Cold/heat storage systems in Flanders
by P Dirven & B Gysen, Vito, Flemish Institute for Technological Research

Aquifer Thermal Energy Storage (ATES) was first introduced in Flanders (the northern region of Belgium) in 1996. Since then, several projects have been completed or are under construction. Each has a different design, illustrating the technical flexibility of ATES.

Introduction

Aquifer thermal energy storage (ATES) uses cold water pumped from an underground well to cool a building or industrial process through a heat exchanger. Once the water has absorbed the excess heat, it can be stored in a second well. In winter, the heat can be recovered and used (for example to pre-heat ventilation air for the same building). Introducing energy storage increases the use of renewable energy by allowing winter cold and summer heat to be stored until it can be utilised. 

The most profitable application of ATES is for air conditioning in large buildings, where it can replace chillers that run on electricity. Extra savings can be gained from using ATES to preheat ventilation air. In industry, processes that require water for cooling all year round can use ATES to ‘recycle’ cooling water, thus saving groundwater resources. ATES can also compete with mechanical cooling equipment, and has a less damaging impact on the environment than conventional cooling installations.

ATES in buildings

In large buildings, ATES air-conditioning is provided by air-handling units. Cooling is usually provided by chillers and cooling towers, and heat by fossil fuel burners. ATES uses groundwater layers for cold and heat storage. In summer, when cooling is needed, cold groundwater is withdrawn from the cold well to cool the building’s water system through a heat exchanger. The water system in turn cools the incoming air in the air-handling units. The same groundwater, now warmed by the heat-exchange process, is injected into a warm well. In winter, the system acts in reverse. Groundwater is withdrawn from the warm well, heating up the building’s water system for pre-heating the cold ventilation air. The cooled groundwater is then injected into the cold well for use in the summer (Figure 1).

Industrial process cooling

A similar method can be used in industrial processes that require cooling all year round. In this case, instead of using the warmed ground water for pre-heating purposes, it is stored in an injection well. Natural cooling is then used during the winter months to lower the temperature of the water below that of the natural groundwater by, for example, a wet cooling tower. So a zero thermal balance between the injected heat in summer and the injected cold in winter is realised (Figure 2).

A few of Belgium’s ATES projects are outlined below.

KBC Bank

The KBC Bank in Leuven was constructed in 1991 and, after five years, the building needed additional cooling facilities because increased staff and computer numbers caused a rise in internal heat production. The original cooling system consisted of ice buffers and chillers. The chillers’ condensers were cooled using pond water, but eventually the pond could not meet demand and the water temperature increased. This increased the temperature of the condensers and the chillers failed.

An ATES system has now been incorporated in the existing infrastructure and cooling capacity has increased by 1,000 kW. In summer, the incoming ventilation air is cooled by groundwater from the cold well, which also cools the pond water flowing to the condensers. In winter, when the temperature falls below 6°C,

the heat in the groundwater is recovered and used to preheat the ventilation air. Installation of the ATES system resulted in substantial energy savings because the chillers were used less, ran at a higher “coefficient of performance”, and the building’s heating costs were reduced in winter because ventilation air is pre-heated.

The system became operational in the winter of 1996/97 and this was the first building in the world to incorporate ATES in a cooling system based on chillers and ice buffers.

Janssen Pharmaceutica

Janssen Pharmaceutica has decided to establish an administrative base in Beerse, Belgium. A cooling capacity of 1,170 kW will be required for one of the office buildings and ATES has been selected to meet this requirement.

In summer, the cold well will provide the air handling unit and the cooling ceilings with cold water. The use of cooling ceilings (ie ceilings that are cooled with water) means that the amount of ventilation air is insufficient to load the cold well in winter; therefore a cooling tower will be needed.

Vito NV laser centre

Vito, the Flemish Institute for Technological Research, plans to build an extension on its laser centre. The lasers have a cooling demand of

390 kWth. In addition, a laboratory next to the centre has its own cooling circuit of 150 kW for various types of equipment.

Vito decided to connect the two different cooling circuits with one ATES system to provide a total cooling capacity of 600 kWth. The cold storage/re-circulation option has been suggested as the best solution to Vito’s requirements. In winter, the water temperature will be lowered below that of the natural groundwater (12°C) using a wet cooling tower, to establish a zero thermal balance in the injection well.

Environmental benefits

Compared with traditional cooling equipment, ATES saves energy and is more environmentally-friendly because no refrigerants (CFCs) or lubricants are used and there are fewer exhaust gas emissions. The ATES projects described in this article, which will be in operation in the year 2000, will provide a total annual reduction of 226,000 kg of CO2, 174 kg of NOx and 4 kg of SO2.

If the growth of the ATES market in Flanders over the next five years goes according to predictions, 35 projects will become operational and the annual reductions in exhaust gases will increase to

8 tonnes of CO2, 6 tonnes of NOx and 140 kg of SO2. The calculated reduction of the exhaust gases is related to a combined-cycle gas turbine production unit with an efficiency of 52%.

Conclusions

ATES can be used to cool buildings and industrial processes, resulting in energy savings of 60–80%. It represents a profitable use of a sustainable technology that has a relatively short payback period on the extra investment of 2–8 years, compared with a standard chiller system.

For further information contact Paul Dirven paul.dirven@vito.be or Bert Gysen bert.gysen@vito.be at Vito. Tel: +32 14 335913; Fax: +32 14 321185; or the Belgian National Team in Mol.

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