Smartly mapped thermal behaviour of walls

News - 18 June 2020 - Communication BK

Buildings partially lose their heat through their external walls, but we can only approximate the extent of heat loss. PhD researcher Arash Rasooli devised a number of methods for measuring this quickly and accurately on the spot, yielding surprising results.

Determining an energy label or making an energy simulation of a house is performed on the basis of different building characteristics. The year of construction of example, where a house built before 1930 will probably be assigned Energy Label F, while a contemporary building has a good chance of attaining Energy Label A. “This is presently determined using assumptions, the actual thermal behaviour of the building is unknown,” says Rasooli. "When the thermal transmittance (U-value) is measured accurately, the assumptions may sometimes turn out to be 400 percent wrong." So anyone wanting an accurate result will have to measure, says Rasooli.

His research 'In-Situ Determination of Buildings' Thermo-Physical Characteristics' focuses on the thermal behaviour of external walls before looking at the whole building. The exact construction of these walls is often unknown. There is a standard method to map the thermal parameters, the ISO 9869 Average Method. However, this is a static method that can easily take a month of research and is not always accurate. Rasooli’s PhD research improves upon this method using simple and straightforward modifications.

Dynamic behaviour

The alternative method developed by Rasooli shows the dynamic behaviour of a wall quickly. It registers the thermal resistance of a wall and also shows how well and for how long a wall can store heat. The so-called Excitation Pulse Method (EPM) uses a triangular surface temperature excitation pulse. For this, temperature and heat flux sensors are placed on the inside and outside of the wall - there is no need to drill into the wall.

Rasooli built an EPM prototype at home, placing an infrared heater, a control panel, and a cooling system, consisting of a fan and a pump that circulates ice water through a system of tubes on one side of the wall and a protective shield on the other side. Monitoring the heat flux response to the triangular pulse gives a clear picture of how the heat behaves through the wall. “This method is used to map the wall’s Response Factors,” says Rasooli. "Once you have determined this, the thermal behaviour of a wall can be modelled right away". The thermal resistance (Rc value), thermal conductivity, and volumetric heat capacity are easy to calculate from this.

Tests in the laboratory and in a number of houses showed that the Rc value based on the year of construction may deviate significantly from the actual value. The new measurement method can therefore be an effective aid in making homes more energy efficient. "If measurements show that the behaviour of a wall is not as bad as was expected, it is better to invest your money in double glazing or roof insulation than in wall insulation," says Rasooli. "Using data from actual measurements, makes it possible to make a balanced judgment.” 

The method proves to be particularly suitable for light and medium-weight brick or concrete walls, approximately 10 to 25 centimetres thick, which includes many of the external walls of Dutch houses. The measurement results are less accurate for thicker walls.

In inhabited houses or in large-scale comparative studies, this kind of costly and intrusive research is not always possible. In that case, Rasooli proposes using existing data, for example from domotica or smart meters. These may give an indication of characteristics such as heat loss or air exchange at the level of building stock. However, they will be less accurate.

With his Excitation Pulse Method, Arash Rasooli won the 1st prize of both the European REHVA Student Competition 2016 and the HVAC World Student Competition 2016 which had entries from twenty countries. He patented EPM with the Dutch patent office, but has allowed the patent to expire. “We are researchers, not business people. Anyone willing to invest in it is welcome to cooperate with us.”