Control of a CHP and Heat Pump for District Heating

Master's assignment
Student Robbert Cornelissen
Supervisors Prof. Dr. J.L. Hurink and Ir. G. Hoogsteen
Programme Sustainable Energy Technology - University of Twente

Context

Electricity networks have provided the vital links between electricity producers and consumers with great success for many decades. The fundamentals of these networks has been developed to meet the needs of large, fossil fuel based generation. For environmental and security of supply reasons generation capacity is shifting towards (more) sustainable generation. For the same reasons, the adaption of electric vehicles and heat pumps, the total energy consumption shifts towards electricity. These two trends increase the strain on the electricity grid and generates challenges for balancing electricity supply and generation.

The simplest way to overcome the strain on the grid is by simply upgrading the electricity grid as has always been done: increasing the capacity of the grid by putting more copper in the ground and installing larger transformers. However applying this for the entire grid is costly and inefficient since the peak capacity is only required occasionally. The same holds for the imbalance in generation and consumption, the simplest way is by installing more renewable generation capacity which is curtailed if the generation is larger than demand.

A smarter way to coop with these challenges is by reducing the load peaks for the grid and matching demand and supply in a better way. Both of these goals can be achieved by shifting demand of electricity, shift demand away from high load times to reduce the peak and shift it from low supply times to high supply times to reduce the imbalance. Another way is to install energy storage in the grid, hence the demand and supply of electricity can be shifted both solving the load and balancing challenges.

This work investigates a relatively large energy storage system, it uses water at elevated temperatures as an energy storage medium. This hot water is generated with a heat pump since it is also the end product there is no need to convert the energy stored back to electricity. However the system can also generate cogenerate electricity and heat with a gas engine. Therefore it can either generate heat from electricity when the supply is high with the heat pump or generate heat and electricity when the demand is high with the combined heat and power system. The hot water produced by this system is used in a district heating system. This system can be seen as a virtual battery as it can both supply and demand electricity depending on the state of the electricity grid. However contrary to a battery when it is fully charged it can neither charge or discharge, while if the buffer is empty it can both be charged and discharged.

This research investigates the different parameters of this system, such as the ratio between heat pump and CHP, the size of the buffer, the control method: price vs. storage, location in the grid, electricity price and gas price. How these parameters affect the (feasibility of the) system and for which goals which parameters are of most importance (e.g. sustainability, cost or balancing).