• Energy Research

Abstract In this paper, a two-step design optimization framework is developed for four low-temperature geothermal combined heat-and-power plant configurations. The economic comparison, including off-design performance, has not been done before. The optimization tool is applied for an existing district heating system and for geothermal and meteorological conditions which are based on the Belgian situation. It is concluded that a combined heat-and-power plant results in an economically profitable project (net present value of 3.46 MEUR), whereas the stand-alone electrical power plant does not (net present value of −3.65 MEUR). Furthermore, the design for the series set-up is optimal, and the best connections during operation are the series and parallel connections for low and high heat demands, respectively. Also, a less detailed (high-level) control optimization model is developed for this series set-up, based on the part-load operating maps which are calculated from the detailed two-step optimization model results. The calculation time is much faster ( ∼ milliseconds) and the errors on the total revenues are smaller than 0.1%. The goal of this high-level model is to optimize the amounts of heat and electricity to produce, so that the plant can be used as a flexibility tool in energy markets driven by price signals for heat and electricity.

Abstract In this work, we investigate the performance of the so-called Preheat-parallel CHP configuration, for the connection to a thermal network (TN). A low-temperature geothermal source (130°C), and the connection to a 75°C/50°C and a 75°C/35°C thermal network are considered. For a pure parallel CHP configuration, the brine delivers heat to the ORC and the thermal network in parallel. However, after having delivered heat to the ORC, the brine in the ORC branch still contains some energy which is not used. The Preheat-parallel configuration utilizes this heat to preheat the TN water before it enters the parallel branch, where the TN water is heated to the required supply temperature. The Preheat-parallel configuration is especially favorable when connected to a thermal network with a low return temperature, a large temperature difference between supply and return temperatures—thereby exploiting the preheating-effect—and for high heat demands. In this paper, we focus on the effect of the pinch-point-temperature difference (∆T pinch ) on the plant performance. ∆T pinch is directly related with the size and cost of the heat exchangers and strongly influences the preheating-effect, which is the most characteristic feature of the Preheat-parallel configuration. First, we present the results of a detailed sensitivity analysis of ∆T pinch . A higher ∆T pinch results in a lower preheating-effect, a lower net power output and, correspondingly, lower plant efficiency. Furthermore, we compare the performance of the Preheat-parallel configuration with the convenient parallel and series CHP configurations. For all three configurations, the performance decreases with an increase of ∆T pinch . For the considered thermal network requirements, the net power generation is the highest for the Preheat-parallel configuration. With respect to the parallel configuration, the gain in net power generation stays approximately constant (75°C/35°C TN) or decreases (75°C/50°C TN) with the imposed pinch-point-temperature difference. With respect to the series configuration, the gain in net power generation increases for a higher value of ∆T pinch . This means that the impact of ∆T pinch is the biggest for the series configuration, followed by the Preheat-parallel configuration, and that the impact on the performance of the parallel configuration is the smallest.

Abstract A novel CHP configuration is presented, which is fueled by low-temperature geothermal energy and delivers heat to a district heating (DH) system.This so-called “Preheat-parallel” configuration has a higher net electrical power output ( W net ) and a higher exergetic plant efficiency ( η ex ) than the convenient series and parallel configurations for the connection to a state-of-the-art 75/50 DH system.For the considered cases, W net and η ex are 1.3–6.4% and 0.4–1.9%-pts higher than for the parallel configuration, respectively.The highest values correspond to the highest heat demand.With respect to the series configuration W net and η ex are 2.1–9.9% and 0.7–3.0%-pts higher, respectively, where the highest values correspond to the lowest heat demand.Furthermore, the optimal CHP configuration - series, parallel or “Preheat-parallel” - is discussed.The optimal configuration depends on the DH system requirements.Supply and return temperatures in the range of T supply = 40 – 110 ° C and T return = 30 – 70 ° C are considered.We conclude that the series and parallel configurations have the best performance for the connection to low-temperature and high-temperature DH systems, respectively.However, for a wide range of T supply and T return , the “Preheat-parallel” configuration is the most appropriate.The preheating-effect is the main feature of the “Preheat-parallel” configuration, and is more useful for a large temperature difference T supply - T return and for low values of T return .Furthermore, we found that for high heat demands and small temperature differences T supply - T return , the “Preheat-parallel” or series configurations might perform better than the parallel configuration for the connection to a high-temperature DH system.

Abstract Geothermal energy is an interesting alternative to polluting fossil energy sources. Therefore, in Belgium, two wells have been drilled for a deep geothermal power plant. However, the environment to which the installations are exposed is challenging. The geothermal brine has 165 g/l total dissolved solids (of which 90% are sodium and chlorine) and the production temperature can be up to 130 °C. To assess their suitability to be used in a geothermal power plant, the corrosivity of the artificial brine to three common construction materials was investigated with exposure and electrochemical tests. The metals under consideration are a low-alloyed carbon steel (S235JR), an austenitic stainless steel (UNS S31603) and a duplex stainless steel (UNS S31803). The carbon steel, that was found to corrode uniformly, could be considered as a constructional material if a sufficient wall thickness is chosen. The austenitic stainless steel and the duplex stainless steel demonstrate very low uniform corrosion rates. They are however susceptible to pitting and crevice corrosion. To guarantee safe operation of the geothermal power plant, the susceptibility of the alloys to stress corrosion cracking should be tested and in situ experiments should be performed.

Sorption heat storage has the potential to store large amounts of thermal energy from renewables and other distributed energy sources. This article provides an overview on the recent advancements on long-term sorption heat storage at material- and prototype- scales. The focus is on applications requiring heat within a temperature range of 30–150 °C such as space heating, domestic hot water production, and some industrial processes. At material level, emphasis is put on solid/gas reactions with water as sorbate. In particular, salt hydrates, adsorbents, and recent advancements on composite materials are reviewed. Most of the investigated salt hydrates comply with requirements such as safety and availability at low cost. However, hydrothermal stability issues such as deliquescence and decomposition at certain operating conditions make their utilization in a pure form challenging. Adsorbents are more hydrothermally stable but have lower energy densities and higher prices. Composite materials are investigated to reduce hydrothermal instabilities while achieving acceptable energy densities and material costs. At prototype-scale, the article provides an updated review on system prototypes based on the reviewed materials. Both open and closed system layouts are addressed, together with the main design issues such as heat and mass transfer in the reactors and materials corrosion resistance. Especially for open systems, the focus is on pure adsorbents rather than salt hydrates as active materials due to their better stability. However, high material costs and desorption temperatures, coupled with lower energy densities at typical system operating conditions, decrease their commercial attractiveness. Among the main conclusions, the implementation within the scientific community of common key performance indicators is suggested together with the inclusion of economic aspects already at material-scale investigations.

Abstract Over a three years period, an aquifer thermal energy storage system was monitored in combination with a heat pump for heating and cooling of the ventilation air in a Belgian hospital. The installation was one of the first and largest ground source heat pump systems in Belgium. Groundwater flows and temperatures were monitored as well as the energy flows of the heat pumps and the energy demand of the building. The resulting energy balance of the building showed that the primary energy consumption of the heat pump system is 71% lower in comparison with a reference installation based on common gas-fired boilers and water cooling machines. This corresponds to a CO2-reduction of 1280 ton over the whole measuring period. The overall seasonal performance factor (SPF) for heating was 5.9 while the ATES system delivered cooling at an efficiency factor of 26.1. Furthermore, the economic analysis showed an annual cost reduction of k€ 54 as compared to the reference installation, resulting in a simple payback time of 8.4 years, excluding subsidies.

Abstract This work compares the performance of four combined heat-and-power (CHP) configurations for application in a binary geothermal plant connected to a low-temperature 65/40 and a high-temperature 90/60 district heating system. The investigated configurations are the series, the parallel, the preheat-parallel and the HB4 configurations. The geothermal source conditions have been defined based on existing geothermal plants in the northwest of Europe. Production temperatures in the range of 110–150 °C and mass flow rates in the range of 100–200 kg/s are considered. The goal is to identify the best-performing CHP configuration for every set of geothermal source conditions (temperature and flow rate) and for multiple values of the heat demand. The electrical power output is used as the optimization objective and the different CHP plants are compared based on the exergetic plant efficiency. The optimal CHP plant has always a higher exergetic plant efficiency than the pure electrical power plant; up to 22.8%-pts higher for the connection to a 65/40 DH system and up to 20.9%-pts higher for the connection to a 90/60 DH system. The highest increase of the exergetic plant efficiency over the pure electrical power plant is obtained for low values of the geothermal source temperature and flow rate.

Abstract In this paper, several cost metrics for application to a combined heat-and-power plant fueled by a zero-marginal cost energy source are studied. The mature levelized cost concepts are extended with some novel metrics such as the levelized cost of exergy. The results are given for a geothermal combined heat-and-power plant, connected to two different types of district heating systems and for two scenarios for the heat and electricity prices (high and low). For a low price scenario, the conventional costing method based on two separate prices for electrical and thermal energy is the most appropriate. Also for a high price scenario, the conventional costing method is the most appropriate for heat demands at low temperature. However, for higher-temperature heat demands, the exergy costing method results in the highest revenues for the combined heat-and-power plant. The authors recommend the use of the novel levelized cost of exergy metric as different types of energy are priced with a single value. Depending on the amount of energy and the usefulness of the energy type, an appropriate cost can be allocated to each product of a multi-energy system.