Energy efficient low-temperature systems in buildings, Swedish Energy Agency

Rising energy prices have contributed to the development of heat pump-based heating systems in Sweden. Low supply temperature from heat pump systems to radiators is a requirement for energy-efficient and sustainable systems solutions. Different technical solutions have been suggested to improve space-heating with low temperature solutions, i.e. low temperature difference between heating unit and surrounding air. Radiator efficiency has been studied by increasing the heat transferring surface and by forced convection.

This project started with a comparison between low temperature radiators and traditional heating systems. Thermal comfort and energy aspects were considered. Identifying advantages and disadvantages with low temperature radiators in relation to floor heating and high temperature radiators give a platform for further work. The main conclusions in the first submitted journal paper are;

• Large sized low temperature radiators are able to create better thermal comfort conditions than traditional heating systems with those of small sized more heated radiators. Here comfort increases mainly because of more stable thermal conditions prevails due to small temperature differences in the room,
• Low temperature heating systems give a larger contribution of radiation heat in comparison to convection heat, which is considered to be favourable for human beings
• Cold downdraught from supply inlets was the single biggest threat for the thermal climate when fresh air was brought directly from outdoor. Radiator heating may counter act this impact better than floor heating

CFD simulations were used for predicting and visualising the thermal comfort. The CFD model used in the project was made according to descriptions of that real life test room, where climate measurements were made. The reliability of the simulations could be established by making comparisons between measurements and simulated results.

At the Healthy Buildings conference in 2006 a full paper was presented on this topic with visualizing methods using CFD. The presentation focused on comfort temperature as a variable to describe thermal comfort instead of the more commonly used operative temperature. Comfort temperature is a variable describing the total perceived thermal climate, including air temperature, heat radiation and air speed. A high ventilation rate was applied in the CFD room from the very beginning. In the Healthy Building conference several new studies confirmed the importance of a high ventilation rate with fresh inlet air to secure productivity and well being of persons. For that reason a high ventilation rate was kept in the CFD model.

Interaction between ventilation and heating systems has become a top priority to deal with. Cold supply air is here an advantage because of a high thermal gradient for better heat transfer. Use of so called ventilation radiators, radiators where cold inlet air is introduced through a slot between the radiator panels before entering the room, proved to be the key to stop cold draught problems and increase radiator efficiency at the same time. By leading cold air through the radiator the temperature gradient between the radiator surface and ambient air becomes larger. This makes it easier to extract heat from radiator surfaces – the heat transfer process becomes more efficient. When the inlet air has passed through the ventilation slot it is already heated to room air temperature. This results in small temperature gradients in the room and less air movements caused by buoyancy forces. The second journal paper in this project finished in 2007 treats thermal comfort aspects and heat output with ventilation radiators in comparison to traditional radiator systems. In the Climate 2007 conference in Helsinki a new conference paper was also presented on the topic of ventilation radiators. It deals with practical advantages and disadvantages, and give a proposal for utilisation ventilation of radiators to create a dynamic heating and ventilation system which renders energy savings and results in less need for manual control of radiator valves.

From this promising beginning a new project phase begins, with one more PhD student and several other students at engineering level joining the project.