External Costs of ORC Working Fluids for Geothermal Applications

Abstract

This study presents a detailed assessment of the external costs caused by power generation through geothermally driven ORCs with different working fluids. In current operating geothermal applications, ORC working fluids of the hydrocarbons (HC) and hydrofluorocarbons (HFC) groups are dominating. More recently, new working fluid groups have been developed, which are in the hydrofluoroolefines (HFO) and hydrochlorofluoroolefines (HCFO) group. The HFO R1234yf and R1234ze(E) as well as the HCFO R1233zd(E) and R1224yd(Z) are especially promising low-GWP alternatives to the HFC R134a and R245fa. Nevertheless, the evaluation and legal handling of HCFOs with an ODP slightly above zero is controversial. For instance, the German Environment Agency intends to prohibit the application of R1233zd(E), due to its ODP of 0.00024. However, R1233zd(E) has several favorable aspects, such as a very low GWP and no flammability and toxicity (safety classification of A1). A case-study for a hypothetical geothermal system with a brine temperature of 138°C and a flow rate of 122 kg/s is carried out. For the four investigated impact categories global warming, acidification, eutrophication and ozone depletion, external costs between 0.16 €ct/kWhel,net and 1.7 ct/kWhel,net are obtained. For the monetarization factors with the highest probability, the external costs caused by the high-GWP refrigerants R245fa and R134a are 0.69 €ct/kWhel,net and 0.83 €ct/kWhel,net respectively. All derived external costs for the investigated low-GWP working fluids are significantly lower and are in a range between 0.29 and 0.30 €ct/kWhel,net. This proves, that the very small ODP by R1233zd(E) and R1224yd lead to no significant increase of the external costs. Thus, a general prohibition of potentially promising refrigerants with a very small ODP appears not be justifiable based on the presented results.