TPU Researchers Found Out How To Obtain Energy More Efficiently Using An Innovative Geothermal Power Plant

Researchers from Tomsk Polytechnic University have conducted the first comprehensive analysis of the operation features of the binary-type geothermal power plant (GeoPP) developed at the University. Based on the tests performed, they modeled plant operation during the calendar year and found out how to improve the design for a specific geothermal field and achieve the most efficient power generation. The project is implemented with the support of Priority 2030 federal program of the Ministry of Education and Science of the national Youth and Children project.

Earlier, TPU engineers developed a prototype geothermal power plant (GeoPP) which, unlike existing geothermal plants in Russia, is based on the organic Rankine cycle. It is based on the conversion of heat into energy using a working fluid. Depending on the parameters of the heat source, hydrocarbons, silicone oils, and refrigerants can act as working fluids. The use of the Rankine cycle allows the TPU GeoPP to operate at lower temperatures – from 60 degrees Celsius or more. Whereas the existing binary-type geothermal power plants operate at temperatures above 100 degrees Celsius. Moreover, Tomsk GeoPP operates using closed-loop technology, which minimizes emissions into the environment.

"Most geothermal power plants in the world use geothermal source energy with water temperature of 100-200 degrees Celsius. Whereas low- and medium-temperature sources of thermal energy are of particular interest. There are various thermal schemes to generate electric power: when the temperature of geothermal water is above 160 degrees Celsius, a single–loop cycle is used, and at a lower temperature it is more preferable to use a binary cycle with an organic working fluid," says Stanislav Yankovsky, head of the project, associate professor of the I.N. Butakov Research and Educational Center of the School of Energy and Power Engineering.

TPU researchers have modeled the operation of the binary GeoPP with an air–cooled condenser in various climatic operating conditions, the study period is one calendar year. Based on the analysis, the TPU researchers developed a mathematical model that includes a detailed thermal calculation of the geothermal power plant loop diagram, a verification calculation of the cooling surfaces of an air-cooled condenser and the efficiency of the plant depending on various indicators (type of working fluid, mass flow of coolant, coolant temperature, condensation parameters, and others).

The simulation results showed that the amount of electricity generated can vary greatly due to changes in condensation conditions caused by the operating mode of the air-cooled condenser. Moreover, the ambient temperature is a critical parameter for the operation of the binary GeoPPs where an air-cooled condenser is used. Thus, studies of the daily and annual profile of the net output capacity of the GeoPP, taking into account ambient temperature fluctuations, showed that it can decrease by about 36% in summer months as compared to winter.

"The use of an air-cooled condenser in the GeoPP based on the Rankine organic cycle is efficient in climatic regions where the ambient temperature rarely exceeds 20 degrees Celsius during the warm season. At the same time, seasonal temperature changes do have a significant impact on the capacity and absolute efficiency of a binary GeoPP. Thus, during a calendar year, efficiency fluctuations can range on average from 9.3% (for the warm period) to 14.7% (for the cold period). It was found that the design of the air-cooled condenser (the optimal number of modules, the possibility of disconnecting and connecting individual sections, and adjusting the air supply rate to the sections) also affects plant efficiency. All these data will make it possible to improve the design and configuration of the plant equipment for close-together arrangement and the most efficient power generation at a specific geothermal field," the project head emphasizes.

The TPU researchers also conducted numerical studies to identify the optimal working fluids to produce thermal energy. Thus, the analysis showed that it is preferable to use such ozone-safe gases as R245fa, R365mfc, R245ca from the freon group as a working fluid.

The recommendations of the TPU researchers will allow in the future to improve design activities and increase efficiency of binary GeoPPs based on the organic Rankine cycle.