Attic heat harvester systems provide very cheap heat
Attic heat harvesting relies on attic temperatures exceeding room temperatures, the greater the temperature difference the more efficient the system is.
Temperatures have been monitored and recorded at a home in Ohio USA for a number of years, the following chart was produced using data obtained from October 22 to 28 2011.
Heated room temperatures, attic temperatures,
and outside temperatures 22nd - 28th October
During the period a prototype attic heat harvester system was in operation ducting hot air from the attic apex to the heated room when attic temperature exceeded the heated room temperature by more than 4°F.
October 27 was a rainy day, the attic temperature did not exceed the heated room temperature so a 1kW local electric heater was used from 12:30pm until 8:00pm, it consumed 7.5kWHrs.
In October 2012 the attic heat harvester Differential Temperature Controller capability was enhanced by adding the ability to calculate and display attic heat harvesting performance data. The following charts were based on the data obtained.
The system performance data chart following shows the power generated, the fan running hours, the fan power used, the fan running cost, and the cost of an equivalent amount of electrically generated heat for each day.
US Performance, October, Power and Costs - 22nd to 28th October
Attic heat harvester power generated correlates well with the attic to heated room temperature differences in the first chart and with electrical heating energy used on 27 October.
The next chart shows the attic heat harvester power generated and the system efficiency for each day.
Power generated KwHrs - System Efficiency
The efficiency figures achieved correlate with the attic to heated room temperature difference in the first chart.
The following chart shows the cumulative attic heat harvesting performance over the week.
This chart shows the total power generated - 38.3kWHrs, the overall System efficiency - 18.1, the total fan running hours - 35.2, the total power consumed by the fan and control system - 2.11 kWHrs, and the total cost of running the system $US 0.25.
Over the period each kWHr of heat cost 0.6 cents! - this is very cheap heat.
The attic heat harvester system efficiency is somewhat understated as the Differential Temperature Controller calculations give the energy received by the room where the Room Temperature sensor is located, in the prototype system this was the kitchen/dining/den areas. The solar heated attic air blown into the kitchen/dinning/den area pushes the existing warm air into other parts of the home heating them, this heat is not accounted for in the controller calculated performance data. Additionally the heat produced by fan operation has been ignored, if it was included the efficiency figure would increase by 1.
The results shown above were obtained from a prototype system that used a 'home made' zero energy 'gravity closed' one way air valve. Production systems will used a recently tested motorized air valve which increases air flow and hence the system efficiency by some 50% without increasing the energy consumed by the fan. Additionally some changes have now been made in the attic to maximize attic heat. Tests performed in Ohio in the spring of 2012 using a motorized air valve and a more efficient fan, both are supplied with the production system, achieved an efficiency of 37. This improvement reduces operating costs to 0.3 cents per kWHr!
Attic heat harvesting systems are incredibly efficient, inexpensive, relatively easy to install, provide cheap heat, simple to maintain, and very reliable - no other heating system can compete under any of these criteria.