entropy Article Design and Analysis of the Domestic Micro-Cogeneration Potential for an ORC System Adapted to a Solar Domestic Hot Water System Daniel Leal-Chavez 1, Ricardo Beltran-Chacon 1,* , Paola Cardenas-Terrazas 1, Saúl Islas 2 and Nicolás Velázquez 2 1 Centro de Investigación en Materiales Avanzados, S.C., CIMAV, Chihuahua 31136, Mexico; [email protected] (D.L.-C.); [email protected] (P.C.-T.) 2 Instituto de ingeniería, Universidad Autónoma de Baja California, Mexicali 21100, Mexico; [email protected] (S.I.); [email protected] (N.V.) * Correspondence: [email protected]; Tel.: +52-614-439-1155 Received: 30 July 2019; Accepted: 11 September 2019; Published: 19 September 2019 Abstract: This paper proposes the configuration of an Organic Rankine Cycle (ORC) coupled to a solar domestic hot water system (SDHWS) with the purpose of analyzing the cogeneration capacity of the system. A simulation of the SDHWS was conducted at different temperatures, observing its performance to determine the amounts of useable heat generated by the solar collector; thus, from an energy balance point of view, the amount of heat that may be used by the ORC could be determined. The working fluid that would be suitable for the temperatures and pressures in the system was selected. The best fluid for the given conditions of superheated vapor at 120 ◦C and 604 kPa and a condensation temperature of 60 ◦C and 115 kPa was acetone. The main parameters for the expander thermodynamic design that may be used by the ORC were obtained, with the possibility of generating 443 kWh of annual electric energy with 6.65% global efficiency of solar to electric power, or an overall efficiency of the cogeneration system of 56.35% with a solar collector of 2.84 m2. Keywords: solar domestic cogeneration; Organic Rankine Cycle; acetone; evacuated tube solar collector 1. Introduction The growing demand for energy and the current dependence on fossil fuels require more efficient processes and technologies to be developed, to allow a transition into a system with sustainable energy sources. In general terms, the objective is that buildings have zero net energy consumption through an efficient use of energy, and the adoption of renewable sources of energy and other technologies [1]. Photovoltaic, solar, thermal, and biomass technologies can contribute to reducing consumption for heating of spaces in the Mediterranean region [2], in order to reduce high energy costs, create energy security, and mitigate local air pollution [3–5]. Solar energy systems allow consumption to be reduced to almost zero net energy in buildings. For combined heat and power (CHP) systems, return periods are between 5.5–6.5 years, while for photovoltaic systems it takes 7 years [6]. Organic Rankine Cycle technology has intensified in recent years, and is being used as the main alternative to convert energy at low temperatures [7]. Solar water heaters generally capture a greater amount of solar energy during seasons with low hot water consumption, such as summer. In these circumstances, a lot of solar energy is wasted. Extending the utilization of energy captured by solar collectors through a second application would favor a technological alternative for electric production through renewable energies and a reduction of CO2 emissions associated with conventional fossil fuel-based generation techniques. Entropy 2019, 21, 911; doi:10.3390/e21090911 www.mdpi.com/journal/entropy Entropy 2019, 21, x FOR PEER REVIEW 2 of 23 EntropyExtending2019, 21, 911 the utilization of energy captured by solar collectors through a second application2 of 23 would favor a technological alternative for electric production through renewable energies and a reduction of CO2 emissions associated with conventional fossil fuel-based generation techniques. An option to improve the utilization factor of solar systems is by integratingintegrating ORC systems to generate electricity in standard domestic use. Th Thee ORC is a proven technique to convert unused low temperature heat into useable mechanicalmechanical or electric energy [[8],8], and is characterized by its simplicity and flexibilityflexibility at a relatively lowlow operatingoperating temperaturetemperature [[9].9]. This techniquetechnique has has been been widely widely used used in recent in recent years withyears di ffwitherent different heat sources, heat such sources, as geothermal, such as heatgeothermal, recovery, heat biomass, recovery, and solar biomass, systems. and There sola arer manysystems. applications There andare configurationsmany applications at diff erentand temperaturesconfigurations and at capacitiesdifferent temperat [10]. In recentures and years, capacities there have [10]. been In recent many studiesyears, there on ORC, have ranging been many from optimization,studies on ORC, through ranging component from optimization, and working thro fluidugh selection, component to techno-economic and working fluid and experimentalselection, to studiestechno-economic [11]. and experimental studies [11]. Currently, therethere areare aa numbernumber ofof domestic-sized solarsolar ORC systems [[12–16];12–16]; their main operation characteristics are shown in Table1 1.. However,However, nonenone ofof themthem hashas aa configurationconfiguration usingusing directdirect vaporvapor generation with evacuatedevacuated tubetube collectors.collectors. These These configurations configurations generally generally use use an indirect heat exchanger between thethe solar heating systemsystem andand thethe ORCORC systemsystem (see(see FigureFigure1 1).). TheThe aimaim isis toto useuse thethe solar systemsystem only only for for pre-heating pre-heating the workingthe working fluid, andfluid, to laterand evaporateto later evaporate the fluid with the afluid conventional with a heatconventional source under heat controlledsource under conditions. controlled conditions. Figure 1. (a) Proposed configuration, (b) typical configuration. Figure 1. (a) Proposed configuration, (b) typical configuration. Table 1. Performance of different Organic Rankine Cycles (ORCs). Table 1. Performance of different Organic Rankine Cycles (ORCs). Author Output Power Isentropic Efficiency (%) Global Efficiency (%) Output Isentropic Global Helvaci and Khan 2016Author [12] 135.96 W 58.66 3.81 Taccani et al. 2016 [13] 670 Wepower Efficiency 62 (%) Efficiency 8(%) VittoriniHelvaci et al. 2018 and [ 14Khan] 2016 [12] 700 W135.96 W Not shown58.66 3.81 3 * J. FreemanTaccani et al. 2017 et [al.15] 2016 [13] ~1 kWe670 We 75 *62 8 7.3 Cioccolanti, et al. 2017 [16] 2 kWe 60 * 4.98 Vittorini et al. 2018 [14] 700 W Not shown 3 * J. Freeman et al. 2017 [15] * assumed~1 kWe by the author. 75* 7.3 Cioccolanti, et.al 2017 [16] 2 kWe 60* 4.98 There are different studies in the field of optimization of residential solar ORC configurations. * assumed by the author. A system with flat plate collectors with a heat source of 3.542 kW [12] uses a heat exchanger for condensationThere are and different a subsystem studies for in waste the field heating of optimi to heatzation the water of residential in a hot water solar tank.ORC Aconfigurations. configuration ofA 100system m2 of with a parabolic flat plate channel collectors with ORCwith [a13 heat] uses so aurce heat of exchanger 3.542 kW to [12] transfer uses heat a heat from exchanger the parabolic for channel,condensation and operates and a subsystem as an evaporator for waste for heating the ORC. to heat A configuration the water in a of hot an water evaporator tank. SDHWSA configuration used as anof 100 intermediate m2 of a parabolic heat exchanger channel to with heat theORC working [13] uses fluid a heat of the exchanger ORC system to transfer [14] additionally heat from uses the aparabolic system of channel, indirect heatingand operates for the as DHWS an evaporator and three flatfor platethe ORC. solar thermalA configuration collectors. of A an configuration evaporator withSDHWS a solar used area as ofan 15 intermediate m2 and thermal heat energy exchanger storage to heat [15] maintainsthe working and fluid uses of energy the ORC to mitigate system solar [14] energyadditionally variations uses ina thesystem ORC. of A solarindirect system heatin withg for a linear the DHWS Fresnel collector,and three with flat aplate total surfacesolar thermal area of 148collectors. m2, thermal A configuration storage with with phase a solar change, area and of 15 an m ORC2 and [16 thermal], uses the energy tank indirectlystorage [15] for maintains phase change and asuses the energy evaporator to mitigate from the solar ORC. energy variations in the ORC. A solar system with a linear Fresnel The proposed system presents a fundamental difference to other applications reported in the literature, in that the heat transfer is carried out directly from the evacuated tube solar collectors, Entropy 2019, 21, x FOR PEER REVIEW 3 of 23 collector, with a total surface area of 148 m2, thermal storage with phase change, and an ORC [16], uses the tank indirectly for phase change as the evaporator from the ORC. EntropyThe2019 proposed, 21, 911 system presents a fundamental difference to other applications reported in3 ofthe 23 literature, in that the heat transfer is carried out directly from the evacuated tube solar collectors, using it as an evaporator. This improves the cycle efficiency due to the elimination of heat losses in using it as an evaporator. This improves the cycle efficiency due to the elimination of heat losses in the intermediate heat exchanger. When intermediate heat exchangers