Our lab is inside a building in University of Girona, connected to the building’s electricity and air-conditioning installations. However, during lasts years we have been trying to convert our lab in a self-sufficient and sustainable micro-grid. First, last year we introduced a PV solar installation in the roof of the building, which, thanks to the inverter and a Sonnen battery, supplies our electricity demand during the day and night. Moreover, we still have some surplus from our panels during the day but no way to benefit from them when the battery is full. Then, in order to take advantage of this solar energy surplus, this year we coped with a challenge: leave the centralized building’s air-conditioning system behind and use our own one.

This is how we started researching about the most efficient and electric air-conditioning system, which finally resulted to come from underground. We installed a geothermal well with a 2 kW heat pump.

So, first of all, why a heat pump?

A heat pump is a high-efficiency electric device to generate thermal energy. They can work for cooling or heating, and there are different models depending on the input/output energy, for example air-to-air, air-to-water or water-to-water. In our case, we have a water-to-water heat pump, where the input water comes from the geothermal well and the output water goes to our lab’s fan-coil.

That’s fine, but… why geothermal if there are air-to-air heat pumps with a good efficiency and without the cost of doing a well!?

Below 100 meters underground, there’s a constant temperature during the whole year around 14ºC. So if we pipe water in-and-out of the well, in summer we get fresh water in comparison to outdoors 25 ºC and in winter we get hot water in comparison to outdoors 5 ºC. This lets the heat pump to work always with only few degrees of difference between the input water temperature and our lab’s comfort temperature, which results in an ultra-high efficiency. The nominal COP (Coefficient of performance) of a geothermal heat pump is between 4 and 6, which means 4-6 kWh of thermal energy from 1 kWh of electrical energy. In comparison to an electric boiler for example, with an efficiency of 1 kWh of thermal energy from 1 kWh of electrical energy (Joule effect), a heat pump seems magic!

Our goal? To supply the heat pump electrical demand mainly with our solar energy surplus.

Currently, during the day, the heat pump + fan-coil system consumes the following periodical peaks to maintain a 20ºC temperature inside the lab:

Our challenge now is to design a control system to concentrate these small demand periods to solar surplus hours, ensuring, on one hand, the optimization of our own solar energy, and at the same time, a comfort temperature inside the lab.