Wetenschap - 8 oktober 2015

Invisible storage under the campus

tekst:
Roelof Kleis

There is nothing to see, but deep underground, in the middle of the campus, there is a massive heat reservoir that helps us keep a cool head in the summer and keeps us warm in the winter.

Friday, 9 October is National Sustainability Day so Wageningen UR will be showing how sustainable it is on the Thursday before at the info market in Forum. One problem is that some achievements are difficult to visualize. Take ground source heat pumps (GSHPs) for instance, technology that makes use of the ground for heat and cold storage. The campus took the first steps towards ground source heating and cooling in 2007. Now half of its buildings are connected up to the underground reservoir. But how does it work exactly and what makes it so sustainable?

Under the campus, at a depth of about 45-85 metres, there is a well-insulated water-retaining layer of sand known as an aquifer. This layer is used in the summer to store excess heat from the buildings, and in the winter to heat the cold buildings. To avoid any misunderstandings: that layer of water is not a source of heat in itself we turn it into one. That is why you can compare it to an accumulator – a rechargeable battery. The water is a temporary storage site for heat and cold.

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This layer of water is naturally at about 11 degrees Celsius. This cold water (the source of the cooling) is used in the summer to cool the buildings. Heat exchangers transfer the heat from the buildings to the cool water from the ground. The water – heated to 17-18 degrees – is then pumped back into the ground elsewhere on campus, where it serves as a source of heat in the winter. The process is reversed in the winter, with heat exchangers extract ing the stored heat. ‘So you have sources of heat and sources of cold, and the water goes back and forth between the two,’ is how energy coordinator Michiel Van der Wal summarizes the process.

Additional
GSHPs are fundamentally different to geothermal power, technology that involves pumping up hot water from deeper layers many hundreds of metres or even kilometres below the ground. That heat can be used only once. It should be said that the GSHP system is not the only source of heating and cooling in the campus buildings. There are also standard central heating boilers and air conditioning systems. Van der Wal explains that GSHP is an additional facility in most of the buildings. That is the case in Forum, Atlas and Radix-West. ‘GSHP is supplementary there. Orion has its own, more modern system, where GSHP is the main source and the conventional heating and cooling systems are supplementary. The same will apply in Helix. So that’s a fundamental difference.’

Large numbers are involved in all this pumping water back and forth. Van der Wal: ‘Each pump can manage up to 110 cubic litres of water per hour. So that’s 110,000 litres of water. We have a permit for a maximum of 1.4 million cubic litres of water a year with associated thermal energy of 4 megawatt hours. But we’re nowhere near that amount yet: we are about halfway.’ Pumping all that water costs energy too. Even so, GSHP is sustainable as it does not use up fossil fuels and therefore helps reduce CO2 emissions. GSHP also helps towards the achievement of the objective to be energy-neutral by 2030.

Control Engineering project manager Dick van Hoften (left) and energy coordinator Michiel Van der Wal in Orion’s engine room. Orion has its own, modern ground source heat pump system.
Control Engineering project manager Dick van Hoften (left) and energy coordinator Michiel Van der Wal in Orion’s engine room. Orion has its own, modern ground source heat pump system.

The storage of heat and cold is a question of balance. The system works best if you pump as much energy and water from the cold source to the warm source every year as you do in the other direction. That requires an awful lot of figuring out and playing with the controls, if only because no two seasons are alike and each building is different. ‘Orion, for example, systematically requires less cooling than heating,’ says Dick van Hoften, Control Engineering project manager. ‘So that leaves a cooling surplus. We recently resolved this issue by connecting up part of Actio, Vitae (Rikilt) and Zodiac to the GSHP cooling system. Then the air conditioning systems there don’t have to work so hard.’

Since the GSHP system was introduced in 2007, half the buildings on campus have been connected up to the underground reservoir. Van der Wal says that there are ideas for Gaia, Lumen and large parts of Radix but no concrete plans as yet. Today, the info market in Forum will be focusing on the GSHP system because of Sustainability Day. But as was said earlier, there’s not much to see. The reservoir does its sustainable job quietly, hidden deep under the ground.

Photo: Guy Ackermans

LEADING THE FIELD WITH MODEST RESULTS
The Netherlands is the global leader when it comes to ground source heat pumps (GSHPs). According to Statistics Netherlands, there were 2740 GSHP systems here two years ago, with a combined capacity of 1103 megawatt-hours of energy. At the current rate of growth, there will be 3500 such systems in 2020. But the eff ect is modest as yet: those 3500 systems reduce CO2 emissions from buildings by 2 percent a year. GSHP also has only a modest share in Wageningen UR’s total energy consumption. About 3 percent of the self-generated sustainable energy comes from GSHP. On the other hand, it uses a lot of water. All the Dutch GSHP systems combined use 261 million cubic litres of water a year, making them the biggest application for groundwater after drinking water (756 million cubic litres). The diff erence is that the water is not used up; it simply serves as a means of storage. Of those 261 million cubic litres, at most 1.4 million is used by the GSHP system on campus.


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