Influence of the pinch-point-temperature difference on the performance of the Preheat-parallel configuration for a low-temperature geothermally-fed CHP

Available online at www.sciencedirect.com

Available online at www.sciencedirect.com Available online www.sciencedirect.com Available online atatwww.sciencedirect.com

ScienceDirect ScienceDirect Energy Procedia 00 (2017) 000–000 Energy Procedia 129 (2017) 10–17 Energy Procedia 00 (2017) 000–000 Energy Procedia 00 (2017) 000–000

www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia

IV IV International International Seminar Seminar on on ORC ORC Power Power Systems, Systems, ORC2017 ORC2017 13-15 September 2017, Milano, 13-15 September 2017, Milano, Italy Italy

Influence of pinch-point-temperature difference on Influence of the the pinch-point-temperature difference on the the The 15th International Symposium on District Heating and Cooling performance of the Preheat-parallel configuration for performance of the Preheat-parallel configuration for aa Assessing the feasibility of using the heat demand-outdoor low-temperature low-temperature geothermally-fed geothermally-fed CHP CHP temperature function district bheat demand forecast a,c,∗ for a long-term b,c a,c Sarah Sarah Van Van Erdeweghe Erdeweghea,c,∗,, Johan Johan Van Van Bael Baelb,c ,, Ben Ben Laenen Laenenb ,, William William D’haeseleer D’haeseleera,c a KU Leuven, Applied a,b,c Mechanics and aEnergy Conversion a Section, Celestijnenlaan b c Leuven, Belgium c 300 box 2421, B-3001 a KU I. Andrić *, A. Pina , P. Ferrão J. Fournier ., B. , O. Le Belgium Corre Leuven, Applied Mechanics and Energy Conversion,Section, Celestijnenlaan 300Lacarrière - box 2421, B-3001 Leuven, b

a

Institute of Technological Research (VITO), Boeretang 200, B-2400 Mol, Belgium b Flemish Flemish Institute of Technological Research (VITO), Boeretang 200, B-2400 Mol, Belgium c EnergyVille, ThorResearch Park, Poort Genk 8310, B-3600 Genk,Av. Belgium IN+ Center for Innovation, Technology and Policy - Instituto Superior Técnico, Rovisco Pais 1, 1049-001 Lisbon, Portugal c EnergyVille, Thor Park, Poort Genk 8310, B-3600 Genk, Belgium b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France

Abstract Abstract In this work, we investigate the performance of the so-called Preheat-parallel CHP configuration, for the connection to a thermal In this work, we investigate the performance of the so-called Preheat-parallel CHP configuration, for the connection to a thermal Abstract network (TN). A low-temperature geothermal source (130◦ C), and the connection to a 75◦ C/50◦ C and a 75◦ C/35◦ C thermal network network (TN). A low-temperature geothermal source (130◦ C), and the connection to a 75◦ C/50◦ C and a 75◦ C/35◦ C thermal network are considered. For a pure parallel CHP configuration, the brine delivers heat to the ORC and the thermal network in parallel. are considered. For a pure parallel CHP configuration, brine delivers to the ORC the thermal networkdecreasing in parallel. District heating networks are heat commonly addressed inthe the as heat one still of the most and effective solutions However, after having delivered to the ORC, the brine in literature the ORC branch contains some energy which for is not used. Thethe However, aftergas having delivered heat to the ORC, the brine insystems the ORC branch stillinvestments contains some energy which is through not used.theThe greenhouse emissions from the building sector. These require high which are returned Preheat-parallel configuration utilizes this heat to preheat the TN water before it enters the parallel branch, where the TN water heat is Preheat-parallel utilizes heat to preheat the TN renovation water before it entersheat the parallel where TN water is sales.toDue to configuration the changed climate this conditions and building demandbranch, in theconnected futurethe could decrease, heated the required supply temperature. The Preheat-parallel configurationpolicies, is especially favorable when to a thermal heated to the the required supplyreturn temperature. The Preheat-parallel configuration is especially favorable when connected to a thermal prolonging investment period. network with a low return temperature, a large temperature difference between supply and return temperatures—thereby exploiting network with a lowofreturn temperature, a large supply and return temperatures—thereby exploiting The main scope this paper is to heat assess the temperature feasibility ofdifference using thebetween heat demand – outdoor temperature function for heat demand the preheating-effect—and for high demands. the preheating-effect—and for high heat demands. forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 In this paper, we focus on the effect of the pinch-point-temperature difference (∆T pinch ) on the plant performance. ∆T pinch is In this paper, we focus on the effect of the pinch-point-temperature difference (∆T pinch ) (low, on themedium, plant performance. ∆T pinch is buildings that vary in both construction period and typology. Three weather scenarios high) and three district directly related with the size and cost of the heat exchangers and strongly influences the preheating-effect, which is the most chardirectly related with the size and cost of the heat exchangers and strongly influences the preheating-effect, which is the most charrenovation scenarios developed configuration. (shallow, intermediate, deep). To error, obtained heat demand values .were A acteristic feature of the were Preheat-parallel First, we present the estimate results ofthe a detailed sensitivity analysis of ∆T acteristic feature of the Preheat-parallel configuration. First, we present the results of detailed sensitivity analysis of ∆T pinch pinch . A compared with results a dynamic heat demand model, developed anda validated by the authors. higher ∆T pinch results in from a lower preheating-effect, a lower netpreviously power output and, correspondingly, lower plant efficiency. Furtherhigher ∆T pinchshowed results in a lower a lower net power the output and, of correspondingly, plantfor efficiency. FurtherThe we results when preheating-effect, only weather change is considered, margin error could be lower acceptable some applications more, compare the that performance of the Preheat-parallel configuration with the convenient parallel and series CHP configurations. more, we compare the performance of the Preheat-parallel configuration with theconsidered). convenient parallel and after seriesintroducing CHP configurations. (the error in annual demand was lower than 20% for all weather scenarios However, renovation For all three configurations, the performance decreases with an increase of ∆T pinch . For the considered thermal network requireFor the considered thermal network requireFor all threethe configurations, the performance decreases with an on increase of ∆T pinch scenarios, error value increased to 59.5% (depending the weather and. renovation scenarios considered). ments, the net power generation is theuphighest for the Preheat-parallel configuration. With respect to thecombination parallel configuration, ments, the net power generation is the highest for the Preheat-parallel configuration. With respect to the parallel configuration, ◦ ◦ ◦ ◦ The value of power slope coefficient increased on averageconstant within (75 the C/35 range CofTN) 3.8% to 8% (75 per C/50 decade, that with corresponds to the the gain in net generation stays approximately or up decreases C TN) the imposed the gain in net power generation stays approximately constant (75◦ C/35◦ C TN) or decreases (75◦ C/50◦ C TN) with the imposed decrease in the numberdifference. of heatingWith hours of 22-139h during the heating season the combination offor weather and pinch-point-temperature respect to the series configuration, the gain(depending in net powerongeneration increases a higher pinch-point-temperature difference. With respect to the series configuration, the gain in net power generation increases for a higher renovation scenarios considered). the other function intercept increased for 7.8-12.7% per decade (depending on the . This means that theOn impact of ∆Thand, is the biggest for the series configuration, followed by the Preheat-parallel value of ∆T pinch is the biggest for the series configuration, followed by the Preheat-parallel value of ∆T pinch . This means that the impact of ∆T pinch configuration, and thatThe the impact the performance the parallel configuration is the smallest.for the scenarios considered, and coupled scenarios). values on suggested couldpinch beofused to modify the function parameters configuration, and that the impact on the performance of the parallel configuration is the smallest. cimprove  2017 The by Elsevier Ltd. theAuthors. accuracyPublished of heat demand estimations. c 2017 The Authors. Published by Elsevier Ltd.  © Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. Peer-review Peer-review under under responsibility responsibilityof ofthe thescientific scientificcommittee committeeof ofthe theIV IVInternational InternationalSeminar Seminaron onORC ORCPower PowerSystems. Systems. © 2017 The Authors. Published by Elsevier Ltd. Keywords: Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Keywords: CHP; District heating; Low-grade geothermal energy; ORC; Thermal networks. Cooling. CHP; District heating; Low-grade geothermal energy; ORC; Thermal networks. Keywords: Heat demand; Forecast; Climate change ∗ ∗

Corresponding author (Sarah Van Erdeweghe). Tel.: +32 16 328388. Corresponding author (Sarah Van Erdeweghe). Tel.: +32 16 328388. E-mail address: [email protected] E-mail address: [email protected] 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. c 2017 1876-6102  Authors. Published by Elsevier Ltd. of The 15th International Symposium on District Heating and Cooling. Peer-review underThe responsibility of the Scientific Committee c 2017 1876-6102  The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. 1876-6102 © 2017 The Authors. Published by Elsevier Peer-review under responsibility of the scientific committee ofLtd. the IV International Seminar on ORC Power Systems. Peer-review under responsibility of the scientific committee of the IV International Seminar on ORC Power Systems. 10.1016/j.egypro.2017.09.163



Sarah Van Erdeweghe et al. / Energy Procedia 129 (2017) 10–17 S. Van Erdeweghe et al. / Energy Procedia 00 (2017) 000–000

2

11

1. Introduction In the northwest of Europe, binary geothermal power plants are often not economically feasible. Due to the low thermal gradient of about 30◦ C/km, the well drilling costs are very high and the heat source temperature is rather low. On the one hand, the economics of a low-temperature geothermal power plant might be increased by combining multiple (renewable) energy sources in a hybrid plant. Multiple studies on hybrid power plants including geothermal energy have been performed in [1–5]. Another way to improve the plant economics is by the combined production of electrical and thermal power in a combined heat-and-power (CHP) plant. The performance and multiple configurations of CHP plants have already been studied in [5–9]. In this work, we present a novel CHP configuration, which we call the Preheat-parallel configuration. A lowtemperature geothermal source (brine) is considered and the CHP is connected to a thermal network (TN). We prefer using the more general term thermal network, which includes district heating and cooling systems, over the term district heating system. Two state-of-the-art thermal networks are studied: the 75◦ C/50◦ C TN [8] and the 75◦ C/35◦ C TN [10]. For these two cases, we investigate the effect of the pinch-point-temperature difference in the brine heat exchangers on the plant performance. 2. Methodology 2.1. The Preheat-parallel CHP configuration Figure 1 gives an outline of the Preheat-parallel configuration. The heat source is geothermal water (referred to as brine) with a production temperature of T b,prod = 130◦C, a pressure of pb,prod = 40bar(a) and a flow rate of m ˙ b = 194kg/s. Besides electricity production via an Organic Rankine Cycle (ORC), also heat is delivered to a thermal network (TN). The pressure of the water in the TN is pT N = 7bar(a) and the nominal heat demand is Q˙ T N = 6MW. Furthermore, in Figure 1, heat exchangers are succinctly named as HEx; b,prod and b,inj represent the brine production and injection sides, respectively, and also the return, mid and supply sides of the thermal network are indicated. Brine and TN water streams are presented in full and dashed lines, respectively. The electricity is produced via a subcritical ORC with R236ea as the working fluid. From previous work [6], we know that for the investigated thermal network temperatures, the ORC outlet temperature is always constrained. Therefore we consider a recuperated ORC instead of a basic cycle. 2.2. Models and assumptions The models are implemented in Python [11], making use of the CasADi [12] optimization framework together with the IpOpt [13] non-linear solver. Fluid properties are called from the REFPROP 8.0 database [14]. We optimize the net electrical power output of the CHP plant, which is defined as: ˙ net = W ˙ t ηg − W

˙p W ˙ wells −W ηm

(1)

˙ t and W ˙ p the turbine and pump mechanical power, ηg and ηm the generator ˙ net the net electrical power output, W with W ˙ wells = 600kW the power of the well pumps. and motor efficiency and W Table 1: Definition of the fixed parameter values.

Fig. 1: Schematic outline of the Preheat-parallel configuration. The full and dashed lines represent the brine and TN water streams, respectively.

∆T recup,pinch ∆T cond,pinch ηp ηt ηm ηg

5◦ C 5◦ C 80% 85% 98% 98%

T cond T re f pre f Tc pc

25◦ C 15◦ C 1 bar(a) 15◦ C 2 bar(a)

Sarah Van Erdeweghe et al. / Energy Procedia 129 (2017) 10–17 S. Van Erdeweghe et al. / Energy Procedia 00 (2017) 000–000

12

3

The variables in the optimization process are the brine flow rate through the ORC branch (m ˙ b,ORC ), the brine temperature at the ORC outlet (T b,ORCout ), the working fluid temperature in the evaporator (T evap ), the intermediate temperature of the thermal network water (T mid ), the cooling water outlet temperature and the recuperator efficiency. Furthermore, the model results are based on the following assumptions: • no pressure drops in the heat exchangers and piping; • neglect kinetic and potential energy differences; • no superheating. Superheating has negative impact for R236ea, which is an isentropic working fluid. 0.01◦ C of superheating is imposed to ensure numerical stability; • the working fluid state is saturated liquid at the condenser outlet; • fixed ORC parameter values are given in Table 1: for the recuperator and condenser pinch-point-temperature differences ∆T recup,pinch & ∆T cond,pinch , the efficiencies, the condenser temperature T cond , the reference (subscript re f ) conditions and the cooling water inlet (subscript c) conditions. In general, the pinch-point-temperature difference ∆T pinch of a heat exchanger is the smallest temperature difference between the two streams in that heat exchanger. For the condenser, the position of the pinch-point-temperature difference is located where the working fluid is cooled to saturated vapor. For the evaporator, the pinch-point-temperature difference is where the working fluid is heated to saturated liquid. More details regarding the model equations can be found in [6]. 3. Influence of the pinch-point-temperature difference on the Preheat-parallel CHP In this section, we discuss the effect of the pinch-point-temperature difference of the brine heat exchangers on the plant performance. For the considered supply and return temperatures of the thermal network and the assumed (constant) value of the heat demand, the optimal design conditions are calculated. A smaller pinch-point-temperature difference requires larger and more expensive heat exchangers. The nominal design value of the pinch-point-temperature difference is 5◦C. However, we investigate the effect of this assumption on the plant performance. 3.1. Connection to a 75◦ C/50◦ C thermal network The performance of the Preheat-parallel configuration is discussed for the connection to a state-of-the-art 75◦ C/50◦ C thermal network. For the reference pinch-point-temperature difference of 5◦C and a heat demand of Q˙ T N = 6MW, ˙ net = 5.14MW, the exergetic plant efficiency ηex = 38.31% and the heat delivered the net electrical power output is W ˙ in heat exchangers 1 and 2 QT N1 = 3.09MW and Q˙ T N2 = 2.91MW. The exergetic plant efficiency is defined as: ηex,plant =

˙ TN ˙ net + Ex W ˙ b,prod Ex

(2)

˙ T N the exergy delivery to the thermal network and Ex ˙ b,prod the brine flow exergy at the production state. with Ex 3.1.1. Influence of the pinch-point-temperature difference on the performance Now we are considering the effect of the pinch-point-temperature difference on the heat delivery, the net electrical power output and the exergetic plant efficiency. The results are shown in Figure 2. For the moment, we consider only the dashed lines which correspond to a heat demand of Q˙ T N = 6MW. For a higher ∆T pinch , the potential of the preheating-effect in TN HEx 1 decreases. Less heat is transferred to the thermal network in TN HEx 1 and more heat has to be delivered by TN HEx 2 in the parallel branch. As a consequence, the net electrical power output is lower. ˙ net , also the exergetic plant efficiency is Since, for a fixed heat demand of Q˙ T N = 6MW, ηex,plant only depends on W lower for a higher value of ∆T pinch . These effects can be discussed more in detail when considering the operating temperatures and flow rates, which are given in Figure 3. Also here, we only consider the dashed lines for now. In order to keep the mass flow rate through the ORC branch m ˙ b,ORC —hence the electrical power output of the ORC—as high as possible, the ORC outlet temperature (T b,ORCout ) increases with ∆T pinch in order to satisfy the heat demand. Since it is detrimental for the ORC

4

Sarah Van Erdeweghe et al. Procedia / Energy Procedia (2017) 10–17 S. Van Erdeweghe et al. / Energy 00 (2017)129 000–000

˙ net (a) Net electrical power output W

13

(b) Share of the heat delivered in TN HEx 1 Q˙ T N1 /Q˙ T N

Fig. 2: Influence of the pinch-point-temperature difference (∆T pinch ) on the performance of the Preheat-parallel configuration, for a heat demand of Q˙ T N = 3MW, Q˙ T N = 6MW and Q˙ T N = 9MW.

electrical power output to increase T b,ORCout too much, T mid decreases to satisfy the imposed pinch-point-temperature difference. Due to the decrease of T mid , less heat is transferred in TN HEx 1 and the flow rate through the parallel branch increases, or equivalently m ˙ b,ORC decreases with ∆T pinch . Since m ˙ b,ORC decreases and T b,ORCout increases, less heat is added to the ORC and less electrical power is produced. The evaporator temperature (T evap ) decreases as a direct result of the higher imposed pinch-point-temperature difference. The ORC working fluid mass flow rate (m ˙ wf ) decreases as a consequence of the lower heat addition. The brine outlet temperature (T b,in j ) is the mixing temperature of the brine flow through the ORC branch and the brine flow through the parallel branch. For this 75◦ C/50◦ C TN, the brine outlet temperatures of both TN HEx 1 and TN HEx 2 increase with ∆T pinch such that also T b,in j increases. 3.1.2. Influence of the heat demand on the performance In this section we elaborate on the effect of the total heat demand Q˙ T N on the plant performance and operating conditions, still for the connection to a 75◦ C/50◦ C thermal network. We refer back to Figures 2 and 3. The nominal heat demand of Q˙ T N = 6MW is presented by the dashed lines, the high heat demand (Q˙ T N = 9MW) is presented by the full lines, whereas only the markers are shown for the low heat demand (Q˙ T N = 3MW).

(a) Brine injection temperature (T b,in j ), evaporator temperature (T evap ), brine ORC outlet temperature (T b,ORCout ) and intermediate temperature of the thermal network (T mid )

(b) Brine mass flow rate in the ORC branch (m ˙ b,ORC ) and ORC working fluid mass flow rate (m ˙ wf )

Fig. 3: Influence of the pinch-point-temperature difference (∆T pinch ) on the operating conditions of the Preheat-parallel configuration, for a heat demand of Q˙ T N = 3MW (only markers), Q˙ T N = 6MW (dashed lines) and Q˙ T N = 9MW (full lines).

14

Sarah Van Erdeweghe et al. / Energy Procedia 129 (2017) 10–17 S. Van Erdeweghe et al. / Energy Procedia 00 (2017) 000–000

5

From Figure 2a, it is clear that there exists a trade-off between electricity production and heat delivery. The higher the heat demand, the lower the net electrical power output. Besides, we can see from Figure 2b that the preheatingeffect is more useful for higher heat demands: a larger share of the heat is delivered by TN HEx 1 (full lines). This can be explained as follows. If all the heat has to be delivered in the parallel branch (TN HEx 2), this results in a very low m ˙ b,ORC and, correspondingly, the electrical power output of the ORC is low. Therefore, it is beneficial to deliver heat in TN HEx 1. By doing so, the brine flow rate through the ORC branch can be increased and also the ORC electricity production is higher. Especially for high heat demands, the preheating-effect of the Preheat-parallel configuration is very useful. The effect on the operating temperatures and mass flow rates was shown in Figure 3. The trends are the same for different values of the heat demand. However, for high heat demands (full lines), the preheating-effect is more useful and a larger share of the heat is delivered in TN HEx 1. Therefore, the brine temperature difference over TN HEx 1 is as high as possible and the value of T b,ORCout is higher for higher heat demands. Correspondingly, for the same value of the imposed ∆T pinch , also T mid is higher by the same value. Since the ORC outlet temperature is higher, also the evaporator temperature is higher in case of a high heat demand. However, due to a lower m ˙ b,ORC and a higher ˙ w f is lower. The brine injection temperature is slightly higher in case of T b,ORCout , less heat is added to the ORC and m a high heat demand. A larger share of the brine flow rate passes the parallel branch (TN HEx 2) to satisfy the higher heat demand, and the brine outlet temperature of TN HEx 2 is higher than the brine outlet temperature of TN HEx 1, so also T b,in j is higher. 3.2. Connection to a 75◦ C/35◦ C thermal network ˙ net and the share of the heat delivered by TN HEx 1 as a function of ∆T pinch for the reference Figure 4 shows W heat demand of Q˙ T N = 6MW. The results for the connection to a 75◦ C/50◦ C TN and a 75◦ C/35◦ C TN are presented by upper and lower triangles, respectively. From Figure 4a we can see that a lower return temperature of the thermal network gives rise to a higher net electrical power output. A lower T return allows more heat delivery to the thermal network in TN HEx 1, which is shown in Figure 4b. The preheating-effect of the Preheat-parallel configuration is thus more useful in case of a low return temperature. The trends of the operating temperatures (T b,in j , T evap , T b,ORCout and T mid ) and mass flow rates (m ˙ b,ORC and m ˙ w f ) as a function of ∆T pinch are similar to the trends for the connection to a 75◦ C/50◦ C TN (which were shown in Figure 3). If we consider the effect of the heat demand Q˙ T N on the operating conditions, for the connection to a 75◦ C/35◦ C TN, we see that the trends are similar to these for the connection to a 75◦ C/50◦ C thermal network. The only exception is that the brine injection temperature is lower for a higher heat demand. This is due to the preheating-effect which is more useful in case of a large temperature difference between supply and return temperature (i.e. more useful for

˙ net (a) Net electrical power output W

(b) Share of the heat delivered in TN HEx 1 Q˙ T N1 /Q˙ T N

Fig. 4: Influence of the pinch-point-temperature difference (∆T pinch ) on the performance of the Preheat-parallel configuration for the nominal heat demand of Q˙ T N = 6MW, and for the connection to a 75◦ C/50◦ C and a 75◦ C/35◦ C thermal network.

6

Sarah Van Erdeweghe et al. Procedia / Energy Procedia (2017) 10–17 S. Van Erdeweghe et al. / Energy 00 (2017)129 000–000

(a) Connection to a 75◦ C/50◦ C thermal network

15

(b) Connection to a 75◦ C/35◦ C thermal network

˙ net ) of the Preheat-parallel configuration over the series configuration as a function of the pinch-pointFig. 5: Gain in net electrical power output (W temperature difference (∆T pinch ), for a heat demand of Q˙ T N = 3MW, Q˙ T N = 6MW and Q˙ T N = 9MW.

the 75◦ C/35◦ C TN). For the 75◦ C/35◦ C connection, it is profitable to deliver as much heat as possible in TN HEx 1. As a result, the brine is cooled down more in TN HEx 1 and the brine outlet temperature of TN HEx 1 is lower for a higher heat demand. The brine outlet temperature of the ORC branch dominates the brine injection temperature such that T b,in j decreases.

4. Performance improvements of the Preheat-parallel CHP over the series and parallel configurations In this section, we compare the Preheat-parallel configuration with the convenient series and parallel configurations. As for the Preheat-parallel configuration, the ORC outlet temperature of the series configuration is always constrained by the thermal network temperatures. Therefore, the recuperated ORC is considered for the series configuration since, in this case, it performs better than a basic cycle [6]. The ORC outlet temperature of the parallel configuration does not depend on the thermal network temperatures, such that here a basic ORC is chosen. Figure 5 shows the improvements in terms of net electrical power output of the Preheat-parallel configuration in comparison to the series configuration, for the connection to a 75◦ C/50◦ C and a 75◦ C/35◦ C thermal network, respectively. Three values for the heat demand are considered: the reference heat demand of Q˙ T N = 6MW, a lower (Q˙ T N = 3MW) and a higher (Q˙ T N = 9MW) heat demand. We can conclude that the Preheat-parallel configuration performs better than the series configuration for the considered cases (the figures only contain positive values). As is ˙ net decreases with ∆T pinch , however the electrical power output of the Preheat-parallel evident, for all configurations, W configuration is less sensitive to ∆T pinch than the series configuration. Furthermore, the performance improvements over the series configuration are higher for a low heat demand, and for the connection to a thermal network with a low return temperature (Figure 5b versus Figure 5a). Figure 6 shows the performance improvements with respect to the parallel configuration. We can see that the Preheat-parallel configuration also performs better than the parallel configuration for the considered cases. However, for the connection to a 75◦ C/50◦ C thermal network (Figure 6a), the Preheat-parallel configuration is slightly more sensitive to ∆T pinch , especially at high values of ∆T pinch (the gain decreases). This means that the electrical power output of the Preheat-parallel configuration decreases faster as a function of ∆T pinch than the electrical power output of the parallel configuration. For the connection to a 75◦ C/35◦ C thermal network (Figure 6b), the gain slightly increases for low values of ∆T pinch and is almost constant for higher values of ∆T pinch . In general, the improvements over the parallel configuration are higher in case of a high heat demand, and for the connection to a thermal network with a low return temperature.

Sarah Van et Erdeweghe et al. / Energy 129 (2017) 10–17 S. Van Erdeweghe al. / Energy Procedia 00Procedia (2017) 000–000

16

(a) Connection to a 75◦ C/50◦ C thermal network

7

(b) Connection to a 75◦ C/35◦ C thermal network

˙ net ) of the Preheat-parallel configuration over the parallel configuration as a function of the pinchFig. 6: Gain in net electrical power output (W point-temperature difference (∆T pinch ), for a heat demand of Q˙ T N = 3MW, Q˙ T N = 6MW and Q˙ T N = 9MW.

5. Conclusions In this work, we have studied the effect of the pinch-point-temperature difference on the performance—the optimal design conditions and the net electrical power output—of the novel Preheat-parallel CHP configuration. We considered a low-temperature geothermal source (130◦C) and the heat delivery to a 75◦ C/50◦ C and a 75◦ C/35◦ C thermal network. We indicated that the Preheat-parallel configuration is especially favorable when connected to a thermal network with a low return temperature, a large difference between supply and return temperature—thereby exploiting the preheating-effect—and for high heat demands. From the study of the pinch-point-temperature difference, it is clear that the net electrical power output of the ORC decreases with the pinch-point-temperature difference, which was expected. Moreover, in case of a large pinch-point-temperature difference we can make less use of the preheating-effect: the share of the heat which is delivered by TN HEx 1 decreases. We also investigated the effect of the heat demand on the performance. The higher the heat demand, the lower the net electrical power output and the more useful the preheating-effect. Furthermore, we compared the performance of the Preheat-parallel configuration with the convenient series and parallel CHP configurations. We found that the net electrical power output of the Preheat-parallel configuration is always higher than for the series and parallel configurations, for the considered cases. With respect to the series configuration, the benefits are more outspoken for low heat demands, whereas with respect to the parallel connection, the benefits are higher in case of high heat demands. The performance improvements of the Preheat-parallel configuration over the series and parallel configurations are higher for the connection to a thermal network with a low return temperature. We also found that the series configuration electrical power output is more sensitive to the pinch-point-temperature difference than the Preheat-parallel configuration, and that the electrical power output of the parallel configuration is the least sensitive to variations in pinch-point-temperature difference. Or in other words, the performance gain of the Preheat-parallel configuration over the series configuration increases with ∆T pinch , whereas the performance gain over the parallel configuration stays almost constant or decreases slightly with ∆T pinch . Acknowledgments This project receives the support of the European Union, the European Regional Development Fund ERDF, Flanders Innovation & Entrepreneurship and the Province of Limburg. Nomenclature Symbols



Sarah Van Erdeweghe et al. / Energy Procedia 129 (2017) 10–17 8

17

S. Van Erdeweghe et al. / Energy Procedia 00 (2017) 000–000

symbol ˙ [MW] Ex m ˙ [kg/s] p [bar] Q˙ [MW]

description flow exergy mass flow rate pressure thermal power

symbol T [◦C] ˙ W [MW] η [%]

description temperature electrical power efficiency

Subscripts symbol b c cond evap ex g in j m mid net ORC out p par

description brine cooling water condenser evaporator exergetic generator brine injection state motor TN water state between TN HEx 1 & TN HEx 2 net Organic Rankine Cycle outlet pump parallel CHP

symbol pinch plant pp prod recup re f return ser supply t TN wells wf

description pinch-point CHP plant Preheat-parallel CHP brine production state recuperator reference state return state of TN water series CHP supply state of TN water turbine Thermal Network geothermal wells working fluid of ORC

References [1] Astolfi, M., Xodo, L., Romano, M.C., Macchi, E.. Technical and economical analysis of a solar-geothermal hybrid plant based on an Organic Rankine Cycle. Geothermics 2011;40(1):58–68. URL: http://linkinghub.elsevier.com/retrieve/pii/S0375650510000490. doi:10.1016/j.geothermics.2010.09.009. [2] Tempesti, D., Fiaschi, D.. Thermo-economic assessment of a micro CHP system fuelled by geothermal and solar energy. Energy 2013;58:45– 51. URL: http://linkinghub.elsevier.com/retrieve/pii/S0360544213000893. doi:10.1016/j.energy.2013.01.058. [3] Heberle, F., Br¨uggemann, D.. Thermoeconomic Analysis of Hybrid Power Plant Concepts for Geothermal Combined Heat and Power Generation. Energies 2014;7(7):4482–4497. URL: http://www.mdpi.com/1996-1073/7/7/4482/. doi:10.3390/en7074482. [4] Cardemil, J.M., Cort´es, F., D´ıaz, A., Escobar, R.. Thermodynamic evaluation of solar-geothermal hybrid power plants in northern Chile. Energy Conversion and Management 2016;123:348–361. URL: http://linkinghub.elsevier.com/retrieve/pii/S0196890416305131. doi:10.1016/j.enconman.2016.06.032. [5] Ezzat, M., Dincer, I.. Energy and exergy analyses of a new geothermalsolar energy based system. Solar Energy 2016;134:95–106. URL: http://linkinghub.elsevier.com/retrieve/pii/S0038092X16300627. doi:10.1016/j.solener.2016.04.029. [6] Van Erdeweghe, S., Van Bael, J., Laenen, B., D’haeseleer, W.. Preheat-parallel configuration for low-temperature geothermally-fed CHP plants. Energy Conversion and Management 2017;142:117–126. URL: http://linkinghub.elsevier.com/retrieve/pii/S019689041730225X. doi:10.1016/j.enconman.2017.03.022. [7] Guo, T., Wang, H., Zhang, S.. Fluids and parameters optimization for a novel cogeneration system driven by lowtemperature geothermal sources. Energy 2011;36(5):2639–2649. URL: http://linkinghub.elsevier.com/retrieve/pii/S0360544211000831. doi:10.1016/j.energy.2011.02.005. [8] Habka, M., Ajib, S.. Investigation of novel, hybrid, geothermal-energized cogeneration plants based on organic Rankine cycle. Energy 2014;70:212–222. URL: http://linkinghub.elsevier.com/retrieve/pii/S0360544214003788. doi:10.1016/j.energy.2014.03.114. [9] Fiaschi, D., Lifshitz, A., Manfrida, G., Tempesti, D.. An innovative ORC power plant layout for heat and power generation from medium- to low-temperature geothermal resources. Energy Conversion and Management 2014;88:883–893. URL: http://dx.doi.org/10.1016/j.enconman.2014.08.058. doi:10.1016/j.enconman.2014.08.058. [10] EGEC, . Geothermal District Heating. Tech. Rep.; Renewable Energy House; Brussels; 2011. URL: http://egec.info/wpcontent/uploads/2011/03/Brochure-DISTRICT-HEATING1.pdf. [11] van Rossum, G.. Python Tutorial, Technical Report CS-R9526. Tech. Rep.; Centrum voor Wiskunde en Informatica (CWI); Amsterdam; 1995. URL: http://www.python.org. [12] Andersson, J.. A General-Purpose Software Framework for Dynamic Optimization. Phd; Arenberg Doctoral School, KU Leuven; 2013. [13] W¨achter, A., Biegler, L.T.. On the implementation of an interior-point filter line-search algorithm for large-scale nonlinear programming. Mathematical Programming 2006;106(1):25–57. URL: http://link.springer.com/10.1007/s10107-004-0559-y. doi:10.1007/s10107-004-0559-y. [14] Lemmon, E., Huber, M., McLinden, M.. REFPROP - Reference Fluid Thermodynamic and Transport Properties. NIST Standard Reference Database 23. 2007.