Geothermal Energy Systems
Building Blocks of Sustainability
Engineers' Corner
Benefits of geothermal systems
Benefits of geothermal systems
Please choose one of these topics:
Thermal efficiency
The net thermal efficiency of a heat pump should take into account the efficiency of electricity generation and transmission, typically about 40%. Since a heat pump takes heat from the ground, the total heat energy output to the building is greater than the electricity input. This results in thermal efficiencies greater than 100%, up to around 150%.
Net thermal efficiency tends to be confusing to consumers, so heat pump performance is generally expressed as the ratio of heat output to electricity input. An allowance is included for electricity used by the fluid pumps. Cooling performance is typically expressed in units of BTU/hr/Watt as the Energy Efficiency Ratio, (EER) while heating performance is typically reduced to dimensionless units as the Coefficient of Performance. (COP) The conversion factor is 3.41 BTU/hr/Watt. Both of these measures will vary depending on the temperature difference between the ground source and the building, which can vary greatly between installations and over the course of the year.
For the sake of comparing ground source heat pump appliances to each other, independent of installation variations, a few standard test conditions have been established by the American Refrigerant Institute (ARI) and more recently by the International Standards Organization. A Standard ARI 330 ratings were intended for closed loop ground-source heat pumps, and assumes secondary loop water temperatures of 77°F for air conditioning and 32°F for heating. These temperatures are typical of installations in the northern USA. Standard ARI 325 ratings were intended for open loop ground-source heat pumps, and include two sets of ratings for groundwater temperatures of 50°F and 70°F. ARI 325 budgets more electricity for water pumping than ARI 330. Neither of these standards attempt to account for seasonal variations. Standard ARI 870 ratings are intended for direct exchange ground-source heat pumps. ASHRAE transitioned to ISO 13256-1 in 2001, which replaces ARI 320, 325 and 330. The new ISO standard produces slightly higher ratings because it no longer budgets any electricity for water pumps.
Residential ground source heat pumps on the market today have COP's ranging from 2.4 to 5.0 and EER's ranging from 10.6 to 30. To qualify for an Energy Star label, heat pumps must meet certain minimum COP and EER ratings which depend on the ground heat exchanger type. For closed loop systems, the ISO 13256-1 heating COP must be 3.3 or greater and the cooling EER must be 14.1 or greater. Many factors affect the efficiency of ground source heat pumps. COP improves with a lower delta-T, the temperature difference between the input and output of the heat pump. The volume of ground that is used must be large enough that the heat addition or subtraction does not cause a significant change in temperature. Efficient compressors, variable speed compressors and larger heat exchangers produce more efficient heat pumps. Soaker hoses are sometimes used to wet the ground to increase heat transfer.
Soil without artificial heat addition or subtraction and at depths of several meters or more remains at a relatively constant temperature year round. This temperature equates roughly to the average annual air-temperature of the chosen location. It is usually 7-12°C (45-54°F) at a depth of six meters in locations where heating is needed in winter. Ground-source heat pumps rely on this near constant temperature as a base temperature that is raised or lowered minimally to create a desirable indoor temperature. The soil temperature will vary as heat is added or removed. Because this temperature remains more constant than the air temperature throughout the seasons, geothermal heat pumps perform with far greater efficiency and are stressed less during extreme air temperatures than fueled or electric conventional air conditioners and furnaces. A particular advantage is that they can use electricity to heat spaces and water much more efficiently than an electric heater.
Engineering notes
The specified and tested manufacturer that GES uses is Water Furnace International as all their equipment is ARI rated, and meets ISO 13256-1 and Energy Star requirements. For example, the Water Furnace NDH038, has a minimum EER rating of 32.1 which equates to a minimum rating of 6.5-7 COP.
In reality, a heat pump is nothing more than a refrigeration unit. Any refrigeration device (window air conditioner, refrigerator, freezer, etc.) moves heat from a space (to keep it cool) and discharges that heat at higher temperatures. The only difference between a heat pump and a refrigeration unit is the desired effect--cooling for the refrigeration unit and heating for the heat pump. A second distinguishing factor of many heat pumps is that they are reversible and can provide either heating or cooling to the space.
One of the most important characteristics of heat pumps, particularly in the context of heating/cooling, is that the efficiency of the unit and the energy required to operate it are directly related to the temperatures between which it operates. In heat pump terminology, the difference between the temperature where the heat is absorbed (the "source") and the temperature where the heat is delivered (the "sink") is called the "lift." The larger the lift, the greater the power input required by the heat pump. This is important because it forms the basis for the efficiency advantage of the geothermal heat pumps over air-source heat pumps. An air-source heat pump must remove heat from cold outside air in the winter and deliver heat to hot outside air in the summer. In contrast, the GHP retrieves heat from relatively warm soil (or groundwater) in the winter and delivers heat to the same relatively cool soil (or groundwater) in the summer.
As a result, geothermal heat pump, regardless of the season is always pumping the heat over a shorter temperature difference than the air-source heat pump. This leads to higher efficiency and lower energy use. In fact, for every one unit of electricity the system consumes, four units of energy are provided free from the earth, delivered to the site.
1 kW of energy or electricity, equates to 4 units of energy free from the earth (COP of 4)
GSHPs operate at peak COP at lower ground temperatures. What makes this system so efficient is that it is a closed loop system and works on the second law of thermodynamics of hot to cold. Unlike cooling towers and chiller systems, where there is a need to carry huge amounts of latent heat to the evaporation towers, the geoexchange system is known as a balanced passive system. What occurs is that the small difference in the load temperature and the incoming constant temperature in the ground - even if only 1 degree - will result in a small delta-t. The diffusivity of heat at much lower temperature delta-t, (passive system is unaffected by ambient changes such as air temperature, wind etc.) will drive the COP up. In contrast, we need to invest large amounts of energy (large delta-t) to drive the heat up and down in tower and boiler systems, which are also adversely affected by the outside ambient air temperatures , wind etc.
Prior to the implementation of commercial/industrial GSHP systems, conductivity tests are done to determine the ground temperature. The lower the measured temperature, which is typically around 12°C in the Eastern Cape, the higher the resultant COP.
In addition, if one takes a GSHP system running with a lower operating temperature, and linked to part loading operating performance, because the system stays in cooling cycle most of the time without having to deal with off-peak drops in temperatures as is the case with tower systems, but has constant temperatures all the time of 12 to 18 degrees, the system GSHP operating with a basic starting COP of 6.5, and with the right variable speed motors , correct shut off load times on the geoexchange, (which no other system can do), as well, as the fact that on rejected heat, during the summer months, we can part load the desup heaters, and get free hot water.
Furthermore, when GS heat pumps are used instead of air handlers as in a VAV system as well as certain other systems, the GSHP has an advantage over a coil system as such further increase the COP.
It should also be noted that the earth has a built in time delay. During those times of the year when the cooling requirements are the highest, the earth’s temperature is still cool, allowing easy absorption of excess heat into the ground. Towers and other systems, where, when the temperatures are stagnant and get either too hot or too cold, result in either the close cooler circuit dropping the set point or the opposite where the boiler must be fired up to raise the loop temperature. This is not needed in the geothermal system. Wind, fluctuations in temperature or any other factor do not affect it in the least.
There is also an equivalent to cold storage in geothermal terms. An additional loop is designed in the ground for part storage, and by driving the heat pumps at low loads during off peak times and storing the energy, the COP can go as high as 14 to 20.
On the GSHP, the compressors operate much more efficiently than those of any other heat pumps as there are less highs and lows. In addition air is needed to be moved on only one side of the GSHP less power is needed to move the liquid on the other side than is needed to move the air. There is no need for defrost cycles or back up heat either. As the system as described above is balanced, no net air-conditioning is required.
Environmental impact
The U.S. Environmental Protection Agency (EPA) has called ground-source heat pumps the most energy-efficient, environmentally clean, and cost-effective space conditioning systems available. Heat pumps offer significant emission reductions potential, particularly where they are used for both heating and cooling and where the electricity is produced from renewable resources.
Ground-source heat pumps have unsurpassed thermal efficiencies and produce zero emissions locally, but their electricity supply almost always includes components with high greenhouse gas emissions. Their environmental impact therefore depends on the characteristics of the electricity supply. In South Africa most of our electricity is generated through the burning of coal with very high greenhouse gas (GHG) emissions. The consequences for the use of ground-source heat pumps are therefore twofold.
On the negative side, since electricity is needed to drive the pumps, an emission component is present.
On the positive side, since there is no other requirement for electricity and the energy which is being moved to or from the earth does not require any other form of input energy, the savings directly and indirectly on GHG emissions and consequent reduction in carbon footprint, are large and generally calculated as between 40 to 70%.
Therefore ground-source heat pumps always produce less greenhouse gases than air conditioners. In large installations this also may also result in the potential to participate in carbon credit trading.
The fluids used in closed loops are designed to be biodegradable and non-toxic and are usually water or a water/glycol mix. The refrigerant used by GES in the heat pump cabinet is R410A.
Economics
Ground-source heat pumps are characterised by high capital costs and low operational costs compared to other HVAC systems. Their overall economic benefit depends primarily on the relative costs of electricity and fuels, which are highly variable over time. Based on recent prices, ground-source heat pumps currently have lower operational costs than any other conventional heating source almost everywhere in the world. In general, a savings of anywhere from 20% to 70% annually on utilities can be achieved by switching from an ordinary system to a ground-source system
The life span of the system is longer than conventional heating and cooling systems. Many early systems are still operational today after 25–30 years with routine maintenance. Most loop fields are warrantied for 25 to 50 years and are expected to last even longer.
Copyright © GES (Pty) Ltd. 2015