An increase in the number of volatile renewables in the electricity grid enhances the imbalance of supply and demand. One promising candidate to solve this problem is to improve the energy storage. The Ecovat system is a new seasonal thermal energy storage system currently under development. In this paper, an integer linear programming model is developed to describe the behaviour and potential of this system. Furthermore, it is compared with a previously developed model, which is simplifying the behaviour of the Ecovat system much more, but is much less computationally expensive. It is shown that the new approach performs significantly better for several cases. For controlling a real Ecovat system in the future we may incorporate a number of improvements identified by our comparison analysis into the previously developed approach, which may help increase the quality of the obtained results without increasing the computational effort too much.

The Ecovat system is a seasonal thermal storage technology developed to supply the heat demand of a neighbourhood of houses throughout the entire year. It consists of a large subterranean water tank which is divided into a number of virtual segments which can be charged or discharged independently from each other. In this work, we present a fast heuristic approach to model and control the charging (store heat) and discharging (withdraw heat) of the Ecovat system. We compare the results obtained with this approach to results obtained with a previously developed integer linear programming model (ILP) of the Ecovat system, which is too slow to be used in practice. The heuristic is found to reduce the computational time from 12+ hours for the ILP model to one second and is thereby suitable to be used in practice. The price for this reduction is an average decrease in performance of 5.2% and a maximum observed decrease of 12.5%.

Abstract:The Ecovat is a seasonal thermal storage solution consisting of a large underground water tank divided into a number of virtual segments that can be individually charged and discharged. The goal of the Ecovat is to supply heat demand to a neighborhood throughout the entire year. In this work, we extend an integer linear programming model to describe the charging and discharging of such an Ecovat buffer by adding a long-term (yearly) planning step to the model. We compare the results from the model using this extension to previously obtained results and show significant improvements when looking at the combination of costs and the energy content of the buffer at the end of the optimization. Furthermore, we show that the model is very robust against prediction errors. For this, we compare two different cases: one case in which we assume perfect predictions are available and one case in which we assume no predictions are available. The largest observed difference in costs between these two cases is less than 2%.

Developing a Method for the Operational Control of an Ecovat System Proefschrift ter verkrijging van de graad van doctor aan de Universiteit Twente, op gezag van de rector magnificus, prof. dr. T.T.M. Palstra volgens besluit van het College voor Promoties in het openbaar te verdedigen op vrijdag 20 december 2019 om 16.45 uur door

In this paper we present a demand side management control approach for a neighbourhood, which includes a seasonal thermal storage, called the Ecovat system. The Ecovat system is used to satisfy the heat demand of this neighbourhood instead of gas boilers, which are currently used in most Dutch houses for this purpose. We observe that an Ecovat system is capable of supplying the heat demand of such a neighbourhood throughout the year, even if the heat demand is unexpectedly high, for example due to a harsh winter. As benefits we observe an 86.8 to 91.8% reduction in electricity fed back to the grid as well as a reduction in the amount of CO2 emitted yearly by 91.4 up to 138.8 ton, when using an Ecovat system instead of gas boilers to satisfy the heat demand of the neighbourhood.