From the Integration of Renewable Energy on Farms Website

 

http://www.farm-energy.ca/IReF/index.php?page=earth-energy

 

Earth Energy

 

Ground Source Heat Pump 

With the cost of energy steadily increasing, finding ways of reducing energy use and alternate energy sources is becoming increasingly important. Ground source heat pumps have been around since the turn of the century and have been used widely in Europe . The economics are driving the geothermal industry growth by 30-40% per year. Today’s geothermal installations are not only cost-effective, they are also comfortable, quiet and environmentally friendly.

The agricultural sector is an ideal candidate for geothermal technology. Access to a sufficient amount of land is generally not a problem and electricity is available on-site. The energy from the ground loop can be transferred to the farm residence, workshops, storage shops, pig barns and many other applications in the agricultural sector. Installing geothermal energy in the farming sector can reduce heating and cooling costs by as much as 70% in some cases, which will bring down the operating costs and thus increase revenue.

Single-phase power is common for farms, but we are seeing more three-phase power brought to the yards with higher electrical load. Single-phase ground source heat pumps are limited to about 60,000 Btu/h (32°F EWT) output and with the larger farm loads, multiple units must be used. In farmyards that have three-phase power, larger geothermal equipment can be used. (A 1400-sq. ft. bungalow with a full basement would require approximately 30,000 to 35,000 Btu/h or a 40 X 60 shop with R20 walls and R40 ceiling and insulated slab would require approximately 40,000 to 50,000 Btu/h without any ventilation load.)

As energy costs continue to rise and emissions into the environment continue to be of great concern, selecting and installing geothermal equipment is not only a good monetary decision for the owner, but it is also a good environmental decision for the community.

A U.S. Department of Energy (DOE) study found that a geothermal system has lower total emissions, even when the emissions produced at the electric generating plant are included in the calculations.

Today, a geothermal system should be considered for any structure with a heating and/or cooling load.

Technology

Technology

Ground source heat pump systems draw energy from the relatively moderate and consistent temperatures found just below the Earth’s surface. Nearly 47 percent of the sun’s energy that reaches the Earth is absorbed into the ground. This allows the Earth to maintain warmer ground temperatures than the air temperatures in the winter, and cooler ground temperatures in the summer, offering a perfect environment from which to draw energy.

 

There are other heat pumps that draw their energy from the air (air to air heat pumps). The performance of an air source heat pump declines as the temperature of the air decreases; below –7°C, the heat pump performance is equal to the input energy and a backup system must be used for heating. This limitation doesn’t exist for systems extracting heat from the ground, with the groundwater or water body at least 8 feet in depth.

 

Although the heat transfer process may seem complicated, the heat pump system is similar to a refrigerator. It’s simply a process of moving heat energy from one point to another using heat exchangers. During heating, the heat pump extracts heat from the earth loop (or well water) and, using a refrigeration process, the heat is intensified and transferred to the air, which is then delivered into the home. During cooling, the process is reversed. Hot air is removed from the home by the cool refrigerant, and then transferred to the ground.

 

Transferring the low temperature heat from the earth is made easy with the use of refrigerants. As technology improves, newer refrigerants are being designed that provide wider temperature ranges and are more environmentally friendly. Because the ground loop is designed to operate below the freezing point, the closed ground loop must have an anti-freeze solution. A few of them are methanol, ethanol and propylene glycol, each mixed with water to achieve a freeze point of –10°C.

 

Heat Pump Unit

The heat pump unit can heat or cool by forced air. There are also units that heat water for hydronic floor heat or fan coils. The forced air sizing can range from 7,600 BTU/HR to 220,000 BTU/HR (GLHP) and hydronic units range from 5,000 BTU/HR to 462,000 BTU/HR (GLHP). The efficiency of 500 percent or 5 - C.O.P (Coefficient of Performance) can be reached in some installations.

 

Loop Types

The ground loop is a type of heat exchanger that's purpose is to extract heat from the ground, the groundwater or water body at least 8 feet in depth, is referred to as a loop and can be installed in various ways:

 - Vertical loop

 - Horizontal loop

 - Pond loop 

- Open loop

 

Vertical Loop

Vertical loops are used when space is limited and, or soil disturbance is a problem. Holes are bored using a drill rig. A pair of pipes with special u-bend fittings are inserted into the holes. The holes are sealed with a grout. About 200 feet of bore/per 10,000 BTU/HR (heating), (heavy saturated soil conditions).

 

Horizontal loop

Used where adequate land is available and soil conditions are favorable. Horizontal loops involve trenches that are dug using a backhoe, track hoe or chain trencher. Horizontal directional boring is also an option. Polyethylene pipes are inserted and the trenches are backfilled. Some types of horizontal boring have to be grouted to ensure pipe contact with the surrounding ground. About 250 ft of trench/per 10,000 BTU/HR (heating), (7 ft depth, 2 pipe configuration with 2 foot spacing and heavy damp soil).

 

Pond Loop

A pond loop consists of a series of closed loops coiled or slinky style, to be sunk to the bottom of an adequately sized body of water. The pond should be at least 8 feet in depth. About 300 ft of pipe/10,000 BTU/HR (slinky formation).

 

Open Loop

An open loop is used where there is an abundant supply of quality well water. Two wells are required. One for the supply of the water and one for the return of the water. The spacing between wells should be approximately 100 feet. 2 US/GPM @ 44 deg/F per/10,000 BTU/HR. The well supply should be more than required in case the production of the well drops over time.

 

In ground water (open loop) situations where scaling could be heavy or where biological growth such as iron bacteria will be present, a closed loop system is recommended, The heat exchanger coils in groundwater systems may, over a period of time, lose heat exchange capabilities due to a buildup of mineral deposits inside. The ground water temperature should also be a minimum temperature of 6 degrees Celsius. With well water there is a tendency for regular maintinance not only with the heat pump but also with the well pump. It is also important to size the well pump for the system; too much water can be as problematic as not enough water. With the development of variable speed pump, many of the pumping problems like water volume and power consumption have been minimized.

 

Soil & Flow Characteristics

 

Soil Characteristics

It is important to note that the ground loop (source) must be matched for the equipment selected. The soil conditions determine the system's heat transfer capability. Heavy soil performs better than lighter soils. Moisture content is also a factor. Saturated soils perform better than dry or moist soils. During the cooling season, heat rejected to the damp earth will dry the soil and reduce its heat transfer ability (residential cooling loads in Canada have not been a concern). In the winter, freezing soil-moisture around the piping releases a large number of BTU's due to the release of latent heat from the moisture in the soil changing from 0° C water to 0° C ice. Freezing allows the extraction of energy from the soil without the normal drop in soil temperature in the pipe's vicinity. These extra BTU's keeps the loop at or around 0 degree Celsius. Which is very good.

 

Soils can be classified as the follows:

 · Dry, light soil. 

· Damp, light soil. 

· Dry, heavy soil. 

· Damp, heavy soil. 

· Wet soil (rock has similar thermal properties).

 

Flow characteristics

Also important in the design of the loop is flow characteristics. The flow of liquid within a pipe can be considered either Laminar, with the fluid molecules traveling in a straight line, or turbulent, where mixing occurs. The best heat transfer occurs when the flow is turbulent.

 

Air in the pipes

Because the ground loop is connected underground or water, the design must also include the ability to flush the air out of the pipes while filling.

 

The heating/cooling sustainability

The heating/cooling sustainability, of a properly sized ground loop can last indefinably due to the fact that 47% of the suns energy that reaches the Earth is absorbed. This combined with air conditioning (cooling) that injects heat back into the loop will be close to the same earth temperature as first installed or can even be higher if a large air conditioning (cooling) load is present.

 

BOS

Monitoring of geothermal systems is being done throughout the country and databases are being developed at this time. 

The data that is being logged is ground temperatures, loop temperatures; power used (KW input) and average COP.

 

Life Expectancy & Challenges

 

Life Expectancy

Life expectancy for various systems according to ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers http://www.ashrae.org/) is as follows;

 Average life expectancy by equipment type: 

•Water Source Heat Pump 19 Years

•Air-Air HP/Air Conditioner 15 Years

•Furnace (gas or oil) 18 Years

One thing to note is that these numbers are dated and as the need for technology grows the Manufacturing practices are becoming more advanced and we are seeing longer equipment life. The market is also demanding the higher end equipment.

 

The design and manufacturing technology is in the mature stage.

 

The challenges

The challenges with the geothermal technology is in converting the practicing of old technologies (oil, gas) and adopting the new technologies as the standard. Also being that the geothermal industry is relatively new (25 years) there is a shortage of qualified engineers and trades people to support mass deployment at this time.

Benefits

•Heating/cooling and hot water for all types of farm buildings and homes, potential for hybrids

•A heat pump can heat and cool different buildings at the same time such as heating a farm shop and cooling potatoe storage sheds at the same time

•Heat Pumps can be supplemented with solar hot water heating

•The solar can heat the ground loop solution and can possible shorten loop length in high heating load situations. (can be beneficial where drilling cost are extremely high)

 

Examples

A farm shop located 30 miles northeast of Winnipeg , Manitoba , was built in 2001. The dimensions of the shop are as follows:

• 45 X 60 slab on grade with 16.6 ft. ceiling. 10 X 10 bathroom and rear exit added on.

• The insulation values are: walls R20, ceiling R50 and under slab R5.

• There are 2 man doors, one on the north side and one on the south side. It also has a 14 X 40 ft. overhead door on the north side and a 12 X 14 ft. overhead door on the south side. The overhead doors are rated with an R-value of R16.

• There is an open field with a low area 120 ft. east of the shop where the loop was installed. The ground loop is a horizontal type with 6 trenches 300 ft. long and 8 ft. deep. Two ¾” IPS – hidensity SDR 11 pipes were installed in each trench and joined in a header trench and connected to 2 - 11/4” IPS hidensity SDR 13.5 supply and return pipes that run 150 feet back to the shop. The connections were done with a socket fusion tool and fittings.

• Inside the shop, the supply and return lines are connected to a 2-pump flow center that has 2 Grundfos 26-116F pumps and in turn is connected to a WaterFurnace SXW060 water-to-water heat pump. A 20% methanol and 80% water solutions is used in the ground loop. The heat pump is connected and heats a storage tank, and the heated fluid is circulated to the pipe that is installed in the concrete slab.

• The WaterFurnace SXW 060 delivers 50200 BTU/HR @ 40degree F loop fluid temperature and 80 degree F storage tank fluid and operates with a COP of 4.8.

• An independent KW meter was installed on the heat pump system to measure the energy used. Over the past five years the average power (KW) used has been 8,200 kWh/year. The electrical rate is about 6 cents per kWh, which would equal approximately $500.00 dollars a year to heat the shop.

• The cost of the heat pump system with ground loop was $12,000 above a conventional electric boiler.

• With an average COP of about 4, or 400% efficient, the savings of $1,500.00 a year were achieved and a payback period of 8 years was calculated.

Application of ground source heat pumps in the Agrifood sector can offer attractive options in energy efficiency. In addition to residences, it is possible to consider their benefits in greenhouses and other buildings where they may be used in combination with in-slab radiant floor heating.

 

 

 

Comments & Recommendations

Contact local dealers with many years of experience in heat pump installation and services.