Heat generation
Posted on October 21, 2012 by
Heat generation
There are a wide variety of heating systems on the market that cover a wide range of cost and efficiency. Lets separate the discussion in two halves. Heat generation and heat delivery.
Heat generation can typically be broken up into combustion, electrical and solar systems. Combustion systems burn something to release heat chemically (rapid oxidation). Electrical systems can use “resistive heating” or can use the electricity to drive a heat pump and extract the heat thru the phase change of a liquid. There are newer electric heating systems that use other methods. And, obviously, solar systems use various methods to extract energy from the solar radiation that falls freely on your “collector”.
Combustion
Combustion heating is about burning things. It is really an exothermic reaction where the carbon based fuel (wood, pellets, corn, coal, propane, natural gas, money, etc.) is combined with oxygen and “burns” releasing (among other things) carbon dioxide and a lot of energy in the form of heat and light.
Many earth sheltered home owners prefer the idea of the wood stove due to its grid-independence and low cost. Wood stoves are far better than a fire place as a heating system, but they are still far from perfect. Good wood stoves actually cost quite a lot of money (not free heat). They don’t usually distribute the heat evenly over time or space and are not thermostatically controlled, they need to be manually loaded and maintained. They take up living space, both for the stove, the safety region around it and the wood pile (indoor and out). While many people love everything associated with chopping, splitting, hauling and stacking wood, others may find that to be time consuming or tiring and don’t like that the wood brings bugs and dust into the house, etc.
“Chop your own wood, it will warm you twice” ~Henry Ford
Many of these inconveniences are “fixed” with various pellet stove designs that can use a small thermostat-controlled auger or chute to automatically feed pellets into the fire when needed. A quick Google search and you will find many people passionate about this modern take on the old idea. However, it also removes some of the best part about wood stoves… Instead of the independence of chopping your own wood, you must buy the manufactured pellets and you miss the charm of sitting by a nice wood fire.
In the “industrial age”, coal, oil or other “fossil” fuels with very high energy density were used to replace wood. I am assuming that you are not interested in coal or oil, but many people do prefer the convenience and cost of natural gas heating systems. Natural gas is something the United States, Canada and many other countries have been blessed with in great abundance. By some estimates, the United States has about 1000 years worth of the stuff. By comparison, it is estimated that we have less than 30 to 100 years worth of oil. Natural gas, as a utility, is usually only available in urban or suburban areas, and where available, can be hooked up to your furnace, hot water heater, cooking appliances clothes dryer, BBQ, even fancy decorative lamp posts.
Natural gas is less less efficient than electrical resistance because some of the heat energy is wasted up the chimney. In many countries, the cost of natural gas is so low that the $/BTU is less than for electrical resistance heating. Of course, this only works as long as the price of natural gas stays low relative to electricity. In Eastern Europe, the price of natural gas is relatively high and Moscow uses the threat of cutting off the supply to control the leaders of Eastern Europe. It is probable that future advances in electrical power generation will lower the price of electricity. It is also probable that the cost of non-renewable energy sources (like natural gas) will eventually go up.
No matter where you live, installing natural gas appliances is more complicated (gas lines, exhaust ports, flow regulators, burner nozzles, etc.) so they cost more to purchase and should have professional installation and maintenance.
The biggest concern with natural gas appliances is the potential for health problems, including leaks of the methane its self or combustion by-products such as carbon monoxide, nitrogen dioxide (linked to asthma), VOCs, fine soot, and many other trace chemicals, so I don’t recommend them for well sealed earth sheltered homes. If you do go with gas, make sure you add what ever extra ventilation you need to stay safe, plus a bit more.
Electrical
Resistance
Electrical resistance heating is probably the cheapest to purchase, install and maintain. It is clean and easy to control with a thermostat. Electrical resistance is much more efficient than combustion because all the heat goes where it is needed instead of up the chimney. However, due to the lower cost of some fuels (such as natural gas) electrical resistance heating can be more expensive per unit of heat generated ($/BTU).
Many earth sheltered home owners are happy with the cost structure; a very low initial expense and reasonable electrical costs on those few days when they are happy to pay for the additional heat. You can purchase electrical resistance heaters that use forced air distribution, or even in-floor radiant heat. The baseboard radiator method (electrical resistance and radiation/convection) appears to be a cheap way to go, but don’t forget that they draw a lot of amps and will require you to upgrade the wiring throughout your whole home.
The owner of the new earth sheltered home going in near Battle Creek Michigan (see these posts for more on this house) talked to an electrician about baseboard heating and decided the additional wiring costs (heavier gauge wiring is needed for the baseboard heaters) would make it cheaper to go with a centrally located electric furnace that uses the ventilation ducts he already planned to install. He expects his home to need little, if any, additional heat and plans to purchase the smallest mobile home electric furnace he can find. (less than $600)
There is the potential to generate your own electricity (off the grid), via solar or wind or water energy. This is much more likely than being able to generate your own natural gas or even pellet fuel. However, some combinations, such as solar to electric to heat, are much less efficient than directly converting solar energy to heat energy. If you do opt to generate your own electricity for heating, it will be even more imprtant to reduce your heat loss to keep the system practical.
Heat pump and Geothermal
Another way to heat with electricity is the “heat pump“. This device works like a refrigerator or air-conditioner. The electricity powers a pump that forces a “coolant” gas to be compressed into a liquid, releasing heat, in one location, and then forced thru pipes to another location where it is allowed to expand into a gas, absorbing heat, from another location. The gas continues around the loop where it is compressed again. This is really using the concept of the latent heat of evaporation where the phase change is able to absorb a lot of energy. Many homes use air to air heat pumps that extract the heat from outdoor air (the evaporator is outside). These systems cost less than a small woodstove for the unit, plus additional costs for forced air ducting. They can easily be controlled by a thermostat and they can provide heating and cooling (just the heating cycle in reverse). The problem with this approach is that the air outside is coldest when you need the most heat. This makes the system work much harder and reduces the efficiency of the process. Many air to air heat pumps actually have an electrical resistance heater built in as a back up for cold days… In an earth sheltered home in a norther area, those could be the only days when you need the heater to work. Therefore you may end up purchasing a fancy heat pump and only ever using the electrical resistance portion of it.
Earth sheltered homes go better with geothermal heat pumps. These are not truly geothermal (the heat isn’t really from the earth), but they do take advantage of the heat capacity of the earth as one giant solar-assisted thermal capacitor. A “geothermal” heat pump is much more efficient than other heating/cooling systems because the electricity is only used to move heat, not generate it. They take advantage of the same principles as Earth Sheltered homes, the earth is much warmer than the outside air in the middle of winter and much cooler in the heat of summer.
From the buried heat exchange tubes to the compressor unit that goes in your house, it is probably the most expensive heating system to install. Costs are more moderate for new construction though. After a Geothermal system is installed, it has a much longer life than other systems and, due to its lower operating costs, will eventually pay its self off.
For more on the costs of Geothermal, see this sourcing page.
The most efficient types of heat pump work just like the air to air heat pumps, except that instead of wrapping the outside heat exchanging coil around a fan behind your house, it is stretched out and buried in the soil. This site for the International Ground Source Heat Pump Association (a more accurate name for GeoThermal) has a great blurb on the invention and history of this method of heating. Here is a copy…
History: Ground source heat pump technology is the wave of the future, but the concept isn’t new at all. In fact, Lord Kelvin developed the concept of the heat pump in 1852. In the late 1940’s, Robert C. Webber, a cellar inventor, was experimenting with his deep freezer. He dropped the temperature in the freezer and touched the outlet pipe and almost burned his hand. He realized heat was being thrown away, so he ran outlets from his freezer to his boilers and provided his family with more hot water than they could use! There was still wasted heat, so he piped hot water through a coil and used a small fan to distribute heat through the house to save coal. Mr. Webber was so pleased with the results that he decided to build a full size heat pump to generate heat for the entire home. Mr. Webber also came up with the idea to pump heat from underground, where the temperature doesn’t vary much throughout the year. Copper tubing was placed in the ground and freon gas ran through the tubing to gather the ground heat. The gas was condensed in the cellar, gave off its heat and forced the expanded gas to go through the ground coil to pick up another load. Air was moved by a fan and distributed into the home. The next year, Mr. Webber sold his old coal furnace.
In the forties, the heat pump was known for its superior efficiency. The efficiency was especially useful in the seventies. The Arab oil embargo awakened conservation awareness and launched interest in energy conservation despite cheap energy prices. That is when Dr. James Bose, professor at Oklahoma State University, came across the heat pump concept in an old engineering text. Dr. Bose used the idea to help a homeowner whose heat pump was dumping scalding water into his pool. Dr. Bose fashioned the heat pump to circulate the water through the pipes instead of dumping the water into the pool. This was the beginning of the new era in geothermal systems. Dr. Bose returned to Oklahoma State University and began to develop his idea. Since then, Oklahoma has become the center of ground source heat pump research and development. The International Ground Source Heat Pump Association was formed in Oklahoma, and is based on the campus of Oklahoma State University, where Dr. Bose serves as executive director.
The original “Ground Source Heat Pumps” only needed two exchangers one in the ground and one in the home. They pumped the refrigerant (ground safe) directly between the two. This approach has several advantages including less hardware (exchangers, fans, pumps), less electricity required and shorter tube length (less digging, etc.) The down side is that the buried pipes must be made of something like copper, which must then be protected against corrosion in acidic soils. Still, these refrigerant systems tend to be cheaper to manufacture, install and repair and are at least 25% more efficient to operate (some companies like EarthLinked claim 50% more efficient) than Water or glycol based “GeoThermal”, which are already 350 to 400% more efficient than electrical resistance heat.
I am not sure why Dr. Bose decided to go with water-based-GeoThermal, but perhaps it was due to concerns about corrosion of the buried pipe leading to refrigerant leaks. Both problems have been solved now. In these systems, you need 3 exchangers. Two of the exchangers (condenser and evaporator) are in the unit in the house (instead of one being outside or buried), but thermally connected to the earth thru a second pump pushing a fluid (water or glycol, no phase change) thru hundreds of feet of buried pipe. The longer pipe is needed to make up for the less efficient heat exchange, but it may also help prevent the soil from becoming saturated. The additional exchange and extra pump required for the water based systems do reduce their efficiency and increase the cost, size and complexity.
Either way, heat pump systems are much more efficient than electrical resistance heating. Air to Air systems are 2 or 3 times more efficient, and geothermal systems can be 3.5 to 5.5 times as efficient. Air to air systems have been made more efficient with earth tubes to supply the air. Essentially, the condenser is brought underground (inside) and fed earth warmed air thru earth tubes. This is how Russel Finch heats his home and green houses in Nebraska.
Oranges in Nebraska; “Every time an expert tells me it can’t be done, I sit back, peel one of my oranges, and marvel at my ability to eat something that is impossible to grow in Nebraska.” ~ Russel Finch (A very similar article) (Citrus in the Snow website)
For above ground homes, Geothermal companies typically argue that although the systems are more expensive to install, they will save you money over the long run. They usually ask you for a lot more information than they need to design your system because they need that extra info to work out the pay back period on the investment. They seem to spend more time on the this pay back equation than anything else because, without it, the sticker shock would scare away most customers. However, if a normal home has a pay back period (additional initial cost divided by energy savings per month adjusted for inflation and the current interest rate) of ~7 years, that same system in an earth sheltered home, with its much lower energy needs, and therefore proportionally smaller energy savings, would need many more years for payback… The more efficient your home is, the less likely the savings will ever justify the initial costs. Of course, that assumes the same size system… If you can get them to install a much smaller system, you may be able to get the math to work for you, but the cost does not scale much with the size of the sytem.
Solar
Solar can be much more than passive solar. Solar voltaic is very inefficient for home heating, but solar hot water can be very efficient and effective. It has everything your banker is looking for, in terms of reliability and thermostat controls. It can be hooked up to a variety of distribution methods including baseboard radiators, radiant floor or even forced air systems.
The basic concept and setup of a solar hot water system is covered on another page, but it doesn’t take much of a stretch to imagine increasing your solar hot water capacity and pumping it thru the floors to provide heat for an earth sheltered home. Working with solar hot water companies, they have generally increased my array from 2 or 3 to 5 panels when I asked for radiant floor heating. These panels are between $600 and $900 each, which seems reasonable for a perpetual heat source. Back up heat is usually in the form of an electrical resistance or natural gas hot water heater, which hopefully wouldn’t be needed very often in a well designed earth sheltered home where we could store the days heat for use overnight.
For costs of going solar, see the sourcing solar section.
