7. Heat Transfer
The greenhouses receive solar radiation from the mirrors. The total energy of sunlight received inside one greenhouse unit is 65 kW. This results in the optimal light-intensity for photosynthesis of 265 W/m2 and 15000 Lux. Finally, all of this 65 kilowatt radiation power is converted into heat, when it is absorbed by the walls and the ground.
In lunar vacuum the regolith around the greenhouse has a very low thermal conductivity of the order 1/100th to 1/1000 th watt per meter and degree of temperature. Therefore, the heat loss through the regolith cover is almost negligible.
Only the wet soil inside the greenhouse can absorb and emit a considerable amount of heat.
Thus, most of the 65 kW of heat need to be conducted away via a cooling system. The colling system consists of a a convector inside and a radiator outside that radiates the heat into space. Between these two we need a liquid circulating medium.
This cooling system is very similar to that of the International Space Station. With a separate water based cooling circuit inside and a frost proof but toxic ammonia based circuit outside, that exchange heat.
The ammonia flows through the radiator, a permanently shadowed dark surface with high IR-emissivity. From there the heat can be radiated away into deep space
Since the artificial crater in the center of the greenhouse acts as a heat trap, there won’t be much heat losses through the window as long as it receives sunlight. But there is considerable heat loss, when no sunlight is received during the short dark periods.
During the dark phases roller blinds temporarily cover the window. These roller blinds consist of 20 layers of reflective Mylar membranes and are highly heat insulating in the lunar vacuum.
Dark phases can last up to 4 days at the C1- location on the south pole and up to 11 days on the H0-location on the lunar north pole. Even after 11 days the greenhouse still can keep a temperature of 19° Celsius, as our simulation with console software shows.
Temperature curves of air and soil inside the greenhouse during initial warming phase, normal operation at sunshine and during the dark period.