EnergyVille introduces technical solution for better energy utilization of a low-temperature thermal source

13-09-2017

Milano, Italy – 13 September 2017 – During the 4th edition of the International Seminar on Organic Rankine Cycle Power Systems, EnergyVille, a research institute which focuses on sustainable energy and intelligent energy systems, has introduced a new study which investigates the performance of a novel combined heat-and-power (CHP) configuration: the Preheat-parallel configuration. This Preheat-parallel configuration combines the benefits of the conventional series and parallel CHP plants. It uses the remaining heat of the energy source in the series branch to preheat the district heating water, which is subsequently heated to the required supply temperature in the parallel branch. According to the study, the preheat-parallel configuration shows a significantly higher efficiency than the series and parallel configurations and therefore opens new opportunities for low-temperature fueled (e.g. geothermal) CHP plants.

The Preheat-parallel CHP configuration

Especially for low-temperature energy sources, a common way to improve the overall economics of a power plant is by generating multiple energy products. In the present study, electricity and heat are produced in a combined heat-and-power-plant (CHP). The utilization of heat next to electricity results in a higher plant efficiency and might give better plant economics than a pure electrical power plant.

For a traditional parallel CHP, the thermal source delivers heat to both the ORC and the district heating system in parallel. However, some energy is left in the thermal source after having delivered heat to the ORC. The preheat-parallel configuration optimally utilizes this heat by using it to preheat the district heating water before it enters the heat exchanger in the parallel branch, where the district water is heated to the required supply temperature. Figure 1 gives a schematic outline of the Preheat-parallel configuration and illustrates the working principle.

Figure 1: Schematic outline of the Preheat-parallel CHP configuration. Electricity is produced by means of an ORC and heat is delivered to a district heating system. The green lines indicate the energy source and the dashed blue lines the water of the district heating system.

An optimal solution for low-temperature energy sources

The study shows the preheat-parallel configuration is specifically favorable when connected to a district heating system with a sufficiently large temperature difference between supply and return temperatures. Such a network exploits the preheating-effect, which is the most important characteristic of the Preheat-parallel configuration. The new configuration is particularly beneficial in the northwest of Europe, where geothermal plants struggle with economic feasibility due to the low thermal gradient of about 30°C/km, the high drilling costs and the rather low heat source temperature. In these regions, the Preheat-parallel CHP plant coupled to a district heating system opens new perspectives for an economically feasible geothermal plant.