Solar-Driven Distributed Heating System : Upgrading a 200 kW Solar-driven Organic Rankine Cycle Unit for Distributed Heating

Sammanfattning: The University of Tianjin, China is working on a 200 kW solar-driven Organic Rankine Cycle (ORC) plant. Due to difficulties with Chinese energy regulations and legislation, the plant will not be connected to the grid for electricity generation. The university intents therefore to use the solar system for distributed heating at times without ongoing experiments. Since no heat consumer was designated initially, the heating purpose resulting in the most cost-effective usage of the already purchased components was sought. In this context, the plant’s performance in four different heating scenarios was assessed to determine the necessary upgrades, which led to the optimized Levelized Costs of Energy (LCOE). The upgrades considered a thermal energy storage system (TESS), extension of heat exchanger (HE) capacity and redesign of the HE’s hot fluid outlet temperature (HFOT). Scenario 1 (S1) represents the current system on-site for space heating and cooling. The system has been upgraded with a TESS and the HFOT was lowered. Scenario 2 (S2) differs from S1 by also considering upgrading of the HE capacity. In S1, the LCOE for the optimized system based on the original HE capacity and heat demand are shown, whereas the LCOE for S2 indicate the minimum LCOE possible with an optimized system for an increased space heating and cooling demand. In scenario 3 (S3) and 4 (S4), the optimized LCOE of the system used for industrial heat loads was studied. The industrial heat load was assumed to be constant. Two durations were chosen: 24 hours and 7 days a week (24/7) and 16 hours and 7 days a week (16/7) according to a factory working in a three-shift and two-shift system, respectively. In order to obtain the optimized LCOE, the parabolic trough collector (PTC) field and TESS were modeled and simulated in TRNSYS and MATLAB, respectively. For S1 the minimum LCOE of 0.187 $/kWh is achieved providing that none of the analyzed upgrades are made to the current system. In S2 the minimum LCOE of 0.145 $/kWh is obtained at 750 kW HE capacity, 10 m³ TESS and 50°C HFOT. In this setup, the HE capacity is large enough to utilize nearly all solar energy immediately. For S3, the lowest LCOE of 0.106 $/kWh was obtained at 80 kW HE and 40 m³ TESS and for S4, it was 0.098 $/kWh at 130 kW HE and 30 m³ TESS. Based on those results, the following main conclusions are drawn: (1) the low degrees of utilization of the plant in S1 and S2 led to high LCOE which are not competitive with those for traditional heating with air conditioners (0.112 $/kWh), (2) the LCOE can be optimized when the system provides heat continually throughout the year as required, for instance, by industrial processes and (3) for a system with optimized LCOE, the CO2 reductions associated to the upgrades are below 6% for space heating and cooling and 55 – 65% for industrial process heat.