energy efficiency of the solar ORC using TDP increases with the increase in evaporating temperature. When the temperature increases from 75 ◦C to 100 ◦C, the energy efficiency ranges from 11.3% to 12.6%. In order to further illustrate the significance of solar ORC using TDP, the energy efficiencies of two systems are compared with the results from references [1,10,56]. It is indicated that the energy efficiencies in the present work are much higher than the results from references [10,56] which are 8.58% and 5.6% when evaporating temperatures are 78.8 ◦C and 85 ◦C, respectively. It proves that the high efficiency evacuated flat-plate collector used in this study could supply a higher solar collector outlet temperature, which improves the overall energy efficiency of system. The difference of energy efficiency between present work and reference [1] is relatively small which is mainly because different working fluid flow rates. The efficiency of ORC using TDP is quite desirable when considering that the heat source temperature is relatively low, and it represents an advantage of the ORC using TDP. Similar to power output, the energy efficiency of the ORC using TDP becomes higher than that of conventional ORC when the temperature is higher than 90 ◦C. The difference in energy efficiency between the two solar ORCs increases remarkably with the increase in evaporating temperature. The largest percent increment in energy efficiency can reach 3.27% at a 100 ◦C evaporating temperature when compared with the conventional ORC. A similar trend for exergy efficiency can be found for the ORC using TDP, as shown in Figure 14b. Exergy efficiency ranges from 45.8% to 51.3% when the evaporating temperature increases from 75 ◦C to 100 ◦C. Results show that both energy efficiency and exergy efficiency of conventional ORC climb from 75 ◦C to 90 ◦C, followed by a slight increase, and it then decline rapidly from 95 ◦C to 100 ◦C. The main reason for the improved energy efficiency and exergy efficiency is that a higher power output is obtained from the ORC with TDP. Energies 2019, 12, x FOR PEER REVIEW 15 of 21

4.3. Thermal Performance of Solar ORCs throughout the Day This section aims to investigate the thermal performance of solar ORCs using and not using TDP in terms of various evaporating temperatures. Similar with section 4.2, the solar collector outlet temperature is set as 122.6 °C for the maximum power output. Figure 14 indicates the energy and exergy efficiencies of the solar ORC using TDP when compared with those of the conventional ORC, with R134a as the working fluid. Figure 14a demonstrates that the energy efficiency of the solar ORC using TDP increases with the increase in evaporating temperature. When the temperature increases from 75 °C to 100 °C, the energy efficiency ranges from 11.3% to 12.6%. In order to further illustrate the significance of solar ORC using TDP, the energy efficiencies of two systems are compared with the results from references [1,10,56]. It is indicated that the energy efficiencies in the present work are much higher than the results from references [10,56] which are 8.58% and 5.6% when evaporating temperatures are 78.8 °C and 85 °C, respectively. It proves that the high efficiency evacuated flat- plate collector used in this study could supply a higher solar collector outlet temperature, which improves the overall energy efficiency of system. The difference of energy efficiency between present work and reference [1] is relatively small which is mainly because different working fluid flow rates. The efficiency of ORC using TDP is quite desirable when considering that the heat source temperature is relatively low, and it represents an advantage of the ORC using TDP. Similar to power output, the energy efficiency of the ORC using TDP becomes higher than that of conventional ORC when the temperature is higher than 90 °C. The difference in energy efficiency between the two solar ORCs increases remarkably with the increase in evaporating temperature. The largest percent increment in energy efficiency can reach 3.27% at a 100 °C evaporating temperature when compared with the conventional ORC. A similar trend for exergy efficiency can be found for the ORC using TDP, as shown in Figure 14b. Exergy efficiency ranges from 45.8% to 51.3% when the evaporating temperature increases from 75 °C to 100 °C. Results show that both energy efficiency and exergy efficiency of conventional ORC climb from 75 °C to 90 °C, followed by a slight increase, and it then

Energiesdecline2019 rapidly, 12, 829 from 95 °C to 100 °C. The main reason for the improved energy efficiency and exergy16 of 22 efficiency is that a higher power output is obtained from the ORC with TDP.

0.52 0.13 R134a R134a 0.12 Conventional ORC 0.11 ORC with TDP 0.50

0.10 Ref.[1] Conventional ORC 0.09 0.48 0.08 Energy efficiency Ref.[51] Exergy efficiency 0.07 ORC with TDP 0.06 Ref.[7] 0.46 0.05 75 80 85 90 95 100 75 80 85 90 95 100 Evaporating temperature (°C) Evaporating temperature (°C) (a) (b)

FigureFigure 14.14. EnergyEnergy and and exergy exergy efficiencies efficiencies of two of twosolar solarORCs ORCs (a) energy (a) energy efficiency; efficiency; and (b) andexergy (b ) exergyefficiency. efficiency.

FigureFigure 15 15 shows shows thethe exergyexergy destructiondestruction inin each system component for for solar solar ORCs ORCs using using and and not not usingusing TDPTDP underunder thethe conditionsconditions ofof aa 9595 ◦°CC evaporatingevaporating temperature and and a a 122.6 122.6 °C◦C solar solar collector collector outletoutlet temperature. temperature. ConsideringConsidering thethe conventionalconventional ORC system, it is is worth worth noting noting that that the the majority majority of of thethe exergy exergy is is lost lost inin thethe preheater,preheater, expander,expander, evaporatorevaporator and condenser. This This is is inevitable inevitable when when solar solar thermalthermal isis converted converted toto mechanicalmechanical work.work. The maxi maximummum amount amount of of heat heat can can be be transferred transferred in in the the preheater,Energiespreheater, 2019 which, which12, x FOR results results PEER in REVIEWin a largesta largest difference difference between between the the output output and and inlet inlet exergy exergy of theof the preheater. preheater.16 of It is21 indicatedIt is indicated that the that exergy the exergy destruction destruction of the ORC of the using ORC TDP using can TDP be reduced can be 2.86 reduced W when 2.86 eliminating W when theeliminating pump. Additionally, the pump. Additionally, the exergy destruction the exergy is reduceddestruction in the is evaporatorreduced in and the condenserevaporator by and up tocondenser 92% and by 2%, up respectively. to 92% and In 2%, contrast, respectively. the exergy In contrast, destruction the increasesexergy destruction in the expander, increases preheater, in the superheaterexpander, preheater, and regenerator. superheater The increased and regenerator. exergy destruction The increased in the heatexergy exchangers destruction is probably in the heat due toexchangers the larger temperatureis probably due difference to the betweenlarger temper the enteringature difference and exiting between ﬂuids of the the component.entering and Another exiting reasonfluids of is thatthe component. the preheater Another has a lower reason inlet is that temperature, the preheater and thehas regeneratora lower inlet and temperature, superheater and could the haveregenerator lower outletand superheater temperatures could of thehave working lower outlet ﬂuid duringtemperatures the pre-expansion of the working process fluid of during the ORC the usingpre-expansion TDP. Since process no power of the is generatedORC using from TDP. the Since expander no power during is generated the pre-expansion from the process,expander it during brings aboutthe pre-expansion a higher exergy process, destruction it brings in the about expander a hi whengher comparedexergy destruction with that ofin the the conventional expander when ORC operatingcompared within with that the of same the cycleconventional time. ORC operating within the same cycle time.

Conventional ORC 80 R134a ORC with TDP

40 Exergy destruction (W) 20

0 r r r e mp te te ser u ea rator ea P h o h den erator re er n P ap ge Expand v up Con e E S R FigureFigure 15.15. Exergy destructiondestruction ofof eacheach componentcomponent betweenbetween twotwo solarsolar ORCsORCs usingusing R134a.R134a.

The exergy efficiency of each system component for both solar ORCs when using R134a is shown in Figure 16. It can be observed that the expander has the maximum exergy efficiency of 76% for the conventional ORC. The fluid pump also has a relatively higher exergy efficiency of 74.9%. An improved exergy efficiency of the evaporator can be observed for the solar ORC using TDP. The highest exergy efficiency of the evaporator can reach 96.9% for the ORC, and this is improved by 62% when compared with that of the conventional ORC, i.e., 59.9%. In contrast, the other components show relatively lower exergy efficiencies, especially for the preheater, which has the lowest exergy efficiency of 21.5%. This is mainly because the temperature of the working fluid increases gradually during the pre-expansion process, which may result in a lower inlet temperature of the working fluid.

1.0 Conventional ORC R134a ORC with TDP 0.8

0.6 gy efficiency

0.4

Component exer 0.2

0.0 r r er ter o mp at nd u ense r pa P hea orator e re p erheater P gen Ex up Cond e Eva S R Figure 16. Exergy efficiency of each component between two solar ORCs using R134a.

Energies 2019, 12, x FOR PEER REVIEW 16 of 21

eliminating the pump. Additionally, the exergy destruction is reduced in the evaporator and condenser by up to 92% and 2%, respectively. In contrast, the exergy destruction increases in the expander, preheater, superheater and regenerator. The increased exergy destruction in the heat exchangers is probably due to the larger temperature difference between the entering and exiting fluids of the component. Another reason is that the preheater has a lower inlet temperature, and the regenerator and superheater could have lower outlet temperatures of the working fluid during the pre-expansion process of the ORC using TDP. Since no power is generated from the expander during the pre-expansion process, it brings about a higher exergy destruction in the expander when compared with that of the conventional ORC operating within the same cycle time.

Conventional ORC 80 R134a ORC with TDP

40 Exergy destruction (W) 20

0 r r r e mp te te ser u ea rator ea P h o h den erator re er n P ap ge Expand v up Con e E S R Energies 2019, 12, 829 17 of 22 Figure 15. Exergy destruction of each component between two solar ORCs using R134a.

The exergy efficiencyefficiency ofof eacheach system component forfor bothboth solar ORCs when using R134a is shown inin FigureFigure 1616.. It can be observedobserved that the expanderexpander has the maximum exergy efficiencyefficiency of 76% for the conventional ORC.ORC. The The fluid fluid pump pump also also has ahas relatively a relatively higher higher exergy efficiencyexergy efficiency of 74.9%. of An 74.9%. improved An exergyimproved efficiency exergy of efficiency the evaporator of the canevaporator be observed can forbe observed the solar ORCfor the using solar TDP. ORC The using highest TDP. exergy The efficiencyhighest exergy of the efficiency evaporator of canthe reachevaporator 96.9% can for thereac ORC,h 96.9% and for this the is ORC, improved and this by 62% is improved when compared by 62% withwhen that compared of the conventional with that of ORC, the conventional i.e., 59.9%. In ORC, contrast, i.e., the 59.9%. other In components contrast, the show other relatively components lower exergyshow relatively efficiencies, lower especially exergy for efficiencies, the preheater, especially which hasfor the lowestpreheater, exergy which efficiency has the of lowest 21.5%. exergy This is mainlyefficiency because of 21.5%. the This temperature is mainly of because the working the temperature fluid increases of the gradually working during fluid increases the pre-expansion gradually process,during the which pre-expansion may result process, in a lower which inlet may temperature result in a of lower the working inlet temperature fluid. of the working fluid.

1.0 Conventional ORC R134a ORC with TDP 0.8

0.6 gy efficiency

0.4

Component exer 0.2

0.0 r r er ter o mp at nd u ense r pa P hea orator e re p erheater P gen Ex up Cond e Eva S R Figure 16. Exergy efﬁciencyefficiency of each component between twotwo solarsolar ORCsORCs usingusing R134a.R134a.

4.4.Further Exploration and Comparison In order to have a comprehensive understanding for small-scale household application, the solar ORC using TDP is compared with the conventional solar ORC in terms of system type, system compactness, power output stability, thermal efficiency and capital cost, as shown in Table6. The conventional ORC is a continuous system, whereas the ORC using TDP is an intermittent system. The system compactness of the solar ORC with TDP is superior to that of the conventional ORC due to the elimination of the fluid pump. In many small-scale ORC experimental rigs, the fluid pump accounts for 5–10% of the system volume, which can then be reduced by using the ORC with TDP. Considering power output stability, the conventional ORC has a better performance than the solar ORC with s TDP. It is worth noting that the relatively large fluctuation of the ORC with TDP may happen when the condenser and evaporator are mutually altered. However, this only lasts for a very short period, i.e., less than 10% of the cycle time when considering the total working process. For the solar energy utilisation of a household, efficient battery technology is expected to solve this problem. This means that the electricity is stored and then output constantly. Another possible solution is the introduction of more heat exchangers to the ORC with TDP, with the aim of ensuring that the evaporator is always generating power while other heat exchangers are either used for supplementing the working fluid or for condensation. However, this method may have a small influence on the system compactness, which could be the focus of our future work. Based on the above thermal analysis, the energy and exergy efficiencies of the solar ORC with TDP are higher than those of the conventional ORC in most operation cases. This improvement will become more advantageous to household application when the heat source temperature and scale of the system decrease. As for cost, the main difference between the ORC with TDP and the conventional ORC lies in the working fluid pump. The cost of the working fluid pump is usually 2–3 times higher than that of the heat exchanger in small-scale ORC applications. Thus, the total cost of the ORC using TDP could be reduced by at least 10% with regard to that of the Energies 2019, 12, 829 18 of 22 whole system. In general, the solar ORC using TDP is superior to the conventional ORC, which reveals the vast potential for small-scale household application, especially for places that have relatively low solar radiation.

Table 6. Comparison between the conventional ORC and ORC with TDP driven by solar energy.

System Output Energy Type System Type Cost Compactness Stability Efﬁciency Conventional ORC Continuous Moderate High Low High ORC using TDP Intermittent High Low Moderate At least 10% less

5. Conclusions In this paper, small-scale ORC systems driven by solar energy are presented to investigate the overall performance in terms of the power output, energy efficiency, exergy efficiency and exergy destruction of the components. Additionally, the thermal performance of the ORC using TDP is compared with that of the conventional ORC. The parametric analysis of two solar ORCs demonstrates that ORC with TDP has a superior performance to that of the conventional ORC under most working conditions. The power output of the solar ORC using TDP ranges from 72 W to 82 W when R134a is adopted as the working fluid and the evaporating temperature varies from 75 ◦C to 100 ◦C. The performance is improved by up to 9.5% at a 100 ◦C evaporating temperature when compared with that of the conventional ORC. The energy and exergy efficiencies of the ORC using TDP increase from 11.3% to 12.6% and from 45.8% to 51.3% when the evaporating temperature increases from 75 ◦C to 100 ◦C. For the ORC using TDP, the exergy destruction in the evaporator and condenser can be reduced by up to 92% and 2%, respectively. The highest exergy efficiency of the evaporator is 96.9% by using R134a, and this is an improvement of 62% compared with 59.9% of the conventional ORC. For small-scale solar utilisation, the ORC using TDP is almost superior to the conventional ORC in terms of system type, system compactness, power output stability, thermal efficiency and capital cost. The large fluctuation of the ORC using TDP may last a very short period during the switch of heat exchangers, which could be solved by using efficient battery technology. This technology could be an alternative to household power supply and also serves as the main part of solar heating and power generation.

Author Contributions: Conceptualization: R.W. and L.J.; methodology: R.W. and A.G.D.; supervision: L.J., Y.W., and A.P.R.; project administration: L.J., Z.M., and A.P.R. Funding: This research was funded by the National Natural Science Foundation of China under grant number 51606118 and Heat-STRESS project EP/N02155X/1. Acknowledgments: This research was supported by National Natural Science Foundation of China under contract number 51606118 and Heat-STRESS project EP/N02155X/1 funded by Engineering and Physical Science Research Council of UK. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

A Area of solar collector, m2 c Specific heat capacity, J·kg−1 -2 −1 c1 Collector heat loss coefficient, W·m ·K −2 −2 c2 Temperature dependence of the heat loss coefficient, W·m ·K −3 −1 c3 Wind speed dependence of the heat loss coefficient, J·m ·K c4 Long-wave irradiance dependence of the heat loss coefficient −2 −1 c5 Solar collector effective thermal capacity, J·m ·K −1 c6 Wind-dependence of the collector optical (zero-loss) efficiency, s·m E Radiation (outside solar spectrum) onto the collector plane, W·m−2 . E Exergy flow rate, W Energies 2019, 12, 829 19 of 22 e Specific flow exergy, W·kg−1 F’ Collector efficiency factor h Specific enthalpy, J·kg−1 I Irradiance, W·m−2 . I Irreversibility rate, W Kθ Solar collector Incident Angle Modifier M Mass, kg . m Mass flow rate, kg·s−1 . Q Heat flow rate, W s Specific entropy, J·kg−1·K−1 t Time, s T Temperature, K u Wind speed in (parallel to) the collector plane, m·s−1 V Valve W Work, W Greek Letters η Efficiency θ Incidence angle, ◦ ρ Density, kg·m−3 (τα) Effective transmittance-absorptance product Subscripts 0 Dead state ave Average b Direct (beam) radiation c Cooling water d Diffuse radiation cond Condenser eva Evaporator ex Exergy exp Expander ext External environment h Heat in Inlet ins Instantaneous power output L Long wavelength net Net power output out Outlet p Isobaric process pu Pump preheat Preheater regen Regenerator s Isentropic sc Solar collector sol Solar superheat Superheater superheated Superheated temperature v Vapor w Water wf Working fluid Abbreviations ORC Organic Rankine cycle RV Refrigerant valve TDP Thermal driven pump Energies 2019, 12, 829 20 of 22

References

1. Freeman, J.; Hellgardt, K.; Markides, C.N. Working fluid selection and electrical performance optimisation of a domestic solar-ORC combined heat and power system for year-round operation in the UK. Appl. Energy 2017, 186, 291–303. [CrossRef] 2. Chen, Z.-G.; Shi, X.; Zhao, L.-D.; Zou, J. High-performance SnSe thermoelectric materials: Progress and future challenge. Prog. Mater. Sci. 2018, 97, 283–346. [CrossRef] 3. Yang, L.; Chen, Z.-G.; Dargusch, M.S.; Zou, J. High Performance Thermoelectric Materials: Progress and Their Applications. Adv. Energy Mater. 2018, 8, 1701797. [CrossRef] 4. Cioccolanti, L.; Tascioni, R.; Arteconi, A. Mathematical modelling of operation modes and performance evaluation of an innovative small-scale concentrated solar organic Rankine cycle plant. Appl. Energy 2018, 221, 464–476. [CrossRef] 5. Freeman, J.; Hellgardt, K.; Markides, C.N. An assessment of solar-powered organic Rankine cycle systems for combined heating and power in UK domestic applications. Appl. Energy 2015, 138, 605–620. [CrossRef] 6. Freeman, J.; Hellgardt, K.; Markides, C.N. An Assessment of Solar–Thermal Collector Designs for Small-Scale Combined Heating and Power Applications in the United Kingdom. Heat Transf. Eng. 2015, 36, 1332–1347. [CrossRef] 7. Tzivanidis, C.; Bellos, E.; Antonopoulos, K.A. Energetic and financial investigation of a stand-alone solar-thermal Organic Rankine Cycle power plant. Energy Convers. Manag. 2016, 126, 421–433. [CrossRef] 8. Petrollese, M.; Cocco, D. Robust optimization for the preliminary design of solar organic Rankine cycle (ORC) systems. Energy Convers. Manag. 2019, 184, 338–349. [CrossRef] 9. Casartelli, D.; Binotti, M.; Silva, P.; Macchi, E.; Roccaro, E.; Passera, T. Power Block Off-design Control Strategies for Indirect Solar ORC Cycles. Energy Procedia 2015, 69, 1220–1230. [CrossRef] 10. Quoilin, S.; Orosz, M.; Hemond, H.; Lemort, V. Performance and design optimization of a low-cost solar organic Rankine cycle for remote power generation. Sol. Energy 2011, 85, 955–966. [CrossRef] 11. Canada, S.; Cohen, G.; Cable, R.; Brosseau, D.; Price, H. Parabolic Trough Organic Rankine Cycle Power Plant; No. NREL/CP-550-37077; National Renewable Energy Lab (NREL): Golden, CO, USA, 2005. 12. Bellos, E.; Tzivanidis, C. Parametric analysis and optimization of an Organic Rankine Cycle with nanofluid based solar parabolic trough collectors. Renew. Energy 2017, 114, 1376–1393. [CrossRef] 13. Manolakos, D.; Kosmadakis, G.; Kyritsis, S.; Papadakis, G. On site experimental evaluation of a low-temperature solar organic Rankine cycle system for RO desalination. Sol. Energy 2009, 83, 646–656. [CrossRef] 14. Freeman, J.; Guarracino, I.; Kalogirou, S.A.; Markides, C.N. A small-scale solar organic Rankine cycle combined heat and power system with integrated thermal energy storage. Appl. Therm. Eng. 2017, 127, 1543–1554. [CrossRef] 15. Manfrida, G.; Secchi, R.; Stańczyk,K. Modelling and simulation of phase change material latent heat storages applied to a solar-powered Organic Rankine Cycle. Appl. Energy 2016, 179, 378–388. [CrossRef] 16. Jing, L.; Gang, P.; Jie, J. Optimization of low temperature solar thermal electric generation with Organic Rankine Cycle in different areas. Appl. Energy 2010, 87, 3355–3365. [CrossRef] 17. Shi, L.; Shu, G.; Tian, H.; Deng, S. A review of modified Organic Rankine cycles (ORCs) for internal combustion engine waste heat recovery (ICE-WHR). Renew. Sustain. Energy Rev. 2018, 92, 95–110. [CrossRef] 18. Braimakis, K.; Karellas, S. Energetic optimization of regenerative Organic Rankine Cycle (ORC) configurations. Energy Convers. Manag. 2018, 159, 353–370. [CrossRef] 19. Wang, X.D.; Zhao, L.; Wang, J.L. Experimental investigation on the low-temperature solar Rankine cycle system using R245fa. Energy Convers. Manag. 2011, 52, 946–952. [CrossRef] 20. Chen, H.; Goswami, D.Y.; Stefanakos, E.K. A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. Renew. Sustain. Energy Rev. 2010, 14, 3059–3067. [CrossRef] 21. Bao, J.; Zhao, L. A review of working fluid and expander selections for organic Rankine cycle. Renew. Sustain. Energy Rev. 2013, 24, 325–342. [CrossRef] 22. Shu, G.; Zhao, J.; Tian, H.; Liang, X.; Wei, H. Parametric and exergetic analysis of waste heat recovery system based on thermoelectric generator and organic rankine cycle utilizing R123. Energy 2012, 45, 806–816. [CrossRef] Energies 2019, 12, 829 21 of 22

23. Wei, D.; Lu, X.; Lu, Z.; Gu, J. Dynamic modeling and simulation of an Organic Rankine Cycle (ORC) system for waste heat recovery. Appl. Therm. Eng. 2008, 28, 1216–1224. [CrossRef] 24. Mago, P.J.; Chamra, L.M.; Srinivasan, K.; Somayaji, C. An examination of regenerative organic Rankine cycles using dry fluids. Appl. Therm. Eng. 2008, 28, 998–1007. [CrossRef] 25. Nguyen, V.M.; Doherty, P.S.; Riffat, S.B. Development of a prototype low-temperature Rankine cycle electricity generation system. Appl. Therm. Eng. 2001, 21, 169–181. [CrossRef] 26. Wang, X.D.; Zhao, L. Analysis of zeotropic mixtures used in low-temperature solar Rankine cycles for power generation. Sol. Energy 2009, 83, 605–613. [CrossRef] 27. Wang, J.L.; Zhao, L.; Wang, X.D. A comparative study of pure and zeotropic mixtures in low-temperature solar Rankine cycle. Appl. Energy 2010, 87, 3366–3373. [CrossRef] 28. Shu, G.; Wang, X.; Tian, H.; Liu, P.; Jing, D.; Li, X. Scan of working fluids based on dynamic response characters for Organic Rankine Cycle using for engine waste heat recovery. Energy 2017, 133, 609–620. [CrossRef] 29. Yang, J.; Sun, Z.; Yu, B.; Chen, J. Experimental comparison and optimization guidance of R1233zd(E) as a drop-in replacement to R245fa for organic Rankine cycle application. Appl. Therm. Eng. 2018, 141, 10–19. [CrossRef] 30. Ayachi, F.; Ksayer, E.B.; Neveu, P.; Zoughaib, A. Experimental investigation and modeling of a hermetic scroll expander. Appl. Energy 2016, 181, 256–267. [CrossRef] 31. Jiang, L.; Lu, H.; Wang, R.; Wang, L.; Gong, L.; Lu, Y.; Roskilly, A.P. Investigation on an innovative cascading cycle for power and refrigeration cogeneration. Energy Convers. Manag. 2017, 145, 20–29. [CrossRef] 32. Garg, P.; Karthik, G.M.; Kumar, P.; Kumar, P. Development of a generic tool to design scroll expanders for ORC applications. Appl. Therm. Eng. 2016, 109 Pt B, 878–888. [CrossRef] 33. Gao, P.; Jiang, L.; Wang, L.W.; Wang, R.Z.; Song, F.P. Simulation and experiments on an ORC system with different scroll expanders based on energy and exergy analysis. Appl. Therm. Eng. 2015, 75, 880–888. [CrossRef] 34. Hou, X.; Zhang, H.; Yu, F.; Liu, H.; Yang, F.; Xu, Y.; Tian, Y.; Li, G. Free piston expander-linear generator used for organic Rankine cycle waste heat recovery system. Appl. Energy 2017, 208, 1297–1307. [CrossRef] 35. Baccioli, A.; Antonelli, M.; Desideri, U. Dynamic modeling of a solar ORC with compound parabolic collectors: Annual production and comparison with steady-state simulation. Energy Convers. Manag. 2017, 148, 708–723. [CrossRef] 36. Papes, I.; Degroote, J.; Vierendeels, J. New insights in twin screw expander performance for small scale ORC systems from 3D CFD analysis. Appl. Therm. Eng. 2015, 91, 535–546. [CrossRef] 37. Ziviani, D.; Suman, A.; Lecompte, S.; De Paepe, M.; van den Broek, M.; Spina, P.R.; Pinelli, M.; Venturini, M.; Beyene, A. Comparison of a Single-screw and a Scroll Expander under Part-load Conditions for Low-grade Heat Recovery ORC Systems. Energy Procedia 2014, 61, 117–120. [CrossRef] 38. Kang, S.H. Design and experimental study of ORC (organic Rankine cycle) and radial turbine using R245fa working fluid. Energy 2012, 41, 514–524. [CrossRef] 39. Song, P.; Wei, M.; Liu, Z.; Zhao, B. Effects of suction port arrangements on a scroll expander for a small scale ORC system based on CFD approach. Appl. Energy 2015, 150, 274–285. [CrossRef] 40. Hou, X.; Zhang, H.; Xu, Y.; Yu, F.; Zhao, T.; Tian, Y.; Yang, Y.; Zhao, R. External load resistance effect on the free piston expander-linear generator for organic Rankine cycle waste heat recovery system. Appl. Energy 2018, 212, 1252–1261. [CrossRef] 41. Jiménez-Arreola, M.; Pili, R.; Wieland, C.; Romagnoli, A. Analysis and comparison of dynamic behavior of heat exchangers for direct evaporation in ORC waste heat recovery applications from fluctuating sources. Appl. Energy 2018, 216, 724–740. [CrossRef] 42. Dong, J.; Zhang, X.; Wang, J. Experimental investigation on heat transfer characteristics of plat heat exchanger applied in organic Rankine cycle (ORC). Appl. Therm. Eng. 2017, 112, 1137–1152. [CrossRef] 43. Borsukiewicz-Gozdur, A. Pumping work in the organic Rankine cycle. Appl. Therm. Eng. 2013, 51, 781–786. [CrossRef] 44. Tahir, M.B.I.N.M.; Yamada, N. Characteristics of Small ORC System for Low Temperature Waste Heat Recovery. J. Environ. Eng. 2009, 4, 375–385. [CrossRef] Energies 2019, 12, 829 22 of 22

45. Kim, D.K.; Lee, J.S.; Kim, J.; Kim, M.S.; Kim, M.S. Parametric study and performance evaluation of an organic Rankine cycle (ORC) system using low-grade heat at temperatures below 80 ◦C. Appl. Energy 2017, 189, 55–65. [CrossRef] 46. Yamada, N.; Minami, T.; Anuar Mohamad, M.N. Fundamental experiment of pumpless Rankine-type cycle for low-temperature heat recovery. Energy 2011, 36, 1010–1017. [CrossRef] 47. Akbarzadeh, A.; Johnson, P.; Nguyen, T.; Mochizuki, M.; Mashiko, M.; Sauciuc, I.; Kusaba, S.; Suzuki, H. Formulation and analysis of the heat pipe turbine for production of power from renewable sources. Appl. Therm. Eng. 2001, 21, 1551–1563. [CrossRef] 48. Yamada, N.; Watanabe, M.; Hoshi, A. Experiment on pumpless Rankine-type cycle with scroll expander. Energy 2013, 49, 137–145. [CrossRef] 49. Gao, P.; Wang, L.W.; Wang, R.Z.; Jiang, L.; Zhou, Z.S. Experimental investigation on a small pumpless ORC (organic rankine cycle) system driven by the low temperature heat source. Energy 2015, 91, 324–333. [CrossRef] 50. Jiang, L.; Lu, H.T.; Wang, L.W.; Gao, P.; Zhu, F.Q.; Wang, R.Z.; Roskilly, A.P. Investigation on a small-scale pumpless Organic Rankine Cycle (ORC) system driven by the low temperature heat source. Appl. Energy 2017, 195, 478–486. [CrossRef] 51. Aspen Plus, version 8.8; Aspen Technology, Inc.: Cambridge, UK, 2018; Available online: https://www. aspentech.com/ (accessed on 7 November 2018). 52. Freeman, J.; Hellgardt, K.; Markides, C.N. Towards the Optimization of a Domestic-Scale Organic Rankine System for Combined Heating and Power Provision in the UK; Department of Chemical Engineering; Imperial College: London, UK, 2013; p. 53. 53. Perers., B.; Bales, C. A Solar Collector Model for TRNSYS Simulation and System Testing. Reporet of IEA SHC—Task 26: Solar Combisystems; International Energy Agency, Solar Heating & Cooling Programme, 2002. Available online: https://www.aee-intec.at/0uploads/dateien55.pdf (accessed on 7 November 2018). 54. BS EN 12975-2: Thermal solar systems and components. Solar collectors. Test Methods 2006. 55. Ka¸ska,Ö. Energy and exergy analysis of an organic Rankine for power generation from waste heat recovery in steel industry. Energy Convers. Manag. 2014, 77, 108–117. [CrossRef] 56. Bekilo˘glu,H.E.; Bedir, H.; Anla¸s,G. Multi-objective optimization of ORC parameters and selection of working ﬂuid using preliminary radial inﬂow turbine design. Energy Convers. Manag. 2019, 183, 833–847. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).