Notes from Windward: #66

Low-grade Geothermal Energy

A glimpse of things to come

     Strictly speaking, all matter has some level of energy, but when we talk about an energy source, or about useful energy, we're really talking about energy differentials that can be tapped and used for some purpose. It would be really nice if Windward had a source of high-grade geothermal energy such as a hot springs, but we don't. What we do have is over a half-mile of buried water lines that provide water at a temperature of about 55° year round.

     In the spring and fall, that doesn't represent an energy source in and of itself, but when the summer heat peaks at a hundred degrees, the vast geothermal mass that is Windward can provide some 45° of cooling, and in winter when the night time temps drop into the teens, that represents some 40° of heating capacity.

     That's the energy differential that we'll use to heat our workshops in winter, and cool our housing in summer. Right now we have about 3,000' of buried water lines, and when the non-potable water system comes on-line, that will increase to 7,000 feet.

     Long-term readers of these Notes will remember that when we installed the floor slabs in the new workshop building, we underlaid them with two inches of dense foam, made them six inches thick and embedded hydronic tubing in the three slab floors, all so that we could use this low-grade geothermal heat source to provide a baseline heat that would keep that building from freezing in winter.

one of the workshop bays ready for the concrete pour

     Not only is a cold shop a poor and damp place to work in, materials such as wood glue and latex paint are damaged, often irreversably, by freezing temperatures. While we're going to want to raise the work room temps by another ten degrees or so when we're in there working, it doesn't take anywhere near the time or energy we'd need to warm up a shop that's gotten down below freezing.

     In effect we'll be using the ground as a humongeous thermal battery where we can store heat in the summer, and draw back heat in the winter.

      While one our primary research objectives involves developing the community-scale capacity to convert wood waste into automotive fuel, we don't intend to rely on biomass to meet the bulk of our energy needs. For that, we'll be using a community sized version of the solar trough power systems that are already working very successfully in the Mohave desert.

     In such a system, parabolic reflectors concentrate sunlight on a tube that has a heat-transfer oil circulating through it. To get an idea of what that sort of collector looks like, Click Here and scroll about half-way down the page to the section on Parabolic Troughs.

     A flash boiler uses the hot oil to generate the high-pressure steam needed to drive the engine that turns the generator. In order to get the greatest efficiency out of a steam engine, you'll want to hook up the exhaust to a condensor that recovers heat from the spent steam and stores it somewhere such as the workshop's hydronic slabs or a greenhouse during the cold half of the year, or in the ground during the rest of the year via the 4,000' of non-potable water line.

     What brought all this to mind today was something I noticed outside of Vermadise. For the last week we've been seeing temperatures in the teens, so most everything above ground was frozen solid--except for the mini-aquaponics system.

the mini-aquaponics tank is completely ice free

     Our water system is designed to feed and over-fill our main water tank, with the result that the water we drink is always fresh; the surplus water is diverted to showers, gardens, etc. This fall we hooked up a series of hoses to bring the surplus water down to Vermadise where it's being used to steadily flush the concrete tank that we installed this fall to be the heart of our mini-aquaponics system.

     The process of curing concrete involves a chemical reaction that affects the pH of the water, something which would harm fish living in the tank. By keeping fresh water flowing through the tank this winter, the chemical reaction should be pretty much finished by spring and the tank sufficently cured to take its first stocking of fish.

     As you can see in the pic, even though most everything else here was turned into a sheet of ice by the cold, the low-grade geothermal heat in the water was enough to not only keep the water running through the hose, but to melt back the snow for a couple of inches on each side.

the "heat" in the hose has melted the snow around it

     I understand that it's hard to think of water that's 55° F as "hot" but when the outside temps are down in the teens, that water is delivering 320 BTUs per gallon, and since it's running at about 12 gallons an hour, we're talking about a kilowatt of heat per hour around the clock.

     And what's especially nice about this sort of energy source is that the more you need it (i.e. the colder it gets outside), the more effective it is.

Notes From Windward - Index - Vol. 66