Binary Geothermal Combined Heat-and-Power Plants — Optimal Design and Control of Low-Temperature ORC-based Systems

Deep-geothermal energy is readily available all over the world as long as one is willing to drill deep enough. However, in northwest European countries, the geothermal gradient is low (~30 °C/km) which leads to high drilling costs and low geothermal source temperatures (110-150 °C). As a result, the stand-alone electrical power generation from a low-temperature geothermal energy source is not economically viable without some kind of support scheme. The goal of this PhD research is to investigate whether the economics of a binary geothermal electrical power plant can be increased by providing heat in addition to electricity. Four promising alternative Combined Heat-and-Power (CHP) plant configurations are studied, referred to as the series, the parallel, the so-called preheat-parallel and the HB4 configurations, and the connection to third- (current technology) and fourth-generation (near-future technology) district heating systems is considered for the heat delivery. The developed computer tool can be applied to current and (near-)future deep-geothermal projects.

The main outcome of this PhD research is a two-step thermoeconomic optimization framework which allows indicating the most suited CHP plant configuration or stand-alone electrical power plant for a certain application. In the first step of the optimization framework, the design of the heat exchangers and the air-cooled condenser is optimized towards maximum net present value. The net present value is considered since it takes into account the thermodynamic performance, the size and cost of the components and the time value of money (as reflected in the discount rate). In the second step, the operating conditions are optimized for the installed CHP plant design in order to maximize the net electricity generation depending on the fluctuating heat demand and environment conditions. It is found that for a certain CHP layout which has been installed, the off-design operational performance might be improved by changing the physical connection of the heat and electricity generation parts; e.g., to allow a CHP plant which was designed for the series layout to be configured in parallel to be able to provide high heat demands. Finally, the actual (or real) performance indicators can be calculated, taking into account the off-design operational performance. The thermoeconomic design optimization of binary geothermal CHP plants, taking into account the off-design operational behavior and allowing the configuration to change during operation, is novel compared to the existing literature.

The results of a detailed sensitivity analysis have indicated that the (site-specific) geological conditions, the electricity selling contract, the type of investor and the experience of the well drilling company are the most important criteria with respect to the project feasibility. Of course, also economic support schemes might trigger the roll-out of deep-geothermal energy utilization. Furthermore, and based on the off-design optimization results, the parallel CHP plant configuration is indicated as the most flexible layout since the ORC operation does almost not depend on the temperature levels of the heat demand. The off-design optimization model is also capable of calculating the optimal amounts of electricity and heat to be produced for an installed CHP facility, depending on the electricity and gas price signals. However, due to the high level of technical detail, this approach is rather slow (~100 s/data point) and part-load maps for the heat versus electricity production are derived from the detailed off-design optimization model results. These part-load maps are implemented in a high-level control optimization model, which is very fast (~milliseconds) and which can calculate the optimal amounts of heat and electricity to be produced for real-time wholesale price signals and environment conditions.

The thermoeconomic optimization framework is applied to a case study. An existing third-generation district heating system is considered and the geological, environment and economic conditions are based on the Belgian context. For this case study, the series CHP plant design is optimal and the parallel configuration is used for providing high heat demands during operation. The considered CHP plant is economically feasible without some kind of support scheme. Note, however, that this conclusion cannot be generalized due to the site-specific input parameters, especially for the district heating system operation.

In conclusion, although the net electricity generation is lower, the economic viability of a binary geothermal CHP plant might be better than for a binary geothermal stand-alone electrical power plant. The largest improvements are possible for low geothermal source temperatures and low flow rates. Furthermore, also the geothermal energy source utilization is generally better in a CHP plant. From this finding, it is recommended to pay more attention to the use of deep-geothermal energy for heat purposes in northwest European countries.