Intensive interest in final results of Koppert-Cress Project

It was late 2019 when we completed the project “transforming ATES to HT-ATES, and environmental management strategies”.  Unfortunately, COVID restrictions meant we were unable to organise a final conference in recent years. But all good things can wait. There was overwhelming enthusiasm for our afternoon conference, which was even attended by Jeannette Baljeu, the member of the provincial executive board member of Zuid-Holland with responsibility for water, who opened the event! With the consortium partners, we gave the participants, who numbered more than sixty, an overview of the project and the results. And, of course, we organised a tour.

Colleague Stijn Beernink explains about the monitoring system during the tour (photo: Koppert-Cress)”

Evaluation of energy savings and effects on groundwater

Aquifer thermal energy systems (ATES) are a technology that uses groundwater and the subsurface for the temporary storage of excess heat and/or cold. Because about 25% of all energy consumption in the Netherlands is required to heat and cool buildings, ATES can make an important contribution to the energy transition. The current policy frameworks allow groundwater to be injected only at temperatures not exceeding 25°C. Heat storage at higher temperatures is allowed in pilot projects because of limited knowledge about the effects on the local groundwater quality. In many places such as the Koppert-Cress horticultural company, much warmer water is available in the summer. That means that more energy gains could be achieved by injecting the water with this higher temperature as well. The Provincial Authority of Zuid-Holland has given Koppert-Cress permission through a “green deal” to infiltrate water with a temperature of up to 45°C into its existing ATES. The parties involved in this study – Koppert-Cress, Vyverberg, Bart van Meurs, the Provincial Authority of Zuid-Holland, Brabant Water and KWR – have been monitoring this trial jointly in recent years.

Improved performance

The transition from a conventional ATES to an ATES with a higher maximum injection temperature, which started in 2015, greatly increased the effectiveness, performance and sustainability of the heating system at Koppert-Cress. The reduction in emissions and costs are attributed to the fact that, at the higher injection temperatures more heat can be stored and that the ATES can therefore meet a larger proportion of the heat demand,  up-to 100% for the greenhouse sections where the heating system was modified (Figure 1). As a result, less gas needs to be used for heating. In addition, the conversion of energy from the ATES to the greenhouse was much more efficient because the heat pump is more efficient at higher temperatures and it therefore consumes less electricity. Raising the storage temperature further in the warm well (and therefore increasing the amount of heat captured and improved heat pump performance) will further enhance energy performance. Moreover, Koppert-Cress also makes frequent use of short-cycle heat storage. This is an efficient form of heat storage, and therefore an effective way of optimising the ATES without increasing the spatial impact of the subsurface heat. The remaining carbon emissions come primarily from the areas of the greenhouses that are still linked to a gas boiler. In the future, these emissions can be reduced by a better use of the available residual heat in the company, and by drawing on supplementary, external, sustainable heat sources such as geothermal energy. 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 carbon emissions decreased by about 70%.

Figure 1. The required pumped volume of groundwater for heating dropped sharply over the years, while the ΔT between the warm and cold wells became much greater, resulting in a much larger net supply of heat with the ATES.

Effects on the groundwater quality

The analysis of the monitoring data at the Koppert-Cress ATES shows that the increased injection temperature has a limited effect on the groundwater quality:

• ·   The heat dispersion from the warm wells is limited, both horizontally and vertically (Figure 2). This is because of the relatively small storage volumes and the imbalance in the system. At the end of the winter, the warm well is empty, and the well temperature is only a few degrees above the ambient groundwater temperature. This means that a limited part of the aquifer is exposed to temperatures above 25°C, and only temporarily.
• No growth was observed in the set of opportunistic pathogens studied in the water samples and their presence is not related to the higher storage temperatures.

The changes in the chemical composition of the groundwater that were 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 small to the point where it was not possible to distinguish them from other processes.

Figure 2. Heat distribution around a warm well from the late 2019 to the summer of 2020. Above: the temperature at 2.5m from the well; below: the temperature at 8.5m from the well. All the heat is extracted from the well so that the temperature change in the clay layers (above and below the dashed lines) is reversed.

Conclusions

• The application of higher temperatures led to a considerable improvement in the performance of the ATES system at Koppert-Cress (reduced emissions and energy consumption, and therefore lower costs), a decrease in the thermal imbalance, and a reduction in carbon emissions.
• It is expected that comparable performance improvements can also be achieved in other ATES projects where there is high heat demand by increasing the injection temperature of the warm well.
• This study shows that raising the injection temperature of an ATES to 40°C has no significant effects on the chemical and biological water quality.
• The governing 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 main recommendation with regard to the monitoring for comparable projects is that the thermal distribution can be adequately measured with DTS, although the pumping rates also provide a good proxy of this. For the chemical and microbial groundwater quality, it is particularly important to select the sampling location carefully and take the mixing effects into consideration in the analysis.