All the features of a building have a direct influence on energy consumption and on the overall energy efficiency of the building. Normally, this concept tends to be associated only with air conditioning equipment and building materials, but the ventilation system plays a key role in the energy efficiency of the building.

"The need for ventilation to extract polluted or stale air from residential buildings means that air renewal is responsible for a large proportion of thermal energy losses. These losses due to ventilation are becoming more and more important because buildings are increasingly better insulated and more airtight. Therefore, as the short-term objective is to construct buildings with almost zero energy consumption, it is necessary to focus on optimising ventilation," said Joseba Gainza-Barrencua, a member of the CAVIAR (Quality of Life in Architecture) group at the UPV/EHU.

As Gainza explained, "the installation of greenhouses on roofs to reduce the energy consumption of buildings has not been studied in depth, although they offer important advantages: their installation on the roof allows for glazing at any desired angle, which enables the solar gains to be greater; fewer aesthetic and urban planning requirements; the possibility of introducing pre-heated air into the building through the building's mechanical ventilation system, etc.".

So, in this work "we have designed and built a prototype greenhouse and we have seen how it works on sunny days, cloudy days, etc.". Its behaviour has also been analysed by means of computer simulation and this has been validated using the measurements we have taken in the prototype. We have used the simulation to analyse the thermal behaviour of the greenhouse and calculate the energy savings should it be installed in different locations and climates," said the UPV/EHU researcher.

Greenhouse design

"The greenhouse designed is in the shape of a right-angled triangle; firstly, it is essential that it faces south because this is the position in which it receives the most solar radiation, and secondly, it is important that the glazing is as perpendicular as possible to the sun. The position and tilt of the sun varies throughout the year, so we have obtained what the optimum angle would be and we have seen that the angle at which the highest temperatures are achieved in the greenhouse in most of Spain is 55 degrees," said the lead author of the study.

"Our results indicate that greenhouses can significantly improve the energy behaviour of buildings. The effectiveness of these systems is clearly climate-dependent. The greenhouse is not recommended in areas with little need for heating, whereas in colder areas the energy savings are substantial, even more so if the ventilation system is combined with heat recovery. In the coldest climate zone in Spain, in the Burgos area, it is possible to achieve heating savings of 58 %," said Joseba Gainza-Barrencua.

“What is more," he said, "there is a possibility of fitting a heat recovery unit between the expulsion of stale air from the building and the air to be pumped into the greenhouse. This results in heating the air twice and improves the performance of the system significantly".

The researchers believe that this work provides a useful insight into the potential benefits of greenhouses, particularly when combined with a ventilation system. In these cases, the preheated air is easily distributed throughout the building, which improves the potential for energy savings. “For us," added Joseba Gainza-Barrencua, "this is one of the most important contributions of the work, as the advantages of using greenhouses with ventilation systems deserve further research. We are now looking at the use of these greenhouses for other purposes or to complement other systems.