We will be living in a desert. Most people's idea of a desert is that they are HOT. Actually, what makes it a desert is not temperature, but the amount of precipitation.
So, since we will be living at about 4000', the nights can get rather cool, and in the winter, it can get cold enough over night to snow occasionally.
So, in addition to figuring out ways to COOL the HexHouse, we will also need to be able to heat it. We won't need to use it all the time, so installing a dedicated heating system would be wasteful.
I've decided to provide the cooling through a simple geothermal setup, and heat through thermal storage.
Since our water will be stored in tanks that are exposed to the outside temperatures and sun, we will want the capability to both heat and cool our water, as well as the air in the house. I will use the same system for water temperature conditioning as air temperature conditioning.
Once I have better designs committed to paper and have some specs for the pumps I'll need, I'll create separate blog entries with all the details. For now, here's the basic outline.
Both hot and cold systems will use heat exchangers to change the temperature of either the water or the air.
The cold system will be based on a simple geothermal loop. There will be a large loop or group of loops buried under the ground that will cool a water/bleach solution. Im adding the bleach as a way to keep microbes from growing in the solution. I could use antifreeze, but I'd like to keep the system nontoxic.
There are two main ways to construct the "earth" part of a geothermal system. One uses deep bore holes in which the lines containing the solution are run. This is usually done in areas where the amount of usable land area is low. It's also very expensive to drill the holes. The other way is to lay out the lines in a "field" that is buried 6-10 feet below the surface. To provide sufficient heat transfer, it requires a large amount of land in which to create the field. This is far less expensive than drilling.
Since we will have 40 acres on one plot and 10 on another, available space isn't an issue for this project, so we will be using a field-style geothermal setup.
Another thing that drive up the expense of current geothermal setups is the heat pump. The heat pump is capable of creating temperatures that are higher or lower than that of the heat transfer fluid. These heat pump systems can also use a fair amount of energy to operate. We won't be using a heat pump in our system, so we will be limited to the temperature of the transfer fluid. We're hoping to get fluid temperatures into the 50-60F range. If we have sufficient thermal transfer fluid, we should be able to get the temperature in the space into the 70-85F range during the hottest part of the summer.
The cold system will have two heat exchangers. One for cooling the air, and one for cooling water.
For cold water production, my plan is to build a small reservoir that is insulated and have some copper tubing coiled up inside. I'll circulate the geothermal transfer fluid into the reservoir, and the warmer water in the copper coils will transfer its heat into the fluid and head back out to the ground loops.
For cool air production, the geothermal heat transfer fluid will come from the reservoir, through the pump and into a water to air heat exchanger, possibly a car radiator or an a-coil from a dead HVAC unit. We will then blow air over the heat exchanger and into duct work to cool the space.
The Hot system will work very much the same way as the cold system, except that the heat transfer fluid in the hot system will be oil, and that oil will be heated by a solar heater array.
Basically, I will build a large box that I run copper tubing through in a serpentine arrangement. Then, paint everything a flat black and cover it all in glass or Plexiglas. The solar rays will heat the oil in the copper tubing and head to a large reservoir of oil (probably a couple hundred gallons.) I'd like to use vegetable oil if possible so that if there's a leak it's not toxic.
However, vegetable oils that can handle the heat levels I'm thinking of generating without breakdown could be very expensive in the volumes I'm talking about using. If I must, I will use new, clean motor oil. I need to do some testing to determine how hot the solar array can get the oil, since it will determine much of the design parameters, and even which materials I choose.
This summer, I will build a small mock up. I want to see how hot I can get oil that is in direct sunlight. Granted, in Texas, it will get even hotter, but this test will give me a baseline idea of the temperature range. Since my test system won't actually flow, the oil can continue to gather heat for a long time. This will give me an idea of the maximum temperature that the oil can get to. Even if the temperatures I get are less than boiling temperature, I'm still inclined to use oil since it can store heat energy much longer than water can.
The reservoir of hot oil will be very well insulated to reduce thermal losses. I hope to keep the oil as hot as possible for as long as possible.
From this point, the set up will be identical to the cold system. Copper tubing coiled up in the reservoir will take heat from the reservoir and transfer it to the water. From there, the oil will circulate through a heat exchanger and a fan will blow the air through it and into the duct work.
These are just my basic ideas for the design. I need to locate pumps, oil, reservoirs, heat exchangers, insulation, gauges to monitor things, etc. I've never built any of this stuff before, so I'm sure this design will have to change somewhat as it is implemented. If you have any experience in HVAC, especially with off-grid or alternative energy sources, I'd love to hear your comments.