Thermal Energy Storage at a Shopping Mall

by W van Zanten, CADDET Dutch National Team

Introduction

Seasonal energy storage offers immense opportunities to profit from renewable energy for heating and cooling of buildings. In Europe, the outside air temperature in summertime is high enough that extra space heating is not required. Similarly, the low outside temperature in winter means that mechanical cooling is rarely needed .

 Thermal energy storage uses relatively simple technology to store summer's heat for use in winter, and winter cold to provide cooling in summer. The principle is that warm water, heated in summer, can be stored deep in underground wells (aquifers) until winter, when it is pumped up through heat exchangers for space heating. In the same way, cold water can be stored in winter to be pumped up in summer to provide cooling. This has a great environmental advantage over conventional air-conditioning because thermal energy storage uses only pumps and heat exchangers instead of refrigerators with harmful CFCs or HFCs. The technology could be used wherever the subsoil conditions are appropriate, and this is the case in large parts of Europe.

Picture

The HuevelGalerie shopping complex in Eindhoven

The HeuvelGalerie

In the Netherlands, thermal energy storage is becoming quite common. The emphasis is on storing winter cold for cooling in summer, because cooling is currently more expensive than heating. The HeuvelGalerie, a multi-functional shopping mall complex in the centre of Eindhoven, uses long-term thermal energy storage in soil for its cooling and heating. The mall, which opened in 1992, has a total area of 100,000 m2 and a gross surface of about
35,000 m2. It contains shops, a music centre, a casino, offices, a residential area and a car park. The thermal storage installation reaches a capacity of 1,600 kW for both cooling and heating, with an average temperature difference of 14 C.

The Installation

The diagram above shows a broad outline of the installation with its various components.

Picture

Schematic of the thermal storade system

  • The aquifer storage system consists of a warm (32 C) and a cold (18 C) well. Each well can deliver a maximum capacity of about 100 m3/hour.
  • In the mall there is a circulation network with reversible heat pumps to supply both cooling and heating to the shops. The network can simultaneously supply different parts of the mall with heating and cooling, according to local requirements, partly equalising the demand for heat and cold. Any shortage of cooling is supplied by an evaporation cooling tower.
  • Heating is supplied to the ventilation air by the thermal storage system. Any heat shortage is met by a central heating system.

The thermal storage system links the separate subsystems and supplies most of the demand for heat and cooling

When cooling and heating requirements balance each other, the heat pump circulation network transports surplus heat directly to the ventilation system, bypassing the thermal storage facility. The warm water is cooled by the ventilation system and then returned to the heat pump circulation network.

When the system is not in balance, the thermal storage system meets the shortage of heat or cold. Additional peak requirements are met by a central heating system and an evaporation cooling tower.

An initial feasibility study assumed that the cooling demand would increase, on the premise that only a few shops would use the cooling system when it was introduced with others becoming connected later. A running-in period of five years was allowed for, with the temperature of the wells expected to increase from an initial 12 C to a target of 18 C for the cold well and 32 C for the warm well.

Summer mode:

 

Winter mode:

 

Extraction Temperature

18oC

Average extraction temperature

32-34oC

Average injection temperature

32oC

Average injection temperature

18oC

Heat surplus

11,900 GJ

Cooling stored

7,600 GJ

Direct useful heat

4,300 GJ

Losses

4,500 GJ

Storage

7,600 GJ

Available

3,000 GJ

 

 

Directly used

4,300 GJ

'Shortage'

11 GJ

'Shortage'

3,200 GJ

Table 1: Details of the Thermal Storage System after the Running-in Period

Energy Savings

A (fictional) conventional system is used as a reference to determine the savings in energy, capital costs and running costs. This reference system consists of a cooling system which uses only cooling towers and a ventilation system which recovers heat from return air. On this basis, thermal energy storage will result in an annual energy saving of  3,000 GJ heat after the five-year running-in period.

Capital outlay on the cold storage system

875,000

Capital outlay on the cold reference system

845,000

Investment surplus

30,000

Savings on running costs

35,000*

Break-even point

 

Table 2: Costa and profits of the Cold Storage System (Dutch Guilders)
* Savings in the first few years are less because of the running-in-period

Results

During the first few summer seasons the system has operated adequately. The heat pump network stored more heat than predicted owing to the unexpectedly high number of users in the early years. This resulted in greater energy savings and reduced the payback period.

In winter, the energy storage offered the opportunity to pre-heat the ventilation air, reducing natural gas usage and leading to a corresponding reduction in emissions. In summer the stored winter-cold meant that there was no need for mechanical cooling and this eliminated the use of environmentally unfriendly materials (such as Freon R22, glycol and lubricants) used in conventional chillers.

For more information contact the CADDET Dutch National Team in Sittard.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

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