Department of Physics, University of Oslo, P.O.Box 1048 Blindern, N-0316 Oslo, Norway

Passive cooling of buildings takes place by emission of radiation from the surfaces facing the cold and dry night sky atmosphere.

The concept for a combined system for solar heating and radiative cooling is presented. The solar heating system is described in another conference paper (J.Rekstad et. al.: Effective solar energy utilisation...), and is composed of a large and unpressurized heat store, the solar collectors, an additional heating source, a heat distribution system in terms of floor heating and a controller unit. The solar loop and the floor heating loop are directly connected to the heat store avoiding temperature gaps by heat exchangers and favouring a low temperature level.

These components constitute the infrastructure in the building necessary for the cooling function described below.

A simplified version of the solar collector without the transparent cover sheet, have good properties as a radiative cooling surface. The large heat store is only required during the heating season. Hence the main store volume can be utilised as a store for cold water during the summer season, and only a small volume for the supply of solar heated domestic hot water is connected to the solar heating system.

The floor tube system is connected directly to the cold store, and water with temperature of 2-3 K below the desired room temperature circulates in the floor. Due to the good thermal coupling between the floor tubes and the room, a considerable cooling effect (approx. 10 W/m²K) can be obtained. Due to the small temperature gap the risk for water condensation is negligible.

The first small scale experiments revealed a potential for a low cost and simultaneously effective radiative cooling system.

The experiments are carried out at a small test house at the University of Oslo. The house is equipped with two radiative cooling systems mounted on the tilted roof. The systems are connected to two insulated and pressureless reservoirs which store the „cooled liquid" during daytime. The indoor cooling system is a waterbased cooling loop installed and integrated in the floor.

An important aspect of the concept is that it provides a soundless and hidden air conditioning system.

Preliminary results were obtained for the performance with and without cover plate for the radiators, studying the impact of the flow rate in the radiator system.

The radiator is a modular building element for roofs and facades, based on a 60 cm module width and four standard lengths. Polycarbonate twin-wall are used for the radiator. These are equipped with a channel structure which is filled with light-weight clay granulates. The granulate filling causes a capillary effect when water is flowing through the channels leading to deposition of the air present in the collector during standstill of the circulation pump.

Polycarbonate has a high emissivity in the infrared spectrum, but no significant selective properties. The first series of measurements during clear sky conditions, shows a stagnation temperature for the radiator 5 - 7 degrees below the air temperature during night. At the air temperature a cooling power in the range of 40-60 W/m² has been obtained. Due to the low specific cost and the savings because of building integration, the radiator area can be large and adapted to the cooling demand in the building.

This cooling possibility represents an added value to the system with very limited extra costs. The applicability is strongly dependent on the climatic conditions. A minimum night temperature close to the desired indoor temperature is necessary. Also the air humidity has influence on the performance and should be significantly under 100 % in order to gain the full benefit of this simple and inexpensive cooling device.

Keywords: Radiative cooling, integrated systems, radiator/ collector in plastic materials, water based distribution system, one system for heating and cooling