Performance Evaluation of a Solar-Powered [618353]

energies
Article
Performance Evaluation of a Solar-Powered
Regenerative Organic Rankine Cycle
in Different Climate Conditions
Emily Spayde, Pedro J. Mago * and Heejin Cho
Department Mechanical Engineering, Mississippi State University, Starkville, MS 39762, USA;
[anonimizat] (E.S.); [anonimizat] (H.C.)
*Correspondence: [anonimizat]; Tel.: +1-662-325-3260
Academic Editor: Roberto Capata
Received: 10 October 2016; Accepted: 10 January 2017; Published: 13 January 2017
Abstract: A model to evaluate the performance of a solar powered regenerative Organic Rankine
Cycle (R-ORC) using five dry organic fluids: RC318, R227ea, R236ea, R236fa, and R218, is presented
in this paper. The system is evaluated in two locations in the U.S.: Jackson, MS and Tucson, AZ.
The weather data for each location is used to determine the heat available from the solar collector that
could be used by the R-ORC to generate power. Results from the R-ORC performance are compared
with a basic ORC using first and second law criteria as well as primary energy consumption (PEC)
and carbon dioxide emission (CDE) savings for both locations. An economic analysis to determine
the maximum capital cost for a desired payback period is presented in this paper. A parametric
analysis is also performed to study the effect of the turbine efficiency as well as the open feed organic
fluid heater intermediate pressure on the system performance. Results indicate that the R-ORC is
able to generate more power than the basic ORC for some of the selected working fluids. For the
R-ORC, R236ea is the working fluid that show the best performance among the evaluated fluids
under the modeled conditions. On the other hand, the basic ORC with R236ea as the working fluid
outperformed three of the fluids in the R-ORC. Also, the R-ORC evaluated in Tucson, AZ is able to
generate more power, to provide more PEC and CDE savings, and had a higher available capital cost
than the R-ORC evaluated in in Jackson, MS.
Keywords: solar ORC; regenerative ORC; Organic Rankine Cycle; primary energy consumption;
carbon dioxide emissions
1. Introduction
Organic Rankine Cycles (ORCs) have become an area of increased interest and study because
of their ability to use heat transfer from low to medium temperature sources to generate power.
Various sources of heat can be used to power ORCs such as waste heat, geothermal energy, and solar
energy [ 1–5]. ORCs typically generate small amounts of electricity, so they are ideal for small scale
applications. The working fluid greatly affects the performance of ORCs, therefore fluid selection and
performance has been widely studied by several researchers [ 6–11]. Rayegan and Tao [ 12] evaluated
34 fluids for several solar powered ORC configurations based on thermal efficiency, exergetic efficiency,
vapor expansion ratio and network output. Mago et al. [ 13] modeled a regenerative ORC using four
dry fluids and performed a first and second law analysis. They found that of the evaluated fluids,
R113 had the highest thermal efficiency and that the regenerative cycle increased the thermal efficiency
while decreasing the irreversibility.
Regenerative ORCs (R-ORC) have been studied for various applications as well [ 14–18].
Roy and Misra [ 19] evaluated a R-ORC using R134a and R123 as the working fluid for waste heat
recovery. They found that for a turbine inlet pressure of 2.5 MPa, R123 performed better than R134a.
Energies 2017 ,10, 94; doi:10.3390/en10010094 www.mdpi.com/journal/energies

Energies 2017 ,10, 94 2 of 20
Li et al. [ 20] studied an experimental R-ORC using geothermal heat as the heat source and R123 as
the working fluid. They found that the R-ORC had a higher efficiency than a basic ORC cycle for
the case they evaluated. In addition, they investigated the effect of mass flow rate on the system
performance and found out that as the mass flow rate increased the turbine inlet pressure and turbine
rotational speed increased while the regenerator efficiency decreased. Mago et al. [ 21] examined exergy
destruction in basic and R-ORCs using the network topological methodology. It was found that the
evaporator contributed the most to the exergy loss of both cycles, but the loss of exergy was reduced
in the regenerative cycle. In a study by Imran et al. [ 22], the authors performed a thermo-economic
analysis on a basic ORC, a single stage R-ORC, and a double stage R-ORC. They found that of the five
fluids they investigated R245fa under the modeled conditions had the lowest specific investment cost,
and the evaporator pressure had a significant impact on thermal efficiency and specific investment cost.
Primary energy consumption (PEC), carbon dioxide emissions (CDE), and economic analyses
have been performed on power systems to determine their energy, environmental and economic
viability. Fumo and Chamra [ 23] analyzed a combined cooling, heating, and power (CCHP) system to
determine what conditions are needed to generate PEC savings. In another study by Mago et al. [24],
combined heat and power (CHP) systems and CHP-ORC systems were evaluated for a small office
building in six different cities in different climate zones and were compared to purchasing electricity
from the grid while using a boiler to cover the thermal load of the facility. They reported that using
a CHP-ORC resulted in PEC savings, CDE savings, and cost savings when compared to operating a
CHP system alone. The savings from operating a CHP-ORC system when compared to the reference
case depended on the location of the office building. Lecompte et al. [ 25] used an optimization strategy
to minimize the specific investment cost of an ORC under fixed and part load conditions. They applied
this strategy to a case study of a retail company utilizing waste heat from a CHP system to power
the ORC. Calise et al. [ 26] studied off-design performance of an ORC. They modeled a solar powered
ORC and performed a thermoeconomic analysis by optimizing the design parameters of the heat
exchangers in the system. After the optimization was performed, the ORC performance under off
design conditions was investigated by changing the heat source mass flow rate and temperature.
They found that the heat source mass flow rate greatly affected the performance of the ORC. In a
study by Quoilin et al. [ 27], an economic optimization as well as a thermodynamic optimization
was performed for different working fluids for a waste heat recovery ORC. They found that the
operating conditions were different for the working fluids when they optimized based on economic
criteria versus thermodynamic criteria. Feng et al. [ 28] performed a thermodynamic and economic
analysis to compare an ORC to a R-ORC with R123 as the working fluid. A non-dominated sorting
genetic algorithm was used to find a set of optimum operating conditions when optimizing both
exergy efficiency and levelized energy cost by varying the evaporator outlet temperature, condenser
temperature, the degree of superheat, the degree of supercooling, and the pinch point temperature
difference. The authors found that in order to maximize the exergy efficiency the levelized energy cost
was at a maximum but in order to minimize the levelized energy cost then the exergy efficiency would
be reduced for both a basic and regenerative cycle. They concluded that the Pareto-optimal solution
was in between the maximum exergy efficiency operating conditions and the minimum levelized
energy cost. They also found that while the regenerative cycle had a higher exergetic efficiency, it
also had a higher levelized energy cost when compared to a basic ORC under a specific design and
operating condition.
Several authors have studied solar powered ORCs [ 29–31]. Rayegan and Tao [ 32] modeled a solar
powered R-ORC that provides electricity for a geothermal air conditioned net zero energy commercial
building located in Pensacola (Florida, USA) using TRNSYS software. They evaluated 11 fluids and
3 types of solar collectors to determine the optimum combination under the modeled conditions.
The authors found that the low temperature evacuated tube collectors using cyclohexane, isopentane,
benzene, and R245ca as the working fluid provided the lowest solar collector areas necessary.
Marion et al. [ 33] developed a theoretical model based on heat balance equations for a flat plate

Energies 2017 ,10, 94 3 of 20
solar collector in line with an ORC and validated their proposed model with experimental results.
They also performed a parametric study to optimize the system configuration and showed that
the optimized configuration can produce net mechanical power with an efficiency up to 11%.
In another study by Wang et al. [ 34] a flat plate solar R-ORC was analyzed using R123, R245fa,
R134a, and isobutane as working fluids. In addition, they used a thermal storage tank to prevent the
fluctuations in heat supplied to the ORC from the solar collector and used a regenerator to transfer
heat from the working fluid as it left the turbine to the working fluid before it enters the evaporator.
Pei et al. [ 35] modeled a solar powered R-ORC using a two stage compound parabolic concentrator
system with thermal storage using phase change materials. They used a two stage compound parabolic
concentrator and evaporator system to heat the ORC working fluid first to a saturated liquid in the
first stage and then vaporize the working fluid in the second stage. They found that the regenerator
increased the efficiency of the ORC and slightly increased the overall system efficiency. The authors
found the regenerator temperature had a significant effect on the solar collector efficiency as well as
the ORC, i.e., as the regenerator temperature increased, the ORC efficiency increased, but the solar
collector efficiency decreased.
In a previous study, Spayde and Mago [ 36] studied a basic solar powered ORC using five dry
organic fluids in Jackson (Mississippi) and Tucson (Arizona). In their study, hourly solar irradiation
values were obtained from ASHRAE by latitude for the 21st day of each month. They performed a
parametric study to investigate the effect of temperature, pressure, solar collector area, and turbine
efficiency on the overall system performance. The effect of producing electricity by the ORC versus
purchasing electricity from the grid on PEC and CDE was also investigated as well as the available
capital cost. They reported that R236ea performed the best of the evaluated fluids under the modeled
conditions and generated the most PEC and CDE savings and available capital cost. For all the
evaluated fluids, the system showed the best performance in the middle of the day. They also reported
that increasing the solar collector area increased the net energy produced and total exergy destroyed
whereas increasing solar collector pressure and condensing temperature decreased both the net energy
generated and total exergy destroyed.
The present study expands the previous study by the authors by investigating the performance of
a solar powered R-ORC using several dry working fluids, RC318, R227ea, R236ea, R236fa, and R218,
in two different locations in the U.S. The results are compared with a solar powered basic ORC to
establish the benefits of the proposed R-ORC. A thermo-economic analysis is performed to determine
the fluid that provides the best performance under the modeled conditions for the evaluated fluids.
In this study a flat plate solar collector is modeled as the evaporator of the R-ORC system. Because
the flat plate collector is in line with the ORC, the irradiation collected from the flat plate collector,
which is therefore available to the ORC, will vary with the local weather. The solar irradiation data
is calculated hourly from local typical meteorological year 3 (TMY3) weather data available from
National Renewable Energy Laboratory. A parametric analysis is also performed to study the effect of
the turbine efficiency as well as the open feed organic fluid heater intermediate pressure on the system
performance. In addition to the parametric analysis, the proposed system is evaluated in two locations
because weather has a direct effect on the performance of the proposed R-ORC. Jackson, MS and
Tucson, AZ were chosen because they have approximately the same latitude but are in different climate
zones. The performance of the proposed R-ORC is analyzed based on the PEC and CDE savings, and
the available capital cost for both locations.
2. System Model
This section describes the model used to evaluate the performance of the solar powered R-ORC.
Figure 1 shows a schematic of the evaluated R-ORC and the associated T-s diagram. As can be seen
in Figure 1, the system is a direct vapor generation (DVG), since the organic working fluid is directly
heated to the vapor state at the evaporator [37].

Energies 2017 ,10, 94 4 of 20
The organic fluid at State 3 is pumped to State 4 and enters the flat plate solar collector where heat
is transferred to the fluid. The organic working fluid at State 5 enters a two stage turbine, as a saturated
vapor, where power is generated. In the first stage of the turbine, the fluid is expanded to State 6,
where a fraction of the organic fluid is extracted into an open feed organic fluid heater operating at the
extraction pressure (Intermediate pressure). The remainder of the working fluid expands through the
second stage of the turbine to State 7 and then passes through a condenser (State 1). This portion of the
fluid is pumped to the intermediate pressure and introduced into the open feed organic fluid heater
at State 2. For this model, this fluid is then mixed and heated with the fraction of the fluid that exits
the first stage of the turbine so that the working fluid leaving the open feed organic fluid heater is a
saturated liquid at the intermedia pressure. After the two fluid streams combine, the organic working
fluid enters the second pump, at State 3, to increase the pressure before entering the solar collector at
State 4 and repeating the cycle. Assumptions for this model include: a constant irradiation rate for
each hour, a steady state system, no pressure losses, the intermediate pressure is the average of the
high and low pressures for each working fluid, and constant isentropic efficiencies for the pumps and
the turbine. In the model presented in this paper dry organic fluids are used since the saturated vapor
curve has a negative slope which has been proven to provide improved results verses fluids that have
a positive slope for the saturated vapor curve (wet fluids) [13].
Energies 2017 , 10, 94 4 of 19
operating at the extraction pre ssure (Intermediate pressure). Th e remainder of the working fluid
expands through the second stage of the turbine to State 7 and then passes through a condenser
(State 1). This portion of the fluid is pumped to the intermediate pressure and introduced into the
open feed organic fluid heater at State 2. For this model, this fluid is then mixed and heated with the
fraction of the fluid that exits the first stage of the turbine so that the working fluid leaving the open feed organic fluid heater is a satu rated liquid at the intermedia pres sure. After the two fluid streams
combine, the organic working fluid enters the second pump, at State 3, to increase the pressure before
entering the solar collector at State 4 and repeating the cycle. Assumptions for this model include: a constant irradiation rate for each hour, a steady state system, no pressure losses, the intermediate
pressure is the average of the high and low pressure s for each working fluid, and constant isentropic
efficiencies for the pumps and the turbine. In th e model presented in this paper dry organic fluids
are used since the saturated vapor curve has a ne gative slope which has be en proven to provide
improved results verses fluids that have a positive slope for the saturated vapor curve (wet fluids)
[13].

Figure 1. (a) Schematic of modeled solar powered R-ORC; ( b) T-s diagram for the solar powered R-
ORC.
a. Pump 1 (Process 1–2) : The pump power can be determined as:
ܹሶ௣ଵ=ܹሶ௣ଵ௦
ߟ௣ଵ=݉ሶହ ሺ1−)ܺሺℎଵ−ℎଶ௦)
ߟ௣ଵ=݉ሶହሺ1−)ܺሺℎଵ−ℎଶ) (1)
where ܹሶ௣ଵ௦ is the ideal power of Pump 1, ߟ௣ଵ is the isentropic efficiency of Pump 1, ݉ሶହ is the
mass flow rate of the wo rking fluid at State 5, ܺ is the fraction of the extracted fluid from the
first stage of the turbine, and ℎଵ, ℎଶ௦, and ℎଶ are the enthalpy values of the working fluid at
the pump inlet, the ideal enthalpy value at the pu mp exit, and the actual value at the pump exit,
respectively. The exergy destruction rate for Pump 1 is:
ߎ
௣ଵ=ܧሶ௣ଵ−൫ܧሶଶ−ܧሶଵ൯ (2)
where ܧሶ௣ଵ is the pump exergy rate and ܧሶଶ and ܧሶଵ are the exergy rates for State 2 and State 1,
respectively.

Figure 1. (a) Schematic of modeled solar powered R-ORC; ( b) T-s diagram for the solar powered R-ORC.
a. Pump 1 (Process 1–2) : The pump power can be determined as:
.
Wp1=.
Wp1s
hp1=.m5(1X)(h1h2s)
hp1=.m5(1X)(h1h2) (1)
where.
Wp1sis the ideal power of Pump 1, hp1is the isentropic efficiency of Pump 1,.m5is the
mass flow rate of the working fluid at State 5, Xis the fraction of the extracted fluid from the
first stage of the turbine, and h1,h2s, and h2are the enthalpy values of the working fluid at
the pump inlet, the ideal enthalpy value at the pump exit, and the actual value at the pump
exit, respectively.

Energies 2017 ,10, 94 5 of 20
The exergy destruction rate for Pump 1 is:
Pp1=.
Ep1(.
E2.
E1) (2)
where.
Ep1is the pump exergy rate and.
E2and.
E1are the exergy rates for State 2 and
State 1, respectively.
The exergy rate of Pump 1 is given by:
.
Ep1=.
Wp1 (3)
The exergy rate change from State 1 to State 2 can be estimated as:
.
E2.
E1=.m5(1X)(h2h1To(s2s1)) (4)
where To,s1, and s2are the temperature at the dead state (298 K), and entropy values of the
working fluid for States 1 and 2, respectively.
b. Pump 2 (Process 3–4): The power required for Pump 2 is given by
.
Wp2=.
Wp2s
hp2=.m5(h3h4s)
hp2=.m5(h3h4) (5)
where.
Wp2sis the ideal power of Pump 2, hp2is the isentropic efficiency of Pump 2, and h3and
h4are the enthalpy values of the working fluid at State 3 and 4, which are the inlet and outlet of
Pump 2, respectively.
The exergy destruction rate for Pump 2 can be determined as:
Pp2=.
Ep2(.
E4.
E3) (6)
where.
Ep2is the exergy rate of Pump 2 and.
E4and.
E3are the exergy rates for State 4 and
State 3, respectively.
The exergy rate of Pump 2 is given by:
.
Ep2=.
Wp2 (7)
The change in exergy rate between States 4 and 3 is:
.
E4.
E3=.m5(h4h3To(s4s3)) (8)
where s4and s3are the entropy values of the working fluid at States 4 and 3, respectively.
c. Solar Collector (Process 4–5): This is an isobaric process where heat is supplied to the organic
working fluid before the turbine inlet after the fluid exits the second pump. The flat plate solar
collector replaces the evaporator in a typical R-ORC. The solar collector heat transfer rate into the
working fluid follows:.
Qin=.m5(h5h4) (9)
where h5is the enthalpy of the working fluid at the exit of the solar collector.

Energies 2017 ,10, 94 6 of 20
The heat transfer rate from the solar collector can also be expressed as a function of irradiation:
.
Qin=hsolarIA (10)
where hsolar is the solar collector efficiency, Iis irradiation, and Ais the collector area.
The solar collector efficiency is determined using the relationship below:
hsolar=yintmTinTamb
I
(11)
where yintis the y-intercept and mis slope. These two terms are provided by the manufacturer
or a third party certification. In the proposed model, m= 4.910 W/m2
C and yint= 0.706 [ 38].
The equation for solar collector efficiency is the Hottel-Whiller-Bliss equation [ 39] where yintand
mcorrespond to:
yint=FRta (12)
m=FRUL (13)
where FRis the collector heat removal factor, tis the transmissivity of the glass cover plates, ais
the absorptivity of the absorber plate, and ULis the losses due to conduction and radiation.
The irradiation values can be determined using the following equation:
It=IDNcosq+IdH1+cosS
2
+ItHr1cosS
2
(14)
where Itis the total irradiation, IDNis the direct normal irradiation, qis the incidence angle,
IdHis the diffuse horizontal irradiation, Sis the surface tilt angle, ItHis the total horizontal
irradiation, and ris the ground reflectance. Direct normal irradiation, diffuse horizontal
irradiation, and total horizontal irradiation can be found in TMY3 data from the National
Renewable Energy Laboratory [ 40]. The value for ground reflectance used in the paper is
0.2 which was taken from literature [ 41]. The incidence angle and surface tilt angle are dependent
on the solar collector configuration (placement). In this study the solar collectors were modeled to
be 2 axis tracking solar collectors. This gives the maximum solar irradiation. A two axis tracking
system maintains the incidence angle at zero. The two axis tracking system leads to the following
surface tilt equation:
S=90b (15)
where bis the solar altitude which is given by [41]:
b=sin1(cosLcosdcosH+sinLsind) (16)
where Lis latitude, dis declination, and His the hour angle. Declination can be found by using
the following equation [42]:
d=23.45 sin
360284+n
365
(17)
where nis the day of the year.
The exergy destruction rate of the solar collector is:
Ps=.
E.
Qs(.
E5.
E4) (18)

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where.
E.
Qsis the exergy rate due to the heat input to the solar collector and.
E5is the exergy rate
of State 5.
The change in exergy rate across the solar collector is found by:
.
E5.
E4=.m5(h5h4To(s5s4)) (19)
where s5is the entropy of the working fluid at the collector exit.
The exergy rate of the solar collector can be estimated as:
.
E.
Qs=.
Qin
11
3(To/T)44
3(To/T)
(20)
where Tis the solar radiation temperature which is assumed to be 6000 K [43].
d. Turbine (Process 5–6, 7): The power of the two stage turbine is determined by:
.
Wt=ht.
Wts=ht.m5(h5h7s+X(h7sh6s))=.m5(h5h7+X(h7h6)) (21)
where.
Wtsis the power of the ideal turbine, htis the turbine isentropic efficiency, h6and h6sare
the enthalpies of the working fluid for the exit of the first stage of the turbine for the real and
ideal cases respectively, and h7and h7sare the enthalpies of the working fluid of the outlet of the
second stage of the turbine for the real and ideal cases, respectively.
The exergy destruction rate of the turbine is expressed as:
Pt=.
E5.
E6.
E7.
Et (22)
where.
Etis the exergy rate of the turbine and.
E6and.
E7are the exergy rates for State 6 and State
7, respectively.
The change in exergy rates from the inlet to the outlets of the turbine is:
.
E5.
E6.
E7=.m5[(h5h7)X(h6h7)To((s5s7)X(s6s7))] (23)
where s6and s7are the entropy values at State 6 and 7, respectively.
The turbine exergy rate is:.
Et=.
Wt (24)
e. Open feed organic fluid heater (Process 6, 2–3) : The extraction fraction is defined as:
X=h3h2
h6h2=.m6.m5(25)
where.m6is the mass flow rate of the working fluid at the exit of the first stage of the turbine.
The open feed organic fluid heater exergy destruction rate is determined by:
Pf=.
E6+.
E2.
E3 (26)
The exergy balance is:
.
E6+.
E2.
E3=.m5[(h2h3)X(h2h6)To((s2s3)X(s2s6))] (27)

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f. Condenser (Process 7–1): The heat transfer rate leaving the condenser follows:
.
Qc=(1X).m5(h1h7) (28)
The exergy destruction rate of the condenser is:
Pc=.
E7.
E1.
E.
Qc(29)
where.
E.
Qcis the exergy rate of the condenser.
The exergy balance across the condenser is given by:
.
E7.
E1=.m5(1X)(h7h1To(s7s1)) (30)
The exergy rate of the condenser is given by the following equation:
.
E.
Qc=.
Qc
1To
TL
(31)
where TLis the low temperature heat sink which is assumed to be 303 K.
g. R-ORC Net Power: The equation for the net power of the R-ORC is:
.
Wnet=.
Wt.
Wp1.
Wp2 (32)
h. R-ORC Efficiencies: The R-ORC thermal efficiency and R-ORC exergetic efficiency can be
expressed as:
hI=.
Wnet.
Qin=(h5h7+X(h7h6))(1X)(h2h1)(h4h3)
(h5h4)(33)
hII=.
E.
Wnet.
E.
Qin=(h5h7+X(h7h6))(1X)(h2h1)(h4h3)
(h5h4)
11
3
To
T4
4
3
To
T (34)
where.
E.
Wnetand.
E.
Qinare the exergy of the products and the exergy input to the ORC..
E.
Wnetcan
be estimated as:.
E.
Wnet=.
Wnet (35)
i. R-ORC Component Exergetic Efficiencies: The exergetic efficiency of each component of the R-ORC
can be found by defining exergetic efficiency as the used exergy divided by the available exergy
for each component. Equations (36) through (41) define each component’s exergetic efficiency:
hx,p1=.
E2.
E1.
Ep1(36)
hx,p2=.
E4.
E3.
Ep2(37)
hx,s=.
E5.
E4.
E.
Qin(38)

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hx,t=.
Et.
E5.
E6.
E7(39)
hx,f=.
E3.
E2+.
E6(40)
hx,c=.
E.
Qc.
E7.
E1(41)
The contribution of each component to the total exergy destruction rate of the R-ORC can be
estimated as:
%Pi=Pi
Ptotal(42)
where Pirepresents the exergy destruction for each component and Ptotalis the total exergy
destruction rate of the R-ORC system:
Ptotal=Pp1+Pp2+Ps+Pt+Pf+Pc (43)
j. Primary Energy Consumption (PEC) Savings: Generating electricity on site using solar energy
instead of purchasing electricity from the grid results in PEC savings and can be estimated by
subtracting the PEC from the R-ORC ( PEC RORC)from the PEC from the conventional system
(PEC Conv)as follows:
PEC savings =PEC ConvPEC RORC=.
WnetECF PEC.
WnetSCF PEC (44)
where PEC savings is the PEC savings, ECF PECis the site-to-source conversion factor for electricity
(grid purchase), which varies depending on location [ 23,44], and SCF PECis the site-to-source
conversion factor for electricity (solar), which was chosen to have a value of 1 [45,46].
k. Carbon Dioxide Emission (CDE) Savings: Since the R-ORC utilizes solar energy, there are no
CDE associated with the electricity generation. This results in CDE savings when compared to
purchasing electricity from the grid. The CDE savings can be found by subtracting the CDE from
the R-ORC ( CDE RORC=0)from the CDE from the conventional system ( CDE Conv)as follows:
CDE reduction =CDE convCDE RORC=.
WnetECF CDE (45)
where CDE reduction is the CDE savings and ECF CDE is the carbon dioxide emissions factor for
electricity which is location dependent [47].
l. Cost Savings and Available Capital Cost: Available yearly capital cost (ACC) is a means to determine
the maximum capital cost for a given payback period based on the cost of purchased electricity
from the grid. It is based on the purchased electricity savings that result from using the solar
powered R-ORC to generate electricity. The available capital cost can indicate the feasibility of
implementing a solar powered ORC for a specific location. The following equation determines
the cost savings and maximum capital cost available depending on the desired payback period:
Cost savings =.
WnetCost e (46)
ACC =å
PBPCost savings (47)
where Cost eis the yearly forecasted cost of electricity and PBP is the desired payback period.
The cost of electricity is forecasted by plotting the average retail price of residential electricity

Energies 2017 ,10, 94 10 of 20
from 2004 to 2015 [ 48] and performing a linear regression to estimate the future average
retail price.
3. Results and Discussion
The model described in Section 2 was employed to simulate an hourly solar powered R-ORC
in Jackson, MS and Tucson, AZ. The solar collector was modeled as a two axis tracking collector, so
that the maximum irradiation values would be used. The hourly irradiation and ambient temperature
data that was used to determine the solar collector efficiency and the heat transferred from the solar
collector was found from TMY3 data for both locations selected for this study. Jackson and Tucson
were chosen because they are at approximately the same latitude but are in different climate zones.
Figure 2 depicts the yearly irradiation data used for both Jackson, MS and Tucson, AZ calculated
from TMY3 weather data. Figure 2 illustrates that Tucson receives more irradiation than Jackson
throughout the year. Tucson also had in general higher ambient temperatures which affected the solar
collector efficiency.
Energies 2017 , 10, 94 9 of 19
from 2004 to 2015 [48] and performing a linear re gression to estimate the future average retail
price.
3. Results and Discussion
The model described in Section 2 was employed to simulate an hourly solar powered R-ORC in
Jackson, MS and Tucson, AZ. The solar collector was modeled as a two axis tracking collector, so that
the maximum irradiation values would be used. The hourly irradiation and ambient temperature
data that was used to determine the solar collector efficiency and the heat transferred from the solar
collector was found from TMY3 data for both locations selected for this study. Jackson and Tucson were chosen because they are at approximately the same latitude but are in different climate zones.
Figure 2 depicts the yearly irradiation data used for both Jackson, MS and Tucson, AZ calculated
from TMY3 weather data. Figure 2 illustrates that Tucson receives more irradiation than Jackson
throughout the year. Tucson also had in general higher ambient temperatures which affected the
solar collector efficiency.

Figure 2. Yearly irradiation data for Jackson, MS and Tucson, AZ calculated from TMY3 weather data [40].
Five dry organic working fluids were used in this paper: R236ea, R236fa, RC318, R227ea, and
R218. As mentioned before, in this study, the stat e of the working fluid was assumed to be fixed as a
saturated vapor as the fluid leaves the solar collect or; therefore, the maximum available energy from
the solar collector was harvested by changing the mass flow rate of the working fluid. Thus the
changing irradiation values and ambient temperatures affected the amount of power generated by the R-ORC. In addition, for each of the selected fl uids, the fluid enters both pumps were assumed to
be saturated liquid. For all fluids , the condensing temperature was assumed to be 30 °C, the solar
collector pressure was 2 MPa, and the pressure of the open feed organic fluid heater was set as the
average pressure between the solar collector and the condenser. The pump and turbine isentropic
efficiencies were assumed to be 80%. Since the mass flow rate of the organic working fluid changes
depending on the amount of irradiation from the solar collector, the efficiencies of the pump and
turbine will vary during the process, however, they were assumed constant to simplify the proposed
model. To account for this, the effect of turbine efficiency on the net energy produced and the total
exergy destroyed is investigated in a parametric study presented later in this paper. The pressure and
temperature ranges for each fl uid are given in Table 1 [49].

Figure 2. Yearly irradiation data for Jackson, MS and Tucson, AZ calculated from TMY3 weather
data [40].
Five dry organic working fluids were used in this paper: R236ea, R236fa, RC318, R227ea, and
R218. As mentioned before, in this study, the state of the working fluid was assumed to be fixed as
a saturated vapor as the fluid leaves the solar collector; therefore, the maximum available energy
from the solar collector was harvested by changing the mass flow rate of the working fluid. Thus the
changing irradiation values and ambient temperatures affected the amount of power generated by
the R-ORC. In addition, for each of the selected fluids, the fluid enters both pumps were assumed to
be saturated liquid. For all fluids, the condensing temperature was assumed to be 30C, the solar
collector pressure was 2 MPa, and the pressure of the open feed organic fluid heater was set as the
average pressure between the solar collector and the condenser. The pump and turbine isentropic
efficiencies were assumed to be 80%. Since the mass flow rate of the organic working fluid changes
depending on the amount of irradiation from the solar collector, the efficiencies of the pump and
turbine will vary during the process, however, they were assumed constant to simplify the proposed
model. To account for this, the effect of turbine efficiency on the net energy produced and the total
exergy destroyed is investigated in a parametric study presented later in this paper. The pressure and
temperature ranges for each fluid are given in Table 1 [49].

Energies 2017 ,10, 94 11 of 20
Table 1. Critical pressure [49] and pressure and temperature ranges for each of the evaluated fluids.
FluidCritical
Pressure
(MPa)Low
Pressure
(MPa)Low
Temperature
(C)Intermediate
Pressure
(MPa)High
Pressure
(MPa)High
Temperature
(C)
RC318 2.7775 0.36556 30 1.18278 2 98.75
R236fa 3.2 0.32101 30 1.160505 2 101.47
R236ea 3.502 0.24437 30 1.122185 2 111.65
R227ea 2.926 0.52866 30 1.26433 2 83.423
R218 2.671 0.99165 30 1.495825 2 59.094
3.1. System Performance
The R-ORC was compared with a basic ORC using the same irradiation data and locations for
each of the five evaluated fluids. Table 2 presents the thermal and exergetic efficiencies for each fluid
for both the R-ORC and basic ORC. For all of the evaluated fluids, the R-ORC had higher thermal
and exergetic efficiencies than the basic ORC when both systems operated using the same working
fluid. In Table 3, the net energy generated and total exergy destroyed are presented for the five
fluids in both the R-ORC and basic ORC. The R-ORC model was able to generate more net energy
and had less total exergy destruction than the modeled basic ORC for each of the evaluated fluids.
Among all the evaluated fluids, R236ea performed the best under the modeled conditions for both the
R-ORC and basic ORC with respect to the thermal and exergetic efficiencies, the net energy produced,
and the total exergy destroyed. To study the performance of the proposed solar R-ORC, the system
was evaluated using four collectors with an area of 3.696 m2each. Under the modeled conditions,
the R-ORC using R236ea as the working fluid produced 2852 kWh/year in Jackson, MS. The R-ORC
using R236fa as the working fluid had slightly higher efficiencies, higher net energy produced,
and lower total exergy destroyed than the basic ORC with R236ea as the working fluid. Whereas the
remaining fluids performed worse than R236ea in the basic ORC regardless of the chosen cycle under
the modeled conditions. RC318 had the largest percent increase, 13.27%, of net energy produced per
year, and R236ea had the largest percent decrease, 1.84%, of total exergy destruction per year when
comparing the R-ORC to the basic ORC.
Table 2. Thermal and exergetic efficiencies for each of the evaluated fluids in both the basic ORC
and R-ORC.
Thermal Efficiency Exergy Efficiency
Fluid Basic ORC R-ORC Basic ORC R-ORC
RC318 10.10 11.44 10.82 12.25
R236fa 11.16 12.50 11.95 13.39
R236ea 12.40 13.89 13.28 14.87
R227ea 8.81 9.86 9.44 10.56
R218 5.16 5.65 5.53 6.05
Table 3. The net energy produced and total exergy destroyed for each of the evaluated fluids for the
basic ORC and R-ORC.
Net Energy Produced (kWh/Year) Total Exergy Destroyed (kWh/Year)
Fluid Basic ORC R-ORC % Increase Basic ORC R-ORC % Decrease
RC318 2074 2349 13.27 16,796 16,525 1.61
R236fa 2291 2567 12.07 16,583 16,311 1.64
R236ea 2546 2852 12.01 16,331 16,031 1.84
R227ea 1809 2024 11.88 17,056 16,845 1.24
R218 1060 1160 9.42 17,794 17,695 0.55

Energies 2017 ,10, 94 12 of 20
Table 4 shows the percent contribution to the total exergy destruction rate for each of the
components in the R-ORC for each of the evaluated fluids. The solar collector had the highest
percentage contribution to the exergy destruction rate for all of the evaluated fluids. R236ea had
the lowest percentage contribution for the solar collector among the evaluated fluids with a value of
91.76%. The turbine had the next highest percent contribution to the total exergy destruction rate with
a range of 4.22% to 1.95% for the evaluated fluids. R236ea had the highest percent contribution for the
turbine at 4.22%. The open feed organic fluid heater was the third largest contributor to the total exergy
destruction rate with a percentage range of 3.12% to 0.67% for the evaluated fluids. Again R236ea had
the highest percent contribution for the open feed organic fluid heater among the evaluated fluids.
Since R236ea performed the best of the evaluated fluids under the modeled conditions, it is used
as the organic working fluid for the remainder results presented in this paper. Figures 3 and 4 compare
the basic ORC versus the R-ORC over the course of a year in Jackson, MS. The R-ORC produced more
net energy and slightly less total exergy destruction per month than the basic ORC. The R236ea R-ORC
produced 2852 kWh/year of net energy and destroyed 16,301 kWh/year of exergy while the R236ea
basic ORC generated 2546 kWh/year of net energy and destroyed 16,331 kWh/year of exergy resulting
in a 12.01% increase of net energy production and a 1.84% reduction of total exergy destruction.
Furthermore, the R-ORC was evaluated in Tucson, AZ and the results compared with those
presented in Figures 3 and 4 for Jackson, MS. In Figure 5, the net energy produced and total exergy
destroyed are compared for Jackson and Tucson. Since there was more solar irradiation in Tucson,
the net energy produced and total exergy destruction was higher in Tucson when compared to Jackson.
The increase in both net energy production and total exergy destruction was more pronounced in the
summer months. The net energy generated per year for Tucson, AZ was 4162 kWh/year which was
46% increase from the net energy produced in Jackson, MS.
Table 4. Percentage of contribution for each device to the total exergy destruction rate for the R-ORC.
Fluid PPump 1 (%) PPump 2 (%) PSolar (%) PT urbine (%) PCondenser (%)POpen Feed
Organic Fluid
Heater (%)
RC318 0.13 0.22 92.54 3.56 0.86 2.69
R236fa 0.10 0.16 92.85 3.89 0.37 2.63
R236ea 0.09 0.15 91.76 4.22 0.66 3.12
R227ea 0.11 0.16 94.67 3.11 0.29 1.67
R218 0.12 0.22 96.87 1.95 0.16 0.67
Energies 2017 , 10, 94 11 of 19
Table 4 shows the percent contribution to the total exergy destruction rate for each of the
components in the R-ORC for each of the evalua ted fluids. The solar collector had the highest
percentage contribution to the exergy destruction ra te for all of the evaluate d fluids. R236ea had the
lowest percentage contribution for the solar coll ector among the evaluated fluids with a value of
91.76%. The turbine had the next highest percent contri bution to the total exergy destruction rate with
a range of 4.22% to 1.95% for the evaluated fluids. R236ea had the highest percent contribution for
the turbine at 4.22%. The open feed organic fluid he ater was the third largest contributor to the total
exergy destruction rate with a percentage range of 3.12% to 0.67% for the evaluated fluids. Again
R236ea had the highest percent contribution fo r the open feed organic fluid heater among the
evaluated fluids.
Since R236ea performed the best of the evaluated fluids under the modeled conditions, it is used
as the organic working fluid for the remainder results presented in this paper. Figures 3 and 4 compare the basic ORC versus the R-ORC over the course of a year in Jackson, MS. The R-ORC
produced more net energy and slightly less total exergy destruction per month than the basic ORC.
The R236ea R-ORC produced 2852 kWh/year of net en ergy and destroyed 16,301 kWh/year of exergy
while the R236ea basic ORC generated 2546 kWh/year of net energy and destroyed 16,331 kWh/year
of exergy resulting in a 12.01% increase of net en ergy production and a 1.84% reduction of total exergy
destruction.
Furthermore, the R-ORC was evaluated in Tucs on, AZ and the results compared with those
presented in Figures 3 and 4 for Jackson, MS. In Figure 5, the net energy produced and total exergy destroyed are compared for Jackson and Tucson. Sinc e there was more solar irradiation in Tucson,
the net energy produced and total exergy destruction was higher in Tucson when compared to Jackson. The increase in both net energy prod uction and total exergy destruction was more
pronounced in the summer months. The net ener gy generated per year for Tucson, AZ was 4162
kWh/year which was 46% increase from th e net energy produced in Jackson, MS.
Table 4. Percentage of contribution for each device to the total exergy destruction rate for the R-ORC.
Fluid Π Pump 1
(%) Π Pump 2
(%) Π Solar
(%) Π Turbine
(%) Π Condenser
(%) Π Open Feed Organic
Fluid Heater (%)
RC318 0.13 0.22 92.54 3.56 0.86 2.69
R236fa 0.10 0.16 92.85 3.89 0.37 2.63
R236ea 0.09 0.15 91.76 4.22 0.66 3.12
R227ea 0.11 0.16 94.67 3.11 0.29 1.67
R218 0.12 0.22 96.87 1.95 0.16 0.67

Figure 3. Net energy produced per month in Jackson, MS for the basic ORC and R-ORC configurations
using R236ea as the working fluid.
Figure 3. Net energy produced per month in Jackson, MS for the basic ORC and R-ORC configurations
using R236ea as the working fluid.

Energies 2017 ,10, 94 13 of 20
Energies 2017 , 10, 94 12 of 19

Figure 4. Total exergy destroyed per month in Jackso n, MS for the basic ORC and R-ORC using
R236ea as the working fluid.

Figure 5. Net energy produced and total exergy destroyed in Jackson, MS and Tucson, AZ by the R-
ORC using R236ea as the working fluid.
3.2. Primary Energy Consumption and Carbon Dioxide Emissions
The effect of replacing electricity purchased from the grid with electricity generated using solar
power on the PEC and CDE was also investigated. Producing electricity from solar power has the potential for PEC and CDE savings as compared to purchasing electricity form the utility grid. Table 5 presents the site-to-source conversion factor for electricity (grid purchase) for Jackson, MS and
Tucson, AZ, the site-to-source co nversion factor for electricity (solar), and the carbon dioxide
emissions factor for electricity for both locations.

Figure 4. Total exergy destroyed per month in Jackson, MS for the basic ORC and R-ORC using R236ea
as the working fluid.
Energies 2017 , 10, 94 12 of 19

Figure 4. Total exergy destroyed per month in Jackso n, MS for the basic ORC and R-ORC using
R236ea as the working fluid.

Figure 5. Net energy produced and total exergy destroyed in Jackson, MS and Tucson, AZ by the R-
ORC using R236ea as the working fluid.
3.2. Primary Energy Consumption and Carbon Dioxide Emissions
The effect of replacing electricity purchased from the grid with electricity generated using solar
power on the PEC and CDE was also investigated. Producing electricity from solar power has the potential for PEC and CDE savings as compared to purchasing electricity form the utility grid. Table 5 presents the site-to-source conversion factor for electricity (grid purchase) for Jackson, MS and
Tucson, AZ, the site-to-source co nversion factor for electricity (solar), and the carbon dioxide
emissions factor for electricity for both locations.

Figure 5. Net energy produced and total exergy destroyed in Jackson, MS and Tucson, AZ by the
R-ORC using R236ea as the working fluid.
3.2. Primary Energy Consumption and Carbon Dioxide Emissions
The effect of replacing electricity purchased from the grid with electricity generated using solar
power on the PEC and CDE was also investigated. Producing electricity from solar power has the
potential for PEC and CDE savings as compared to purchasing electricity form the utility grid. Table 5
presents the site-to-source conversion factor for electricity (grid purchase) for Jackson, MS and Tucson,
AZ, the site-to-source conversion factor for electricity (solar), and the carbon dioxide emissions factor
for electricity for both locations.

Energies 2017 ,10, 94 14 of 20
Table 5. Site-to-source conversion factor for electricity and carbon dioxide emissions factor
for electricity.
ECF PEC(Jackson, MS) [44] 3.14 kWh/kWh
ECF PEC(Tucson, AZ) [44] 3.06 kWh/kWh
SCF PEC[45] 1 kWh/kWh
ECF CDE (Jackson, MS) [47] 0.467 kg/kWh
ECF CDE (Tucson, AZ) [47] 0.534 kg/kWh
Figure 6 presents the PEC savings for both the R-ORC and basic ORC in Jackson, MS and Tucson,
AZ. The PEC savings were higher in Tucson, AZ regardless of which cycle is used than Jackson,
MS even though the ECF PECwas slightly higher in Jackson. The higher savings in Tucson can be
contributed from the higher amount of net energy produced by the ORC due to the increased solar
irradiation available and the decreased amount of electricity purchased from the grid. The R-ORC
produced more PEC savings than the basic ORC for each location because there was an increase in
electricity produced in the R-ORC compared to the basic ORC. The yearly PEC savings for the R-ORC
in Jackson and Tucson were 6104 kWh/year and 8574 kWh/year, respectively.
Implementing a solar powered R-ORC has also the potential of reducing the CDE since the
R-ORC emits practically zero CDE. Figure 7 presents the CDE savings for the basic ORC and R-ORC
in Jackson and Tucson. As with the PEC savings, the possible CDE savings were higher in Tucson,
AZ versus Jackson, MS. The R-ORC also produced more potential CDE savings than the basic ORC.
The possible yearly CDE savings for the R-ORC in Jackson and Tucson were 1332 kg/year and 2224
kg/year, respectively. The reason that more CDE savings are obtained in Tucson, AZ is that the carbon
dioxide emission factor for electricity are higher in this location and emissions which indicates that
higher carbon content is present in the flue mix used to generate electricity in that area.
Energies 2017 , 10, 94 13 of 19
Table 5. Site-to-source conversion factor for electricity and carbon dioxide emissions factor for
electricity.
ECF PEC (Jackson, MS) [44] 3.14 kWh/kWh
ECF PEC (Tucson, AZ) [44] 3.06 kWh/kWh
SCF PEC [45] 1 kWh/kWh
ECF CDE (Jackson, MS) [47] 0.467 kg/kWh
ECF CDE (Tucson, AZ) [47] 0.534 kg/kWh
Figure 6 presents the PEC savings for both the R-ORC and basic ORC in Jackson, MS and Tucson,
AZ. The PEC savings were higher in Tucson, AZ regardless of which cycle is used than Jackson, MS even though the ECF
PEC was slightly higher in Jackson. Th e higher savings in Tucson can be
contributed from the higher amount of net ener gy produced by the ORC due to the increased solar
irradiation available and the decre ased amount of electricity purchased from the grid. The R-ORC
produced more PEC savings than the basic ORC for each location because there was an increase in
electricity produced in the R-ORC compared to the basic ORC. The yearly PEC savings for the R-ORC in Jackson and Tucson were 6104 kWh/ year and 8574 kWh/year, respectively.
Implementing a solar powered R- ORC has also the potential of reducing the CDE since the R-
ORC emits practically zero CDE. Figure 7 presents the CDE savings for the basic ORC and R-ORC in
Jackson and Tucson. As with the PEC savings, the possible CDE savings were higher in Tucson, AZ versus Jackson, MS. The R-ORC al so produced more potential CDE savings than the basic ORC. The
possible yearly CDE savings for the R-ORC in Jackson and Tucson were 1332 kg/year and 2224 kg/year, respectively. The reason that more CDE saving s are obtained in Tucson, AZ is that the carbon
dioxide emission factor for electricity are higher in this location and emissions which indicates that
higher carbon content is present in the flue mi x used to generate electricity in that area.

Figure 6. PEC savings from the basic ORC and R-ORC in Jackson, MS and Tucson, AZ using R236ea
as the working fluid.
Figure 6. PEC savings from the basic ORC and R-ORC in Jackson, MS and Tucson, AZ using R236ea as
the working fluid.

Energies 2017 ,10, 94 15 of 20
Energies 2017 , 10, 94 14 of 19

Figure 7. CDE savings from the basic ORC and R-ORC in Jackson, MS and Tucson, AZ using R236ea
as the working fluid.
3.3. Available Capital Cost
Available Capital Cost (ACC) determines the maxi mum capital cost for a given payback period
based on the cost saving potential from grid elec tricity price. The ACC for the proposed system is
shown in Figure 8 for a payback period ranging from 1 to 10 years. The ACC values were determined
based on the current and forecasted national av erage purchased electricity cost until 2025. The
predicted national average for 2016 is 0.13 $/kW h and for 2025 is 0.16 $/kWh. Applying these
predicted electricity costs resulted in ACCs of $4198 for Jackson, MS and $6127 for Tucson, AZ given
a 10 year payback period. Tucson had a higher ACC because the modeled R-ORC produced more
electricity in Tucson than in Jackson. This indicates that for the R-ORC to have a payback period of less than 10 years in Tucson the capital cost must be less than $6127 based on an increasing national
average electricity purchase price.

Figure 8. Available capital cost for Jackson, MS and Tucson, AZ for a payback period of up to ten
years using a forecasted national average electr icity price using R236ea as the working fluid.
Figure 7. CDE savings from the basic ORC and R-ORC in Jackson, MS and Tucson, AZ using R236ea as
the working fluid.
3.3. Available Capital Cost
Available Capital Cost (ACC) determines the maximum capital cost for a given payback period
based on the cost saving potential from grid electricity price. The ACC for the proposed system
is shown in Figure 8 for a payback period ranging from 1 to 10 years. The ACC values were
determined based on the current and forecasted national average purchased electricity cost until 2025.
The predicted national average for 2016 is 0.13 $/kWh and for 2025 is 0.16 $/kWh. Applying these
predicted electricity costs resulted in ACCs of $4198 for Jackson, MS and $6127 for Tucson, AZ given
a 10 year payback period. Tucson had a higher ACC because the modeled R-ORC produced more
electricity in Tucson than in Jackson. This indicates that for the R-ORC to have a payback period of
less than 10 years in Tucson the capital cost must be less than $6127 based on an increasing national
average electricity purchase price.
Energies 2017 , 10, 94 14 of 19

Figure 7. CDE savings from the basic ORC and R-ORC in Jackson, MS and Tucson, AZ using R236ea
as the working fluid.
3.3. Available Capital Cost
Available Capital Cost (ACC) determines the maxi mum capital cost for a given payback period
based on the cost saving potential from grid elec tricity price. The ACC for the proposed system is
shown in Figure 8 for a payback period ranging from 1 to 10 years. The ACC values were determined
based on the current and forecasted national av erage purchased electricity cost until 2025. The
predicted national average for 2016 is 0.13 $/kW h and for 2025 is 0.16 $/kWh. Applying these
predicted electricity costs resulted in ACCs of $4198 for Jackson, MS and $6127 for Tucson, AZ given
a 10 year payback period. Tucson had a higher ACC because the modeled R-ORC produced more
electricity in Tucson than in Jackson. This indicates that for the R-ORC to have a payback period of less than 10 years in Tucson the capital cost must be less than $6127 based on an increasing national
average electricity purchase price.

Figure 8. Available capital cost for Jackson, MS and Tucson, AZ for a payback period of up to ten
years using a forecasted national average electr icity price using R236ea as the working fluid.
Figure 8. Available capital cost for Jackson, MS and Tucson, AZ for a payback period of up to ten years
using a forecasted national average electricity price using R236ea as the working fluid.

Energies 2017 ,10, 94 16 of 20
3.4. Parametric Analysis
The effect of the intermediate pressure (extraction pressure) and the turbine efficiency on the net
energy produced, the total exergy destroyed, and the system mass flow rate was investigated. Figure 9
shows the effect of the intermediate pressure on the net energy produced, total exergy destroyed,
and the average mass flow rate for the months of January and July when the intermediate pressure
varies from 0.3 MPa to 1.9 MPa. The net energy produced, total exergy destroyed, and average mass
flow rate were higher in the month of July compared to those in January. The average mass flow
rates for both January and July increased as the intermediate pressure increased. The maximum net
energy produced and the minimum exergy destroyed occurred at the same intermediate pressure
value of 0.75 MPa. Changing the turbine efficiency only affected the net energy produced and the total
exergy destroyed. The effect of the turbine efficiency on the net energy produced and the total exergy
destroyed for the months of January and July is displayed in Figure 10. The efficiency of the turbine
was varied from 50% to 80%. As the turbine efficiency increased, the net energy increased and the total
exergy destroyed decreased slightly. The percent increase of net energy produced did not change for
the months of January and July. Likewise, the percent decrease of the total exergy destroyed did not
change for January and July. As the turbine efficiency increased from 70% to 80%, the percent increase
of net energy produced was 15%, and the percent decrease of exergy destruction was 2.2%.
Energies 2017 , 10, 94 15 of 19
3.4. Parametric Analysis
The effect of the intermediate pressure (extractio n pressure) and the turbine efficiency on the net
energy produced, the total exergy destroyed, and the sy stem mass flow rate was investigated. Figure
9 shows the effect of the intermediate pressure on the net energy produced, total exergy destroyed,
and the average mass flow rate for the months of January and July when the intermediate pressure
varies from 0.3 MPa to 1.9 MPa. The net energy produced, total exergy destroyed, and average mass
flow rate were higher in the month of July compared to those in January. The average mass flow rates for both January and July increased as the intermed iate pressure increased. The maximum net energy
produced and the minimum exergy destroyed occurred at the same intermediate pressure value of 0.75 MPa. Changing the turbine efficiency only a ffected the net energy produced and the total exergy
destroyed. The effect of the turbine efficiency on the net energy produced and the total exergy
destroyed for the months of January and July is disp layed in Figure 10. The efficiency of the turbine
was varied from 50% to 80%. As the turbine effici ency increased, the net energy increased and the
total exergy destroyed decreased slightly. The percent increase of net energy produced did not change for the months of January and July. Likewise, the percent decrease of the total exergy destroyed did not change for January and July. As the turbine efficiency increased from 70% to 80%,
the percent increase of net energy produced was 15%, and the percent decrease of exergy destruction
was 2.2%.

Figure 9. The effect of intermediate pressure on the net energy produced, total exergy destroyed, and
average mass flow rate for Janu ary and July in Jackson, MS.
Figure 9. The effect of intermediate pressure on the net energy produced, total exergy destroyed, and
average mass flow rate for January and July in Jackson, MS.

Energies 2017 ,10, 94 17 of 20
Energies 2017 , 10, 94 16 of 19

Figure 10. The effect of turbine efficiency on net energy produced and total exergy destroyed for
January and July in Jackson, MS.
4. Conclusions
A solar powered R-ORC was investigated in this study using five dry organic fluids. The solar
panels for the ORC were modeled as two axis trac king panels, and the system performance of the R-
ORC and basic ORC was evaluated for Jackson, MS and Tucson, AZ. The R-ORC was compared to the basic ORC in terms thermal and exegetic efficiencies as well as PEC and CDE savings for both
locations.
RC318, R227ea, R236ea, R236fa, and R218 were the fl uids selected in this st udy. Of the evaluated
fluids R236ea performed the best under the model ed conditions. R236ea produced the most net
energy and destroyed the least amount of total ex ergy. R236ea also had the highest thermal and
exergetic efficiencies. The basic ORC and the R-OR C were compared for each of the five fluids. For
all of the fluids the R-ORC had higher efficiencie s, produced more energy, and destroyed less total
exergy than the basic ORC using the same fluid. The fluid that saw the largest percent increase of net
energy produced was RC318 with a 13.3% increase from the basic ORC net energy output. The fluid
that had the largest percent decrease of the exergy destruction rate was R2 36ea with a 1.84% decrease
when compared with the basic ORC.
The R-ORC was modeled in Jackson, MS and Tucson , AZ to see the effect of climates on the
ORC. From local weather data it was determined that Tucson received higher irradiation than
Jackson. This resulted in a higher production of net energy and total exergy destroyed. Because there
was more net energy produced in Tucson than Jackson, there was a larger potential for PEC and CDE
savings in Tucson, AZ. The potential for PEC and CDE savings was higher for the R-ORC than the
basic ORC. The ACC based on increasing average electricity costs and a payback period was also
investigated for the two cities. Tucson had higher AC Cs than Jackson since the electricity production
from the solar powered ORC was higher in Tucson.
A parametric analysis of the R-ORC was performe d to determine the effect of the intermediate
pressure and the turbine efficiency. As the intermedia te pressure increased, the mass flow rate of the
R-ORC increased. A maximum amount of energy production and minimum amount of exergy
destruction occurred when the intermediate pressu re was 0.75 MPa. As the turbine efficiency
increased, the net energy produced increased while the total exergy destroyed decreased.

Figure 10. The effect of turbine efficiency on net energy produced and total exergy destroyed for
January and July in Jackson, MS.
4. Conclusions
A solar powered R-ORC was investigated in this study using five dry organic fluids. The solar
panels for the ORC were modeled as two axis tracking panels, and the system performance of the
R-ORC and basic ORC was evaluated for Jackson, MS and Tucson, AZ. The R-ORC was compared
to the basic ORC in terms thermal and exegetic efficiencies as well as PEC and CDE savings for
both locations.
RC318, R227ea, R236ea, R236fa, and R218 were the fluids selected in this study. Of the evaluated
fluids R236ea performed the best under the modeled conditions. R236ea produced the most net energy
and destroyed the least amount of total exergy. R236ea also had the highest thermal and exergetic
efficiencies. The basic ORC and the R-ORC were compared for each of the five fluids. For all of the
fluids the R-ORC had higher efficiencies, produced more energy, and destroyed less total exergy than
the basic ORC using the same fluid. The fluid that saw the largest percent increase of net energy
produced was RC318 with a 13.3% increase from the basic ORC net energy output. The fluid that had
the largest percent decrease of the exergy destruction rate was R236ea with a 1.84% decrease when
compared with the basic ORC.
The R-ORC was modeled in Jackson, MS and Tucson, AZ to see the effect of climates on the
ORC. From local weather data it was determined that Tucson received higher irradiation than Jackson.
This resulted in a higher production of net energy and total exergy destroyed. Because there was more
net energy produced in Tucson than Jackson, there was a larger potential for PEC and CDE savings in
Tucson, AZ. The potential for PEC and CDE savings was higher for the R-ORC than the basic ORC.
The ACC based on increasing average electricity costs and a payback period was also investigated for
the two cities. Tucson had higher ACCs than Jackson since the electricity production from the solar
powered ORC was higher in Tucson.
A parametric analysis of the R-ORC was performed to determine the effect of the intermediate
pressure and the turbine efficiency. As the intermediate pressure increased, the mass flow rate of
the R-ORC increased. A maximum amount of energy production and minimum amount of exergy
destruction occurred when the intermediate pressure was 0.75 MPa. As the turbine efficiency increased,
the net energy produced increased while the total exergy destroyed decreased.

Energies 2017 ,10, 94 18 of 20
Author Contributions: All of the authors have contributed toward developing and implementing the ideas and
concepts presented in the paper. All of the authors have collaborated to obtain the results and have been involved
in preparing the manuscript.
Conflicts of Interest: The authors declare no conflict of interest.
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