Attic Heat Harvester
Supplemental home heating for the cost of running a light bulb
Attics with sunny roofs can get hot. The Attic Heat Harvester System
is a newly developed and extremely efficient heating system that uses solar heating energy in attic air to
considerably reduce home heating costs.
A common misconception concerning high attic temperature is that it results from heat rising from the home below, homes should have effective attic floor insulation to prevent this. In reality sunny roofed attics mostly get heated by solar energy conducted through the roof raising attic air temperature to surprising high levels as shown in the following chart of temperatures recorded every 5 minutes by a data logging system.
Over the period outdoor temperature - blue line, changed during each day by from 3 to 15°C and by, on average, 8°C. Attic temperature - red line, almost always exceeded daytime temperatures changing during each day by 5 to 25°C and by, on average, 15°C. The average daily maximum attic temperature exceeded the average daily outdoor temperature by 13°C. In a typical attic heat harvester installation a temperature difference of 10 °C provides 3.6kW of energy and uses just 80 watts achieving an efficiency of 45!.
During the period the amount of solar heating energy available for heating the home is proportional to the difference between the living area and attic air temperature shown by the pink line.
Attic Heat Harvester Performance 2012
The Attic Heat Harvester System controller continually calculates and records system performance, the following
charts, which include the period charted above were generated from this data.
Attic Heat Harvester Performance 2013
Since mid May 2013 the Edinburgh weather has been considerably better than that in 2012.
Also improvements have been made to the attic heat harvesting system to increase the air flow, this impacts the heat transferred and the system efficiency.
Attic Heat Harvester System efficiency
For 2012 the Balerno system was set to transfer hot attic air when the attic room temperature exceeded 3 °C. The efficiency achieved was 18.6, i.e. 18.6 times more heat energy was collected than it took to move the heat from the attic to the living area. Since these results were obtained the system performance has been enhanced and 2013 system efficiency is expected to be around 22. Conventional home heating systems typically achieve an efficiency of between 0.25 and 3!
The Balerno home benefits from some passive solar heating, has a small attic which reduces the energy available, and 3 floors complicating the duct routing to the lower floor, all these factors negatively impact system performance. In a more typical 1 or 2 floor home the heat produced and the system efficiency would be considerably higher.
Converting High Attic Air Temperatures into Solar Heating Energy
A home's latitude is the major factor impacting the efficiency and potential reduction of heating costs achievable from harvesting attic heat.
A second prototype system fitted to a 2 floor Toledo, Ohio, USA, home having a larger attic and roof area achieved an efficiency of 37 during early spring 2012. During the summer this system very rarely operates due to high average outdoor and room temperatures.
Attic heat harvesting generally supplements other forms of conventional and solar based heating which are far less efficient and far more expensive to run. Harvesting high attic air temperatures, (heat) reduces the amount of air pollution and greenhouse gases that result from your use of fossil fuels such as natural gas; oil, wood or propane for heating.
Attic heat harvesting is by far the most effective and efficient way to use solar heating energy in homes that have sunny roofed attics.