Here's a design for a heat sink that I'm going to install in my greenhouse. It can be adapted to use in a grow room/tent. Basically, it works by transferring the heat at the top of the greenhouse/grow area (because heat rises, the temp at the top will be quite a lot hotter than at the bottom) to a heat sink that stores heat well, such as a container holding broken glass, stones/pebbles painted matt black or even water if you use a heat exchanger available on Ebay. When the temperature goes down, such as at night, instead of taking heat from the top of the greenhouse, it starts to release the heat stored into the grow area, reducing the need for heating. The fan taking air from the top of the greenhouse will be powered by a solar panel that charges a lead acid type battery so that I won't need to power it from the mains. Here's the link to details of the
greenhouse solar heat sink. To use it in a tent indoors, you would power the fan with the mains electricity. If you have two grow areas, you could bring in air from one area when the lights are on to heat the second area when the lights are off as well, to save power. I might also add that as well as heating during colder periods, it will also cool the grow area when the temperature gets too high and will condense water from the greenhouse air, acting as a sort of cheap dehumidifier when the air becomes too moist.
The bulk of the powered heating wil probably be fueled by a greenhouse propane heater, since it'll generally be cheaper than electricity. I have to make some calculations, but if the cost of heating the greenhouse with electricity is cheaper than heating with propane during the economy 7 hours at night (when there's less demand for electricity and it is cheaper on my electricity plan), then I'll switch to using an electric fan heater with a timer during this time.
Because the cost of electricity
will sky rocket in the near future, we all have to start thinking of ways to save power. Growing in greenhouses and now powering strong HID lights is one main way of doing this. If you can get away with it, then by all means, do this. Alternatively guerilla growing is a great way of saving power, as Wise will agree.
Another way of storing heat, which is one method I am considering, is by placing a few water butts painted matt black and filled with water. They will passively store heat from the sun/heater and release it at night. This is the more usual way of using a heat sink to heat the grow area when the temps drop.
I also plan to use horticultural fleece at night and/or possibly a 'thermal screen' which is a screen that is pulled out over the top of the plants, which effectively reduces the size of the growing area that is to be heated, which lowers heating costs by quite a bit.