Parametric optimization and performance analyses of geothermal organic Rankine cycles using R600a/R601a mixtures as working fluids

Highlights

• Evaporator and condenser pressures and cooling water temperature rise are optimized.

• ORC power output and parasitic power consumption using R600a/R601a are analyzed.

• A geothermal ORC using R600a/R601a generates 4–11% more power than with pure R600a.

• Heat exchanger areas and turbine sizes are analyzed for R600a/R601a.

Abstract

Organic Rankine cycles (ORCs) are preferred to convert low temperature geothermal energy (<150 °C) to electricity. The working fluid selection and system parameters optimization are the main approaches to improve geothermal ORC systems performance. Zeotropic mixtures are showing promise as ORC working fluids due to the better match between the working fluid and the heat source/sink temperatures. This study optimizes the cooling water temperature rise as well as the evaporation and condensation pressures of isobutane/isopentane (R600a/R601a) mixtures for various mole fractions to maximize the net power output of a geothermal ORC for geothermal water temperatures of 110 °C, 130 °C and 150 °C and reinjection temperatures not less than 70 °C. Two mole fractions maximize the turbine power generation, while the maximum net power output occurs for R600a mole fractions from 0.7 to 0.9 due to the variation of the parasitic power consumed by the working fluid feed pump and the cooling water circulating pump. A geothermal ORC using R600a/R601 can generate 11%, 7% and 4% more power than an ORC using pure R600a for geothermal water inlet temperatures of 110 °C, 130 °C and 150 °C, respectively. Both the evaporator and condenser area per unit power output using R600a/R601a are higher than that using pure R600a or R601a due to the reduction in the heat transfer coefficient and the temperature difference between the mixture working fluid and the heat source/sink. The total heat transfer area per unit power output increases to a maximum at an R600a mole fraction of 0.5 and then decreases with increasing R600a mole fraction. The total heat transfer area can be reduced for the same power output with higher geothermal source temperatures. The turbine using R600a/R601a with higher R600a mole fractions is nearly the same size as a turbine using pure R600a.

Introduction

A very large amount of geothermal energy is stored in the earth, but 70% of the geothermal source is low-enthalpy geothermal water at temperatures lower than 150 °C [1]. Organic Rankine cycle (ORC) systems have been developed to convert low-enthalpy geothermal energy to electricity [2], [3], [4], [5], [6], [7], [8], [9], but the thermal efficiencies of the geothermal ORC systems are generally less than 12% [10]. The working fluid selection and system parameters have been optimized in various studies to improve the thermal efficiencies of geothermal ORC systems [9], [10], [11], [12], [13], [14], [15].

A zeotropic mixture can better match the temperature profiles during evaporation and condensation due to the temperature glides with the changing component concentrations in each phase of the mixture. Therefore, the use of zeotropic mixtures as ORC working fluids has received more attention recently. Angelino and Colonna di Paliano [16] analyzed the performance of a geothermal ORC with mixtures of n-butane and n-hexane. Their results showed that the ORC with a mixture of n-butane and n-hexane produced 6.8% more electricity than with just n-pentane. Chen et al. [17] proposed a supercritical ORC using a zeotropic mixture (R134a/R32) as the working fluid to convert low grade heat to power. They found that the thermal efficiency was increased by 10–30% with the supercritical cycle using R134a/R32 (0.7/0.3) compared to a cycle using R134a. Baik et al. [18] investigated the power enhancement potential of a transcritical ORC with R125-based HFC mixture working fluids using a low-temperature geothermal heat source of about 100 °C. Their results showed that the optimized transcritical ORC with an R125/R245fa mixture working fluid yielded 11% more power than the optimized subcritical ORC with just R134a. Heberle et al. [19] investigated the exergy efficiencies of subcritical ORCs with zeotropic mixtures (isobutane/isopentane and R227ea/R245fa) as the working fluids for conversion of low-enthalpy geothermal sources. Their results showed that the exergy efficiencies increased by 4.3–15% using mixtures compared to the most efficient pure fluid for geothermal source temperatures below 120 °C. They pointed out that the temperature glide during condensation should be fit to the cooling water temperature difference. Yin et al. [20] investigated the thermal efficiencies and the condenser and evaporator sizes of thermodynamic cycles using mixtures (SF₆/CO₂) as working fluids for geothermal power plants. Their results demonstrated how zeotropic binary mixtures can increase the thermal efficiencies of geothermal power cycles. They also pointed out that the heat transfer coefficients in both the evaporator and condenser decreased with increasing SF₆ fraction. Liu et al. [21] investigated the method to determine the mixture condensation pressure and the effect of the condensation temperature glide on the geothermal ORCs performance with zeotropic mixtures as working fluids. Their results showed two maxima in the cycle thermal efficiency, exergy efficiency and net power output when the condensation temperature glide matches the cooling water temperature rise. Use of zeotropic mixtures can also increase the thermodynamic performance of ORCs driven by solar energy [22], [23] or high temperature heat sources [24], [25], [26]. Zhao and Bao [27] discussed the influence of composition shift on ORC performance with zeotropic mixtures (isobutane/pentane) since zeotropic mixtures have composition shifts due to leakage and hold-up of the two phase flow during condensation and evaporation.

These previous investigations of ORC performance with zeotropic mixtures mainly focused on analyses or optimizations of the cycle efficiency or power output with little consideration of the influence of the cooling system performance. The coolant (water for wet cooling systems or air for dry cooling systems) temperature rise and the mixture temperature glide during condensation determine the condensation pressure that then affects the ORC system performance [21]. A lower condensation pressure increases the ORC net power output, but the coolant temperature rise needs to be lower and the coolant flow rate needs to be higher which results in higher power consumption by the cooling system. Therefore, the net plant power output (turbine power output minus the parasitic power consumed by the working fluid feed pump and the cooling system) must be used to optimize the coolant temperature rise. One objective of this study is to optimize the cooling water temperature rise as well as the evaporation and condensation pressures to maximize the net power output of a geothermal ORC with a wet cooling system using isobutane/isopentane (R600a/R601a) mixtures as the working fluid. R600a/R601a mixtures were selected as the working fluid because they have been shown to give good performance for geothermal ORC systems [2], [19], [21].

Furthermore, geothermal power plant investment costs are mainly dependent on the heat exchangers and turbine costs [12]. The heat exchanger costs are related to the heat transfer area while the turbine cost is directly related to the turbine size. Many studies have been done on the heat transfer performance [12], [13], [14], [28], [29], [30], [31], [32], [33] and turbine sizes [7], [12] of ORCs using pure working fluids, but the literature provides little information on heat exchanger areas and turbine sizes of ORCs using mixture working fluids. The boiling and condensation heat transfer coefficients and the temperature differences between the mixture and the heat source/sink during evaporation and condensation are all lower than with pure components; thus, the evaporator and condenser areas for mixtures will be increased for the same heat flow. The heat transfer coefficient for single phase flow and the turbine size are related to the mixture’s thermophysical properties. Also, the heat transfer area with single phase flow of mixtures and the turbine size vary with the mole fraction. Therefore, the other objective of this study is to compare the required heat exchanger areas and turbine sizes with R600a/R601a mixtures. The ratios of the heat exchanger areas to the net power output and the turbine size factors are compared for the optimal working conditions using R600a/R601a mixtures for various mole fractions.