Numerisk modellering av varmeoverskudd i grunnvarmeanlegg – Konsekvenser og forebyggende tiltak

The emission of greenhouse gases must be reduced to meet the objectives of The Paris Agreement. Geothermal heat pumps (GSHP) can reduce energy consumption and emission of greenhouse gases in the building sector. When an annual net amount of energy is added to or extracted from the ground it can cause a lasting temperature change. This can result in a deterioration in the performance of the heat pump because it no longer operates at the intended temperature conditions. This thesis is a case study of a GSHP-system which provides heating and cooling for two hospital buildings at Haukeland University Hospital located in Bergen, Norway. The system is planned by COWI AS and exchanges heat with the ground through 165 borehole heat exchangers. It is estimated that the heat pumps will produce a net amount of heat in the future as result of a large need for process cooling. A numerical model is developed in TRNSYS to investigate the temperature rise in the ground resulting from the excess heat from the heat pumps. The results show a temperature increase in the ground of 15,1 °C over a simulation period of 25 years. After the 25th year a quasi-equilibrum state is reached, where the thermal load does not cause further increase in the mean ground temperature. To avoid heat accumulation the excess heat is going to be utilized for preheating of domestic hot water (DHW) in a nearby building. Three models are developed in TRNSYS to estimate the energy savings that are achievable from this solution. The first model represents a system without preheated water, while the other two represent two different systems that include preheated water. In one of these, preheated water is sent into an after-heater component which is used to produce hot water at 70 °C. The other one additionally includes preheated water into the mixer of the system to reduce the amount of after-heated water that is needed to produce tap water at 55 °C in the mixer. The energy consumptions of the three systems are simulated and compared. The system that only includes preheated water into the water heater reduces the energy need for DHW-production by 44,5 %, while the system that also includes preheated water into the mixer results in a 54,8 % reduction. In addition, the preheating of DHW causes the temperature increase in the ground to reduce from 15,1 °C to 3,15 °C, with the new temperature after 25 years being 12,5 °C. This significantly reduces the risk of efficiency deterioration as a result of increas