Review of experimental research on supercritical and transcritical thermodynamic cycles designed for heat recovery application

Supercritical operation is considered a main technique to achieve higher cycle efficiency in various thermodynamic systems. The present paper is a review of experimental investigations on supercritical operation considering both heat-to-upgraded heat and heat-to-power systems. Experimental works are reported and subsequently analyzed. Main findings can be summarized as: steam Rankine cycles does not show much studies in the literature, transcritical organic Rankine cycles are intensely investigated and few plants are already online, carbon dioxide is considered as a promising fluid for closed Brayton and Rankine cycles but its unique properties call for a new thinking in designing cycle components. Transcritical heat pumps are extensively used in domestic and industrial applications, but supercritical heat pumps with a working fluid other than CO2 are scarce. To increase the adoption rate of supercritical thermodynamic systems further research is needed on the heat transfer behavior and the optimal design of compressors and expanders with special attention to the mechanical integrity

Modeling and investigation of the performance of a solar-assisted ground-coupled CO2 heat pump for space and water heating

The rise in the popularity of heat pumps should be accompanied by the increase in the utilization of environmentally-safe working fluids. To push for wider uptake of hybrid heat pump systems that use natural working fluids, information about their performance and operating characteristics should be made available. This study models a CO2 solar-assisted ground-coupled heat pump (SAGCHP) system and investigates its performance for space and water heating. Sensitivity analysis, parametric study, and long-term performance simulation were implemented, with the system’s seasonal performance factor (SPF), levelized cost of heating (LCOH), and ground temperature change (GTC) considered as performance indicators. It was seen that ensuring a good combination of design and operating specifications can result in an SPF (∼3.5) and LCOH (0.184 USD/kWh) that are comparable with those of SAGCHP systems that use conventional working fluids. The heat pump’s high-side pressure and output temperature exhibited notable effects on all the performance indicators. Below the optimal operating pressure, a 5% increase in the heat pump’s high-side pressure brought about a ∼7–10% improvement to the SPF, a ∼3–5% reduction to the LCOH, and a ∼6–10% increase to the GTC. Reducing the output temperature by 5% increased the SPF by ∼7–8%, decreased the LCOH by ∼3%, and increased the GTC by ∼6%. Parametric studies identified the presence of optimal heat pump discharge pressure and heat source circulation rate that should be used for operations. Long-term simulation shows that managing the GTC ensures the longevity of the system. Rather than oversizing the borehole heat exchangers (BHEs), it is more practical to reduce ground temperature decline by increasing the BHE spacing or by adding solar collectors.publishedVersio

Comparison of different configurations of a solar-assisted ground-source CO2 heat pump system for space and water heating using Taguchi-Grey Relational analysis

This work deals with the comparison of different optimized configurations of solar-assisted ground-source CO2 heat pump systems using the Taguchi method and Grey Relational Analysis (GRA) when used for simultaneous space and water heating in cold coastal climate conditions. The configurations studied include: (1) the solar collectors (SCs) and borehole heat exchangers (BHEs) connected in series, with the working fluid flowing to the SCs first; (2) the SCs and BHEs connected in series, with the fluid flowing to the BHEs first; and (3) the SCs and BHEs connected in parallel to the heat pump. Eight parameters were considered for optimizing the systems’ design, including the BHE length, BHE spacing, BHE number, SC area, BHE-SC mass flow rate, space heating return temperature, heat pump discharge pressure, and heat pump’s outlet temperature. The seasonal performance factor (SPF), levelized cost of heating (LCOH), and the estimated maximum annual ground temperature change (GTC) were chosen as performance indicators to evaluate system performance. The system model was developed using Modelica and 27 simulation runs for every configuration were implemented according to the L27 (93) Taguchi orthogonal array. Single objective optimizations were first performed using the Taguchi method to determine the parameter combinations that would optimize the SPF, LCOH, and GTC, separately. After that, multi-objective optimization was performed using Taguchi-GRA to determine the control factor combination that would give the optimal overall performance when all output variables are considered simultaneously and given equal importance. When the performance indicators were considered separately, simulations show that configuration 2 gave the best SPF (4.025), configuration 3 gave the best LCOH (0.124 USD/kWh), and configuration 1 gave the best GTC (100.257%), respectively. Analysis of variance (ANOVA) showed that the SPF is most sensitive to the heat pump’s discharge pressure and outlet temperature; the LCOH to the BHE length, BHE number, and SC area; and the GTC to all the BHE and SC sizing variables. Multi-objective optimization showed that configuration 1 performs the best, giving a grey relational grade of 0.6875, which is equivalent to an SPF of 3.267, and LCOH of 0.155 USD/kWh, and GTC of 100.021%. BHE length was found to be the most influential parameter to overall performance, irrespective of the configuration.publishedVersio