Open-loop aquifer thermal energy storage (ATES) is a technique in which groundwater in the subsurface is used for the temporary storage of heat and/or cold surpluses. Since about 25% of all energy in the Netherlands is used for heating and cooling buildings, ATES systems can make an important contribution to the energy transition. According to regular policy frameworks, 25°C is the maximum temperature at which groundwater may be injected. Because of our limited knowledge of the effects on the surrounding environment, heat storage at higher temperatures is only permitted in pilot projects. However, in a number of places, such as at the horticulture company Koppert Cress, much warmer water is available in the summer. Potentially, therefore, far greater energy savings could be achieved by injecting the water at these higher temperatures as well. The Province of South Holland, by means of a ‘green deal’, granted Koppert Cress permission to infiltrate water in its existing ATES up to a maximum temperature of 45 °C. The partners in this research project, Koppert Cress, Vyverberg, Bart van Meurs, the Province of South Holland, Brabant Water and KWR, jointly monitored the pilot over the last few years.
The research aimed, by means of the Koppert Cress monitoring programme, to gain an understanding of how to implement the transition of an ATES to higher temperatures of up to 45°C, and to evaluate the effects this has on the system’s performance and on the surrounding environment.
The infiltration with higher temperatures meant that better use was made of the sustainable residual heat available from the Koppert Cress company. As a result, over the last four years, operational costs were about 10% lower than was the case with a conventional ATES system, while the CO2 emissions decreased by about 70%.
The emission and cost reductions were partly due to the fact that the ATES system could meet a bigger portion of the heat demand – up to 100% for the greenhouse sections in which the heating system had been adapted – thanks to the higher injection temperatures combined with short storage cycles. In addition, the conversion of energy from the ATES to the greenhouse was much more efficient because, at the higher temperatures, the heat pump was deployed more than previously, and it also functioned more efficiently. So, it cuts both ways. Further increase in the storage temperature in the hot source (and thereby a greater amount of captured heat and an improved heat pump performance) will further improve the energy performance even more. Moreover, Koppert Cress also makes frequent use of short-cycle heat storage. This is an efficient form of heat storage, and therefore an effective means of optimising the ATES, without increasing the heat dispersion. The remaining CO2 emissions are actually related to the continued connection of some of the greenhouses to a gas heater. In the future, these emissions can be reduced by a better use of the available (residual) heat within the company, and by drawing on supplementary external sustainable heat sources, such as geothermal energy.
Effects on the surrounding environment
The analysis of the monitoring data at the Koppert Cress ATES shows that the increased injection temperature had a limited effect on the surrounding environment:
- The heat dispersion from the hot sources is limited, both horizontally and vertically. This is because of the relatively small storage volumes and the imbalance in the system. At the end of the winter, the hot source is empty, and the source temperature is only a few degrees above the natural groundwater temperature. This means that only a limited portion of the aquifer is exposed to temperatures above 25 °C, and, moreover, only temporarily.
- No growth was observed within the set of opportunistic pathogens studied in the extracted water, and their presence is not related to the higher storage temperatures.
- The chemical composition changes in the groundwater observed at Koppert Cress were dominated by mixing effects, a result of extracting water from both a deep, more saline layer, and a shallow, brackish layer. These two water types were mixed in the ATES, and distributed into both formations upon reinjection. As a result of this mixing, the concentrations of different trace elements decreased over the course of time. The possible influence of the temperature changes was so small, to the point where it was not possible to distinguish them from other processes.
- The application of higher temperatures led to a considerable improvement in the performance of the ATES system at Koppert Cress (reduced emissions and reduced energy consumption, and thus lower costs), a decrease in the thermal imbalance, and a reduction in CO2 emissions.
- It is expected that comparable performance improvements can also be achieved at other ATES projects where there is a high heat demand, by increasing the injection temperature of the hot source.
- This research shows that increasing the injection temperature of an ATES to 40 °C has no significant effects on the chemical and biological water quality.
- The competent authority is advised to examine whether, and under what conditions, it would also approve storage at higher temperatures at other locations.
- The report contains recommendations on how to configure the monitoring of such systems in the future.
- The most important recommendation with regard to the monitoring at comparable projects is that the thermal dispersion can be adequately measured with DTS, although the pumping rates also provide a good picture of this. For the chemical and microbial groundwater quality, carefully selecting the sampling point location and taking account of the mixing effects in the analysis are the most important elements.