NWO subsidy granted for development of proof-of-concept of ATES triplet

More efficient than conventional ATES

Seasonal fluctuations in temperature mean that we in the Netherlands need to cool and heat our buildings. In the summer there is a heat surplus, while in the winter there is heat shortage. The energy used for the cooling and heating can be significantly reduced by temporarily storing the heat and cold. This is already frequently accomplished in the country by means of open aquifer thermal energy storage (ATES) systems, an application that has grown sharply over the last few decades. Indeed, the Netherlands has built up more than 25 years’ practical experience with these energy systems, and approximately 3,000 of them are currently operational.

Conventional ATES is particularly advantageous when it comes to cooling. The cold can actually be directly recovered from the groundwater source. The Coefficient of Performance (CoP) – the ratio indicating the relationship between the energy supplied and the energy used – of this cooling system can be as much as 30. That is ten times higher than the performance of a chiller. However, during the cooling only a limited rise of temperature occurs in the groundwater. This means that if a building needs to be cooled to about 20°C in the summer, the groundwater cannot be heated to more than about 18°C. This stored heat is not sufficient for use in the winter. A supplementary heat pump is therefore needed to boost the temperature from 18°C to 40-45°C. Heat pumps can efficiently bring about this small temperature increase; this heating system is consequently more efficient than one using a gas boiler.

Compared to conventional chillers and gas boilers, the current ATES systems are therefore a big improvement. But these systems still use relatively large amounts of energy, of which the heat pump accounts for about 60%. But there is an intelligent alternative: the ATES triplet system.

Figure 1. Schematic illustration of an ATES triplet system without a heat pump [1]

Very promising technology

KWR’s exploratory research revealed how well an ATES triplet system [ML1] can perform in energy-saving. A comparison was made of the total costs of ownership (TCO) of a gas boiler and a chiller, a conventional ATES, and an ATES triplet system, for a fictional office building with 50,000 m2 of gross floor area. The study was based on five-year (2011-2015) climate data for De Bilt, and the calculations included the energy use of the building/well pumps. Based on the energy use requirements, the number of dry coolers and solar collectors needed to capture and store the required cold and heat was determined, as were the associated investments.

The results show that a triplet system, as a business case, is comparable to an ATES, but that its primary energy use, and thus its emissions, are lower. With regard to operating costs, besides energy costs, it was assumed that maintenance would represent 5 percent of the annual investment costs. In total, about 2,000 m2 of solar panels are required, and the capacity of the dry cooler needed to capture sufficient cold has to be more than 1 MW. All in all, it is a very promising technology.

Figure 2. Total energy use, CO2 emissions and pumping rates for conventional energy supply (fossil), conventional ATES (doublet, with heat pump) and an ATES triplet system (without heat pump) for Heating, Ventilation and Air Conditioning (HVAC) systems [2]

Feasibility study

With the aim of further elaborating the feasibility of the ATES triplet system, NWO has granted a subsidy for a proof-of-concept. This will involve the following elements:

• Closer examination of the energy balance of the system, so as to develop a more detailed understanding of the temperature ranges in the building, circuit diagrams between the sources and system components.
• Research on the optimisation of the extraction temperature from the hot source, through the smart placement of sources, or stratified recovery and extraction. Also, an exploration of the scale (type and dimension of the buildings) for which the ATES triplet system would work particularly well. High-temperature subsurface storage is not yet a proven technology; recovery efficiency challenges certainly remain in the case of temperatures above 45°C.

The project will be carried out by TU Delft and TU Eindhoven, with contributions by the Geothermal Energy Knowledge Platform, Kropman, Royal HaskoningDHV and KWR.