Validation of a Simplified Model Predictive Control of a Low Exergy Embedded Heating and Cooling System

Powerhouse Telemark is a low carbon plus energy project in Porsgrunn, Norway. The building is currently in a commissioning phase, but with most of the building under normal operation. The heating and cooling of the building is primarily done by a low temperature radiant floor heating system which is reversed in summer and used for high temperature cooling. The low temperature heating is provided by a geothermal heat pump and the high temperature cooling is provided by free cooling from the energy wells. The flooring system uses embedded pex-pipes in a 100 mm concrete slab with a polished surface to keep a minimal thermal resistance between the hydronic system and the room interior. This gives the possibility of having a water temperature in the floor just a few degrees above the desired room temperature to supply heating, even during design heating conditions.
However, the thermal mass of the concrete slab makes this system very slow reacting, demanding a control strategy different than conventional radiant floor systems. Several studies show that conventional fast acting control systems using the room temperature sensors work poorly, especially during transition periods in spring and autumn. In the research project Lowex (Low-exergy heating and cooling), a control scheme was developed based on simple ON-OFF control, together with a linear model predictive control (MPC) taking data from weather forecasts for the next 48 hours. The linear MPC-models have been tested against measured floor slab temperatures and room temperatures during varying internal and external conditions. We found that the MPC approach works well, but parameters in the model must be adjusted for the specific usage of the building.