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1142 P. Garg et al. / Energy Procedia 49 (2014) 1138 – 1146 recovered can be more than the net work generated. Secondly, we observe a significant rise in pump discharge temperature for CO2 when compared to near isothermal pumping for steam. In a CSP, direct heating of working fluid in solar receiver may cause fluctuations in power output due to variation in solar radiation. To avoid this, typically an HTF loop aided by a storage unit is used. In this scheme, HTF is first heated in a solar receiver and then is stocked in a storage unit from where it is passed to a heater to heat up the working fluid in the power cycle. Charging of storage unit depends upon the solar radiation available, whereas its discharge is fixed for the given amount of power generation in the cycle. Thus, it is recharged in the peak radiation hours and acts as a buffer in the diminished hours. In this paper, we simulate a CSP with a continuous power output of 100 kW as per the above scheme where fluctuations are damped in the storage unit. Fig. 3 shows the temperature profiles of HTF and working fluid during heat transfer in a counter flow exchanger. Mathematical formulation for ascertaining the temperature profile of an HTF having specific heat constant in the range of operation is provided in [3] and is beyond the scope of this paper. In the case of CO2 cycle, since regenerator contributes significantly to heat addition, the quantity of external heat required is restricted to the high temperature range (state 5 to 3) as shown in Fig. 3a. One may also note that the temperature differential between HTF and CO2 remains more or less constant (~10-20 K). However, in case of steam cycle, this temperature difference can be predominantly higher (> 200 K) as revealed in Fig. 3b. Other noticeable difference is the range of operation of HTF. For example, in case of CO2 cycle, TiHTF = 893 K and ToHTF = 615 K whereas for steam cycle, TiHTF = 1120 K and ToHTF = 320 K. Operation of HTF in a larger temperature range of in case of steam cycle could lead to following issues. a. For the same T3’, required heat source temperature (TiHTF) for steam cycle is higher than corresponding CO2 cycle. This leads to higher concentration ratio requirement in a solar receiver for a TC-steam cycle. b. Wider operational temperature range of HTF in case of TC-steam cycle leaves a narrower band of HTFs for selection. These issues can be mitigated by using a two stage heat addition process for TC-steam cycle. Although, it adds extra HTF loop in the system, issues associated with high TiHTF can be effectively resolved as shown in Fig. 3c. In the proposed double stage heat addition scheme, pump outlet is serially routed through low and high temperature HTF loops as shown in Fig. 3c. First stage of heat addition up to point A in the temperature range ~300-650 K takes place in the low temperature HTF loop (HTF1). The heated steam at point A is then passed to a second HTF loop (high temperature HTF2) where it receives the remaining heat raising its temperature from 650 to 873 K. Comparing single stage (Fig. 3b) with the double stage heat addition (Fig. 3c), we note that the maximum HTF temperature has dropped from 1120 to 900 K. Also, two different HTFs can be used in the latter scheme, making the selection of HTF easier than in the former. However, point A in Fig. 3c needs to be optimized based on minimum irreversibility generation in the heater. a 900 800 700 600 0 b c 900 700 500 300 oHTF 5 100 Heat transfer across heater (kW) 0 100 200 300 Heat transfer across heater (kW) iHTF 3 200 300 1100 900 700 500 300 oHTF 2 0 100 Heat transfer across heater (kW) Fig. 3. Temperature profiles of HTF and working fluid inside heater. (a) single stage HTF-CO2 heater; (b) single stage HTF-steam heater; (c) double stage HTF-steam heater. iHTF 200 300 3 iHTF1 oHTF1 2 iHTF2 oHTF2 A 3 Temperature (K) Temperature (K) Temperature (K)PDF Image | CO2 steam in transcritical Rankine cycles concentrated solar
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