Solar Heat Harvesting

Solar heat harvesting systems reduce heating costs by making use of free solar heated air, they are inexpensive to buy, relatively easy to install, incredibly efficient and cheap to operate, simple to maintain, and very reliable - no other heating system can compete.

Traditionally roofs were designed to protect homes from nature - most importantly preventing water damage to the property and it's contents, and to keep wildlife at bay. Until now roofs have not been considered as a potential source of free heat, the Attic Heat Harvester System can change that.

Attics with sunny roofs can collect 1000's of free kilo-watt hours of heat each year. On sunny and even cloudy days infrared electromagnetic solar energy is absorbed by attic roofs, especially those with slate or vinyl tiled roofs, heating the attic air. When home temperatures are too low this free heat can now be transferred into your home extremely efficiently and reduce heating costs.

Solar heat harvester exploiting solar heated attic air
Attic Heat Harvester System for 1 floor home

Solar Heat Harvester efficiency and energy transfer is directly proportional to: a) the temperature difference between the source air and the area receiving the solar heated attic air, b) the air flow rate, and c) inversely proportional to the energy used to transfer the heat. The greater the temperature difference and the airflow rate achieved the greater the power harvested, and the more efficient the system is.

There are things you can do to your attic space to increase the amount of heat you can harvest and the period of time during the year that you can harvest it, to reduce heating costs even further.

Attic heat harvester using solar heated air reflective insulation
Reflective insulation

Solar heated attics have hot and cool sides, the temperature difference between them can exceed 10°C, heat escaping from the cool side reduces the attic temperature and the system efficiency.

This heat loss can be considerably reduced by adding reflective insulation on the inside on non sunny attic sides. The foil reflects radiated energy, the reflective insulation reduces conducted energy, and the sealed surface reduces convection air flow beneath the cool roof, all three keep attics warmer.

reflective insulation reflective insulation
An attic with and without reflective insulation

Reflective insulation was added to both the Balerno (where the photographs above were taken) and Ohio homes on the internal north roof slope. Care was taken to ensure that ventilating air is able to flow between the insulation and the attic roof by leaving gaps in the insulation at both the bottom and top of the slope.

Reflective insulation will increase attic heat harvesting system efficiency and both day and night time attic temperatures. Increasing night time attic temperatures reduces house cooling and also reduces the delay before the attic heat harvester system next begins operating.

Hot solar heated attic air increases home cooling costs. For locations where attic temperatures can get excessive the Attic Heat Harvesting system can be supplied with a facility to control a separate 'low power' attic cooling fan. If the attic temperature exceeds a user pre-settable maximum value the system Controller turns on the attic cooling fan. The attic cooling fan can be fitted to a soffit or eave, a gable end, or in the roof.

If the attic cooling fan is fitted to a soffit the fan it should be mounted to draw in cool air pushing hot air out of roof vents. If the attic cooling fan is fitted to a gable end or in the roof the fan should suck out hot air drawing in cool air from soffit vents. The small cost of operating the attic cooling fan will be more than offset by savings in home cooling costs, alternatively a solar powered fan could be used. The attic and the home will also benefit from the additional ventilation.

When it is realised what attic heat harvesting can do to reduce heating costs home design aims and regulations and design will evolve to maximise the heat that can be harvested. Roof design will change to optimize the heat available increasing attic heat harvester efficiency and the heat produced.

Most roofs are constructed of waterproof material supported by wood, this delays attic air warm up by 2 to 3 hours, but retains heat for later use. Low sections of the roof that receive morning sun could be made from highly heat conductive material, that will reduce solar heated attic air warm up time and increase the heat transmitted through the roof.