Saturday, August 14, 2010

Heat pumps move heat from one place to another - from outside to inside a home, for example. That's why they're called "heat pumps."
We explained the way that they work in the section "Central HVAC." Here's a simplified version of how a heat pump works:
All heat pumps have an outdoor unit (called a condenser) and an indoor unit (an evaporator coil).
A substance called a refrigerant carries the heat from one area to another. When compressed, it is a high temperature, high-pressure liquid. If it is allowed to expand, it turns into a low temperature, low pressure gas. The gas then absorbs heat.
In the winter the normal heat pump system extracts heat from outdoor air and transfers it inside where it is circulated through your home's ductwork by a fan.
Even cold air contains a great deal of heat; the temperature at which air no longer carries any heat is well below -200 degrees Fahrenheit. But the coldest temperature ever recorded in the lower 48 states was -70 degrees, recorded at Roger Pass, Montana on January 20, 1954. Obviously in such weather, a heat pump would have to work pretty hard to produce 68-degree temperatures inside your home.
That's why geothermal heat pumps are so efficient.
Geothermal heat pumps are similar to ordinary heat pumps, but instead of using heat found in outside air, they rely on the stable, even heat of the earth to provide heating, air conditioning and, in most cases, hot water.
From Montana's minus 70 degree temperature, to the highest temperature ever recorded in the U.S. - 134 degrees in Death Valley, California, in 1913 - many parts of the country experience seasonal temperature extremes. A few feet below the earth's surface, however, the ground remains at a relatively constant temperature. Although the temperatures vary according to latitude, at six feet underground, temperatures range from 45 degrees to 75 degrees Fahrenheit.
Ever been inside a cave in the summer? The air underground is a constant, cooler temperature than the air outside. During the winter, that same constant cave temperature is warmer than the air outside.
That's the principle behind geothermal heat pumps. In the winter, they move the heat from the earth into your house. In the summer, they pull the heat from your home and discharge it into the ground.
Studies show that approximately 70 percent of the energy used in a geothermal heat pump system is renewable energy from the ground. The earth's constant temperature is what makes geothermal heat pumps one of the most efficient, comfortable, and quiet heating and cooling technologies available today. While they may be more costly to install initially than regular heat pumps, they can produce markedly lower energy bills - 30 percent to 40 percent lower, according to estimates from the U.S. Environmental Protection Agency, who now includes geothermal heat pumps in the types of products rated in the EnergyStar® program. Because they are mechanically simple and outside parts of the system are below ground and protected from the weather, maintenance costs are often lower as well.
As an added benefit, systems can be equipped with a device called a "desuperheater" can heat household water, which circulates into the regular water heater tank. In the summer, heat that is taken from the house and would be expelled into the loop is used to heat the water for free. In the winter, the desuperheater can reduce water-heating costs by about half, while a conventional water heater meets the rest of the household's needs. In the spring and fall when temperatures are mild and the heat pump may not be operating at all, the regular water heater provides hot water.
How Do They Compare?
Surveys taken by utilities have found that homeowners using geothermal heat pumps rate them highly when compared to conventional systems. Figures indicate that more than 95 percent of all geothermal heat pump owners would recommend a similar system to their friends and family. 

As a rule of thumb, a geothermal heat pump system costs about $2,500 per ton of capacity. The typically sized home would use a three-ton unit costing roughly $7,500. That initial cost is nearly twice the price of a regular heat pump system that would probably cost about $4,000, with air conditioning.
You will have to, however, add the cost of drilling to this total amount. The final cost will depend on whether your system will drill vertically deep underground or will put the loops in a horizontal fashion a shorter distance below ground. The cost of drilling can run anywhere from $10,000 to $30,000, or more depending on the terrain and other local factors.
Added to an already built home an replacing an existing HVAC unit, an efficient geothermal system saves enough on utility bills that the investment can be recouped in five to ten years. 

Geothermal heat pumps are durable and require little maintenance. They have fewer mechanical components than other systems, and most of those components are underground, sheltered from the weather. The underground piping used in the system is often guaranteed to last 25 to 50 years and is virtually worry-free. The components inside the house are small and easily accessible for maintenance. Warm and cool air are distributed through ductwork, just as in a regular forced-air system.
Since geothermal systems have no outside condensing units like air conditioners, they are quieter to operate.
How Do They Work?
Remember, a geothermal heat pump doesn't create heat by burning fuel, like a furnace does. Instead, in winter it collects the Earth's natural heat through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond or lake. Fluid circulates through the loop and carries the heat to the house. There, an electrically driven compressor and a heat exchanger concentrate the Earth's energy and release it inside the home at a higher temperature. Ductwork distributes the heat to different rooms.
In summer, the process is reversed. The underground loop draws excess heat from the house and allows it to be absorbed by the Earth. The system cools your home in the same way that a refrigerator keeps your food cool - by drawing heat from the interior, not by blowing in cold air.
The geothermal loop that is buried underground is typically made of high-density polyethylene, a tough plastic that is extraordinarily durable but which allows heat to pass through efficiently. When installers connect sections of pipe, they heat fuse the joints, making the connections stronger than the pipe itself. The fluid in the loop is water or an environmentally safe antifreeze solution that circulates through the pipes in a closed system.
Another type of geothermal system uses a loop of copper piping placed underground. When refrigerant is pumped through the loop, heat is transferred directly through the copper to the earth. 

Types of Loops
Geothermal heat pump systems are usually not do-it-yourself projects. To ensure good results, the piping should be installed by professionals who follow procedures established by the International Ground Source Heat Pump Association (IGSHPA). Designing the system also calls for professional expertise: the length of the loop depends upon a number of factors, including the type of loop configuration used; your home's heating and air conditioning load; local soil conditions and landscaping; and the severity of your climate. Larger homes requiring more heating or air conditioning generally need larger loops than smaller homes. Homes in climates where temperatures are extreme also generally require larger loops.
Here are the typical loop configurations:

Horizontal Ground Closed Loops

This type is usually the most cost effective when trenches are easy to dig and the size of the yard is adequate. Workers use trenchers or backhoes to dig the trenches three to six feet below the ground in which they lay a series of parallel plastic pipes. They backfill the trench, taking care not to allow sharp rocks or debris to damage the pipes. Fluid runs through the pipe in a closed system. A typical horizontal loop will be 400 to 600 feet long for each ton of heating and cooling.

Vertical Ground Closed Loops

This type of loop is used where there is little yard space, when surface rocks make digging impractical, or when you want to disrupt the landscape as little as possible. Vertical holes 150 to 450 feet deep - much like wells - are bored in the ground, and a single loop of pipe with a U-bend at the bottom is inserted before the hole is backfilled. Each vertical pipe is then connected to a horizontal underground pipe that carries fluid in a closed system to and from the indoor exchange unit. Vertical loops are generally more expensive to install, but require less piping than horizontal loops because the Earth's temperature is more stable farther below the surface.

Pond Closed Loops

This type of loop design may be the most economical when a home is near a body of water such as a shallow pond or lake. Fluid circulates underwater through polyethylene piping in a closed system, just as it does through ground loops. The pipes may be coiled in a slinky shape to fit more of it into a given amount of space. Since it is a closed system, it results in no adverse impacts on the aquatic system.
Although they are less applicable to California, there are other loop systems described at the Geothermal Heat Pump Consortium's Web Site. These include an Open Loop System in which ground water is pumped into and out of a building, transferring its heat in the process; and Standing Column Well Systems, which can be up to 1,500 feet deep and can also furnish potable water.
In a few places, developers have installed large community loops, which are shared by all of the homes in a housing project.
To date, geothermal heat pumps are an under-used technology, merely because few people are aware of it's potential. The Department of Energy's Office of Geothermal Technologies, however, wants to increase installations of geothermal systems to about 400,000 a year by 2005. If the goal is reached, that would mean that 2 million systems would be in service, saving consumers over $400 million per year in energy bills and reducing U.S. greenhouse gas emissions by over 1 million metric tons of carbon each year.

Are Ground Source Heat Pumps (AKA Geothermal Systems) A Good Choice?

by Lloyd Alter, Toronto
Every time I write about ground source heat pumps, (like in my post Blowing Hot and Cold on Ground Source Heat Pumps) the commenters are enraged, saying " If I were a geothermal contractor or manufacturer I would have asked that this be removed for falsely conveying what geothermal has to offer " and "engineering degree would probably help too."
But I just got my licence to practice architecture thirty years ago this month and obviously don't know what I am talking about, so it is great to find a real expert, like Alex Wilson at Green Building Advisor, saying much the same thing.
jargon watch geothermal image
Right off the bat I am excited about his post, for he agrees with me about calling them ground source heat pumps instead of geothermal. (and if you want to see more people calling me an idiot, read the comments at Jargon Watch: Geothermal vs Ground Source Heat Pump) Alex writes:
A ground-source heat pump (GSHP) is also referred to as a "geothermal" heat pump, though I prefer the former terminology, to avoid confusion with true geothermal energy systems that rely on elevated temperatures deep underground from the Earth's mantle.
After that he could sell me on vinyl replacement windows, let alone GSHPs. Instead he goes on to point out:
They are rarely as efficient as promised; they advertise a Coefficient of Performance of 3.5 times that of straight resistance electric heating, but turn out to be around 2.5. (John Straube at notes that the COP rated efficiency may not include the energy required for the pump, suggesting that "This electrical energy can be significant, particularly if the loop is long, the pipes are small, or the flow resistance within the heat pump unit is large." That may account for the difference.)
Alex also notes that the GSHPs run on electricity, which for much of America comes from coal; if you go back to the source, efficiency the numbers are a lot lower. If you care about your carbon footprint, it is actually worse than running on natural gas.a commenter did the numbers:
1. Burn natural gas at 92% efficiency = 1.09 MMBtu of fuel which makes 130 lb CO2.
2. Burn #2 fuel oil at 83% efficiency = 1.2 MMBtu and 175 lb CO2
3. Run a ground-source heat pump and purchase 0.33 MMBtu of electricity which has made 140 lb CO2.
These numbers do not apply if you live in Quebec or parts of the Continent where you can purchase green energy from hydro or renewables.
But ultimately, Alex's objection is the same as mine: they are really expensive and there are better places to spend your money.
The reason I'm not a huge proponent of GSHPs is that they're really expensive. Most of the expense is due to the cost of digging trenches and laying tubing....It is not unusual to hear about GSHPs in Vermont costing as much as $35,000 for typical homes. For the same investment, one could spend $30,000 reducing heating loads (insulating, air sealing, replacing windows, etc.) and install a state-of-the-art mini-split heat pump.
Not only does natural gas have a lower carbon footprint than a coal-fired GSHP, but America is awash in the stuff, so much so that they are thinking of reversing the flow in the pipelines and exporting it to Canada. The development of shale gas in the States changes everything, and anyone who says that you are going to get a return on investment by switching your heating from gas to electricity is basically lying.
We need to invest in reducing our consumption, not in buying green gizmos. That is why I am so enamoured of Passivhaus design. There, the technology is simple: add a shitload of insulation and eliminate leaks. Reduce demand rather than switching supply. That is the path to energy independence and a lower carbon footprint.

Sustainable Colorado Copyright © 2009 Designed by Ipietoon Blogger Template for Bie Blogger Template Vector by DaPino