Direct or non-electric use of geothermal energy refers to the immediate use of the energy for both heating and cooling applications. Direct geothermal energy uses the ground within a few tens of metres below the surface to extract heat in winter for heating and to sink heat in summer for cooling. “Its ability to replace carbon emitting forms of heating and cooling has the potential to have a huge impact on reducing our carbon footprint,” says Professor Ian Johnston (University of Melbourne, Australia). “For each kilowatt of electrical energy put into a direct geothermal system, about 4 kilowatts of energy is developed for the purposes of heating and cooling, reducing the energy cost by 75%.”
How does it work?
The key element in any direct geothermal system is the ground source heat pump. The system works by circulating fluid, water or refrigerant, down pipes that are installed within building foundations or into purpose-drilled boreholes and then back to the surface again. The pump for an average house is about the size of a small bar fridge.
In winter, heat contained in the circulating fluid is extracted by a heat pump, and used to heat the building. In summer, the system is reversed, with heat taken out of the building and transferred to the fluid, depositing it underground.
Key components in a low-temperature, geothermal direct-use system includes:
- A production facility
- Downhole and circulation pumps and wells
- A mechanical system
- Transmission pipelines and distribution networks
- Heat exchangers
- Heat convectors
- Cooling systems (refrigeration)
- Peaking or back-up plants
- Disposal system
- Storage pond
- Injection well.
Main uses of direct geothermal
- Hot springs for bathing,
- Cooking food,
- Heating swimming pools and baths or therapeutic use,
- Space heating and cooling (including district heating),
- Agriculture (mainly greenhouse heating, crop drying, and some animal husbandry),
- Aquaculture (heating mainly fish ponds and raceways), and
- Providing heat for industrial processes and heat pumps (for both heating and cooling)
Types of Technology
The two most common types of downhole pumps are lineshaft pump systems and submersible pump systems. Both types have been used for many years for cold water pumping and more recently in geothermal wells.
Supply and distribution networks can consist of either a single-pipe or a two-pipe system. The single-pipe system is a once-through system where the fluid is disposed of after use. This distribution system is generally preferred when the geothermal energy is abundant and the water is pure enough to be circulated through the distribution system. In a two-pipe system, the fluid is re-circulated so the fluid and residual heat are conserved.
A two-pipe system must be used when mixing of spent fluids is called for, and when the spent cold fluids need to be injected into the reservoir. Two-pipe distribution systems cost typically 20% to 30% more than single-pipe systems.
Steel piping is used in most cases, but fiberglass reinforced plastic or polyvinyl chloride (PVC) can be used in low-temperature applications. Above-ground pipelines have been used where excavation in rock is expensive and difficult; however, below-ground installations are more common to protect the line eliminate traffic barriers.
The principal heat exchangers used in geothermal systems are the plate, shell-and-tube, and downhole types. The plate heat exchanger consists of a series of plates with gaskets held in a frame by clamping rods. The counter-current flow and high turbulence achieved in plate heat exchangers provide for efficient thermal exchange in a small volume. In addition, compared to shell-and-tube exchangers, they have the advantage of occupying less space, they can easily be expanded when additional load is added, and are typically 40% cheaper. The plates are usually made of stainless steel, but titanium can be used when the fluids are especially corrosive. Plate heat exchangers are commonly used in geothermal heating systems in the United States.
Shell-and-tube heat exchangers may be used for geothermal applications, but are less popular due to problems with fouling, greater approach temperature (the difference between incoming and outgoing fluid temperature), and the larger size as compared to the plate type.
Downhole heat exchangers eliminate the problem of disposal of geothermal fluid, since only heat is taken from the well. However, their use is limited to small heating loads, such as the heating of individual homes, a small apartment, house, or business.
Cooling can be accomplished from geothermal energy using lithium bromide and ammonia absorption refrigeration systems.
The lithium bromide system is the most common because it uses water as the refrigerant. However, it is limited to cooling above the freezing point of water. The major application of lithium bromide units is for the supply of chilled water for space and process cooling in either one- or two-stage units. The two-stage units require higher temperatures (about 320°F), but they also have high efficiency. The single-stage units can be driven with hot water at temperatures as low as 180°F. Lower geothermal water temperatures result in lower efficiency and require a higher flow rate. Generally, a condensing, or cooling, tower is required, which will add to the cost and space requirements.
For geothermally driven refrigeration below the freezing point of water, the ammonia absorption system must be considered. However, these systems are normally applied in very large capacities and have seen limited use. For the lower temperature refrigeration, the driving temperature must be at or above 250°F for a reasonable performance.
Direct geothermal resources fill many needs: space heating and cooling, greenhouse heating, industrial processing, and hot water, to name a few. Geothermal district heating systems can save consumers 30% to 50% of the cost of natural gas heating. It is important to consider using a geothermal fluid several times to maximize benefits. For example, the heat could be used for process heat and the resulting lower-temperature fluid could be used for space heating. This multi-stage utilization, where lower water temperatures are used in successive steps, is called cascading or waste heat use.
The temperature and the flow rate of the resource affect the design of the mechanical systems. Two main temperature differences influence the feasibility, flow requirements, and the design of the heating equipment.
The first is the difference between the available geothermal water and the temperature needed for the intended use or process. This difference determines the feasibility of the system. The greater the difference in temperature, the smaller the size and lower the cost of the heat transfer equipment.
The second is the difference between the temperature of the geothermal water entering the system and leaving the system. This temperature difference determines the geothermal water flow rate necessary to provide the heat to the system. Larger temperature differences enable lower flow rates through the system, but the leaving geothermal temperature cannot be lower than the process temperature.
Current developments and trends
The technology is widely used in other parts of the world. It has been estimated that there are over 2 million direct geothermal installations in operation around the world, principally in northern Europe and North America.
There are many examples of much larger direct geothermal energy applications.
Geothermal potential in Australia
A significant amount of electrical power in Australia is generated with brown coal. Replacing 75% of this with a totally clean renewable energy source would reduce greenhouse gas emissions to as little as 25% of what occurs with current practice.
Currently limited use in Australia.
What are the advantages?
- Renewable and low emissions
- More reliable than wind and solar as it is able to operate continuously with little to non fluctuations of energy flow (constantly available)
- Relatively maintenance free
- Cost of instalment can be recovered through reduced energy bills
- Low emissions
- Significant savings means that capital costs can be recovered in a few short years compared to using traditional heating and cooling systems
- Geothermal energy can be used for a wide array of different purposes
- Unlimited potential
What are the disadvantages?
- Capital costs of installing are high compared to current energy prices
- While geothermal heating and cooling systems can save a lot of money in the long run, the initial costs can be quite expensive. It is important to view an installation like this as a long-term investment and not a quick way reduce energy costs. On the other hand, geothermal heat pumps are often more economical and has a quicker payback time than alternatives such as solar energy
- Need a relatively large area to be able to dig out and lay pipes for a geothermal system. These systems can therefore not be installed in all buildings
- While the potential of geothermal energy is massive, the initial cost has to be compared to how much you potentially could save by investing. Even though the energy is dirt-cheap, there will be a time before the initial investment is paid back.
What are the issues to economically developed?
- Need build industry to tailor to needs, and with better systems of design and installation, prices should fall significantly over the next few years.
What are the technical issues to development?
- Large number of installations worldwide, driven by the heating, ventilation and air conditioning industry but little technical input from geotechnical engineers which results in approximate guidelines on installations to provide the energy required. Better technical information would make systems could more cost effective and competitive.
What are the institutional and political challenges?
- Lack of awareness
- Shortage of qualified workers
What are the social and environmental challenges?
- Health, safety and environmental concerns,
- Public opposition due to visual and odour related impacts