Geothermal heat pumps are one of the green technologies capable of solving the problem of global warming by reducing carbon emissions. But like most other renewables, this energy source comes with its own brand of problems. People who wish to use this technology should have a good understanding of it to avoid unnecessary costs and help prevent any negative environmental impacts.
Geothermal Heat Pumps
Geothermal heat pumps (GHPs), also called geo-exchange, work by exchanging heat with the ground at depths below a few feet, where temperatures are approximately constant year round.
Energy.gov explains the types of GHPs used in home and businesses.
- The closed loop system has three types within it. They are horizontal, vertical, and ground. They use water mixed with an antifreeze circulating in closed pipes to exchange heat with the ground or water. A heat exchanger above ground transfers heat between its refrigerants and the antifreeze solution in the closed loops.
- Direct heat exchange systems use refrigerants directly in the underground closed loops to exchange heat and do not have the intermediary heat exchanger.
- Open loop systems continuously draw water from outside sources like wells or lakes for heat exchange and return it as discharge.
An Oregon University report (p. 6) at the World Geothermal Conference in 2015, estimates there are 1.4 million GHPs in the U.S., of these 90% are closed loop systems and only 10% are open loop systems.
While there are several pros to geothermal heat pumps, there are also many cons. Some are generic and others are system-specific problems.
Everybody agrees that the initial installation cost of a GHP is high, and difficult to calculate, as it depends on the size of the house/building, the pump, the soil, climate and loop field. An experienced contractor is important to ensure successful installation.
Estimates from private enterprise Energy Homes for a 2500 square feet house show that a 6-ton Vertical loop system costs $34,000, a 5-ton Horizontal loop for radiant heating and cooling costs $29,500, and a 5-ton Horizontal loop combined with Solar heating costs $47,500.
Energy Homes breaks down the cost issue, saying, "This is around double the cost of a conventional heating, cooling, and hot water system, but geothermal heating/cooling systems can reduce utility bills by 40% to 60%."
Lack of Qualified Professionals
GHP technology is complex and requires knowledge of various aspects. The Union Of Concerned Scientists notes that many heating and cooling installers are "not familiar with the technology," which in turn hinders its spread and maintenance. It is also difficult to find qualified contractors capable of installing GHP systems in certain regions of the country, further adding to the cost of a geothermal heating system.
Not a DIY Project
The US Department of Energy discourages handling GHP as a DIY project. This technology requires specialized know-how in many areas. To decide the system best suited for a home or business a thorough examination of factors like geology, hydrology, land availability, heating and cooling requirements and other important energy-saving devices in the house is necessary. It is not possible for everybody to calculate the optimal size of loop field or pump needed to get the most out of this system.
Electricity is necessary to run the heat compressor in closed-loop systems, and for pumping up water the whole year in open loop systems, so a GHP is not completely carbon neutral.
Closed Loop System Issues
The closed loop systems share common disadvantages like impact of soils on efficiency and presence of antifreeze. Loop issues related to horizontal or vertical orientation are also present, as are concerns with direct heat exchange systems and pond systems.
Heat storage and transfer is best in heavy soils such as clay or rock. Sandy soils cannot store or transfer much heat so larger loop fields are necessary. Decrease in soil moisture below "12.5% has a devastating impact on the performance of heat pumps" states a 2014 study published in Energies (p. 3), while increase in soil moisture above 25% improves heat transfer. So dry soils are not suitable especially in direct heat exchange systems.
The closed-loop systems use water with an antifreeze for heat exchange. Older models used methanol that evaporates fast and is toxic to people and animals, so it is now banned in many parts of the U.S. Ethanol is not as toxic as methanol but is expensive. Concerns that Ethylene glycol could leak and contaminate groundwater sources has lead to this type of antifreeze also being banned for use in geothermal systems in many states. Brine (calcium chloride) is a good option, however it is corrosive, so it needs cupronickel pipes. Propylene glycol has no adverse effects on people or environment.
As long as the water mixed with the antifreeze is circulating in the closed loops there is no environmental influence. However, even small leaks can be hazardous, so it's best to stick with brine or propylene glycol types of antifreezes.
The Technical News Bulletin finds the Horizontal system requires 1,500-3,000 square feet of land for every ton of heating or cooling.
- Large area necessary - This land is later suitable only for gardening, but not any expansion of home or other building construction. These systems are not suitable for retrofit, as there may not be enough space available.
- Temperature differences - At shallow depths of 3 to 6 feet there can be differences in temperature due to season, depth of burial, and rainfall affecting efficiency, even though limiting the depth reduces cost of soil excavation which is the most expensive part of installing a closed-loop system.
- Soil issues - Rocky or shallow soils are not suitable for these systems, in which case vertical systems are necessary.
This is the most efficient system as the U-shaped loops go 150-450 feet deep in the soil, notes the Technical News Bulletin. Other issues include:
- Expense - The U-shaped loops and their depth make them the most expensive of all the GHS systems.
- Skilled installation and equipment needed - Moreover, drilling to these depths needs skilled drillers and special equipment not available everywhere.
Direct Heat Exchange System (DX)
The DX uses copper pipes filled with refrigerants buried 4 to 6 feet under the ground. This system is the oldest of all the GHP models and has the most environmental impact.
- Corrosion of copper pipes is common in acidic soils, so DX is not suitable for these soils, explains a Green Building expert. To prevent this, soil samples must be collected at the depth they will be installed to check for high concentrations of acids, chlorides, hydrogen sulphides, sulphates or ammonia, making the planning stage expensive. Copper is used instead of PVC as it is a better conductor of heat.
- Refrigerants are the major environmental problem with DX. Even small cracks could release them leading to global warming. Earlier models used chlorofluorocarbons (CFCs) and hydrochlorofluorons (HCFCs). The Montreal Protocol has banned their use as they damaged the ozone layer. Their substitutes fluorocarbons (FCs) and hydrofluorocarbons (HFCs) can cause global warming and are prohibited by the Kyoto Protocol Convention on Climate Change. In 2016, the Environmental Protection Agency (EPA) issued recommendations aimed at phasing out these chemicals and listed them as unacceptable. EPA also does not recommend R410A the latest popular refrigerant as it too causes greenhouse emissions.
- Green Building experts state it is illegal to spill the polluting refrigerants either by intent or accident.
In 2001, scientists at Oregon University (p. 2) declared DX systems an environment risk and do not recommend them. It is prohibited in some parts of the U.S., due to local environmental restrictions, according to Energy.gov.
Pond Closed Loop Systems
Closed loop systems can also use water bodies to exchange heat. However, these also have a few issues.
- Shallow waters show variations in the temperatures, and there are chances that piping could get damaged in public water sources, according to the Technical News Bulletin.
- Only ponds that have a required minimum depth and quantity of water are useful according to Energy.gov. You'll need to find a building spot with just the right conditions to utilize this option.
Open Loop System Concerns
Open systems draw water from a well or shallow waters like lakes and ponds. As noted, they are not as frequently used in the U.S., but people should still be aware of their potential disadvantages.
- Inadequate flow of water can occur if the well dug for the loop is not deep enough, or due to excessive withdrawals from the aquifer, according to a Washington State University Energy Program study (p. 5). Sedimentation clog filters in the absence of sufficient water. An Idaho Geothermal Report noted that seasonal demand for alternate uses like sprinklers in summer can affect water quantities available for the heat pump.
- Water quality is not the same everywhere and year round. Debris in lakes are a problem. Scaling due to lime deposits from heavy water needs treatment with chemicals for removal.
- Biological growth, in particular bacteria, are hard to remove once established without use of chemicals, according to the Washington State University Energy Program (p. 5).
- The Idaho Geothermal Report advises finding a suitable site for discharge before installing the open-loop groundwater system. Sandy soils can easily absorb discharge, but if the soil is hard an additional drill for discharge can double the drilling cost, making it as expensive as a closed-loop system. When water is drawn from lakes, discharge is fed back to it.
- All local restrictions regarding discharge must also be met according to Energy.gov.
- Operational costs are high, according to the Washington State University Energy Program study (p. 5) as pumps have to run the whole year to get water in and out of the system. Their maintenance is a major issue too.
- In case of wells, local environmental and water restrictions must be considered, as water available maybe limited according to the Technical News Bulletin.
- Standing column wells which pump water from an aquifer can lower the water table.
Is There A Bright Side?
While it may seem that geothermal heat pumps are difficult and costly, there are many advantages to the system. Governments and environmental nonprofits such as Greenpeace and Union of Concerned Scientists promote geothermal energy. As performance by geothermal heat pumps is interlinked to many environmental factors, it is not a plug and play technology. When considering a geothermal system, analyzing individual details of buildings and the area to choose the right system, along with proper planning and installation, are necessary steps to enjoying the best from this technology.