Free cooling of an office building with boreholes

Project facts

Type: Office, whole-building studies
Consultant: EQUA, Hans Abicht AG
Client: Roche Diagnostics
Summary: An office building cooled with boreholes. Both borehole fields and ambient air heat exchanger could be reduced significantly in size thanks to the simulation.

Challenge

A new office building with relatively high cooling need (IT services) should be cooled by as much free cooling from boreholes as possible. The heat is provided by a heat pump, taking as much heat as possible from the cooling tanks and the rest from the same borehole field.

Whole building simulation model

The questions were:

How many boreholes are needed in order to cover the cooling need?
How should/can the ground be recovered?
Is an ambient heat exchanger needed (to either recover the boreholes or to cover extra active cooling)? If yes: What size and do we need a wet heat exchanger or not?

Solution

A model was created with:

129 zones
hybrid heating/cooling panels, which are able to precool the concrete ceiling during nights
cooling fan coils for the server rooms
2 air handling units, 3 tanks, 1 heat pump, 1 ambient heat exchanger
a borehole field with 29 boreholes
plant control with 41 controllers

Office and meeting rooms with hybrid heating/cooling panels

The control measures tank temperatures on different heights as well as borehole outlet temperature and ambient air temperature. Depending on these temperature measurements, the system is switched between the following 5 operation points:

Simultaneous heating and cooling (heat pump between cooling and heating tank)
Free cooling (from boreholes directly to cooling tank)
Active heating (from boreholes to heat pump)
Active cooling (from ambient air to chiller)
Borehole regeneration (from boreholes to ambient air)

Plant model with 4 of the 5 operation points

Result

Thanks to the higher planning reliability, the borehole field as well as the ambient air heat exchanger could be significantly reduced in size. The investment costs were reduced by 10 times the costs of the whole simulation service. The created dynamic building model can be reused for fault diagnostics and system optimization during the operation of the building.

Borehole outlet temperatures over 50 years for the system with (above) and without (below) regeneration of the borehole field.