One of the most energy efficient methods of domestic heating is to use heat pumps. Heat pumps use electrical energy to reverse the natural flow of environmental heat from cold to hot. A typical heat pump requires only 100 kWh of electrical power to turn 200 kWh of freely available environmental heat into 300 kWh of useful heat. In every case, the useful heat output will be greater than the energy required to operate the pump itself. Heat pumps also have a relatively low carbon dioxide output, less than half that of oil, electric and gas heat production.
Heat pumps for domestic heating are a relatively new concept in Britain; however the technology is widely used in an industrial capacity. Across Europe, hundreds of thousands of domestic heat pump units are in use, and the technology is tried, tested and reliable.
Ideally, a refrigerant will have the following characteristic
- Non-toxic – for health and safety reasons
- Non-flammable – to avoid risks of fire or explosion
- Operate at modest positive pressures – to minimise pipe and component weights (for strength) and avoid air leakage into the system
- Have a high vapour density – to keep the compressor capacity to a minimum and pipe diameters relatively small
- Easily transportable – because refrigerants are normally gases at SSL conditions, they are stored in pressurised containers.
- Environmentally friendly – non-polluting and non-detrimental to the atmosphere, water or ground
- Easily re-cycleable
- Relatively inexpensive to produce.
- Compatible with the materials of the refrigeration system – non-corrosive, miscible with oil, chemically benign
In practice, the choice of a refrigerant is a compromise, e.g., ammonia is good but toxic and flammable. R12 is very good but detrimental to the ozone layer. An air-source heat pump is convenient to use, and so it is a better method for electric heating. The ambient temperature in winter is comparatively high in most regions, so heat pumps with high efficiency can satisfy their heating requirement. On the other hand, a conventional heat pump is unable to meet the heating requirement in severely cold regions anyway, because its heating capacity decreases rapidly when ambient temperature is below -10oC.
According to the weather data in cold regions, the air-source heat pump for heating applications must operate for long times with high efficiency and reliability when ambient temperature is as low as -15oC. Hence, much researches and developments have been conducted to enable heat pumps to operate steadily with high efficiency and reliability in low temperature environments. For example, the burner of a room air conditioner, which uses kerosene, was developed to improve the performance in low outside temperature. Similarly, the packaged heat pump with variable frequency scroll compressor was developed to realise high temperature air supply and high capacity even under the low ambient temperature of –10 to –20oC. Such a heat pump system can be conveniently used for heating in cold regions. However, the importance of targeting the low capacity range is clear, if one has in mind that the air conditioning units below 10 kW cooling account for more than 90% of the total number of units installed in the EU.
Energy efficiency consideration
Heat exchangers are devices, designed to efficiently transfer heat, from one medium to another, i.e., water-to-air, refrigerant-to-air, refrigerant-to-water, stream-to-water. Heat exchangers are widely used in power engineering, chemical industries, petroleum refineries, food industries and in HVAC technology. Therefore, heat transfer and the design of heat transfer equipment continue to be a centrally important issue in energy conservation. With increasing worldwide awareness of the serious environmental problems due to fossil fuel consumption, efforts are being made to develop energy efficient and environmentally friendly systems by utilisation of non-polluting renewable energy sources, such as solar energy, industrial waste heat or geothermal water. The GSHPs are suitable for heating and cooling of buildings and so could play a significant role in reducing CO2 emissions. Ground source or geothermal heat pumps are a highly efficient, renewable energy technology for space heating and cooling. This technology relies on the fact that, at depth, the Earth has a relatively constant temperature, warmer than the air in winter and cooler than the air in summer.
Heat transfer mechanisms
- Single-phase convection on both sides.
- Single-phase convection on one side.
- Two-phase convection on other side.
- Two-phase convection on both sides.
Examples: condensers, boilers, evaporators and radiators (Figure 1).
Naturally, it would be preferred, for comfort reasons that this index would be small, preferably nil. It may be seen that the variable is directly related to temperature discomfort: the larger the value of the index, the farthest will inside conditions be from expected wellbeing. Also, the use of electricity operated air conditioning systems will be more expensive the higher this variable is. Hence, energy expenditure to offset discomfort will be higher when comparing two index values; the ratio of them is proportional to the expected energy savings.
When the external shade blocks the windowpane completely, the excessive heat gains belong to the lowest values in the set, and the dimensionless index will be constant with orientation. For the climate conditions of the locality, it can be seen that a naked window can produce undesirable heat gains – if the orientation is especially unfavourable, when the index can have an increase of up to 0.3 with respect to the totally shaded window.
Heat sources
The technical and economic performance of a heat pump is closely related to the characteristics of the heat source. An ideal heat source for heat pumps in buildings has a high and stable temperature during the heating season, is abundantly available, is not corrosive or polluted, has favourable thermophysical properties, and its utilisation requires low investment and operational costs. In most cases, however, the availability of the heat source is the key factor determining its use. The Table 1 presents commonly used heat sources. Ambient and exhaust air, soil and ground water are practical heat sources for small heat pump systems, while sea/lake/river water, rock (geothermal) and waste water are used for large heat pump systems.
Geothermal heat pumps
Geothermal heat pumps are the most energy efficient, environmentally clean, and cost effective space conditioning systems available according to the Environmental Protection Agency in the United States of America. Ground source geothermal heating and cooling is a renewable resource, using the earth’s energy storage capability. The earth absorbs 47% of the suns energy amounting to 500 times more energy than mankind needs every year.
The closed loop portion of a ground source heat pump system consists of polyethylene pipe buried in the ground and charged with a water/antifreeze solution. Thermal energy is transferred from the earth to the fluid in the pipe, and is upgraded by passing to a water source heat pump. One 100 metres vertical closed loop borehole will typically deliver 14000 kWh of useful heating energy and 11000 kWh of useful cooling energy every year for life. For typical commercial building early trials indicate annual HVAC energy consumption in the
order of 75 kWh/m² compared with 156 kWh/m² ‘good practice target’, and 316 kWh/m² typical consumptions published by the Department of the Environment (DOE) in Energy Consumption Guide No.19. Low energy consumption means associated lower CO2 emissions than from conventional systems.
Water source heat pump
Water Source Heat Pump (WSHP) systems are one of the most efficient, environmentally friendly ways to heat and cool buildings because of their ability to move energy from where it is not needed to where it is needed. High-efficiency, self-contained WSHP units can be placed in virtually any location within the building and connected via a water loop. Heat is added and rejected from the loop using a boiler and a cooling tower, or using geoexchange from natural sources such as the ground, a pond or a well. Each unit responds only to the heating or cooling load of the individual zone it serves. This provides excellent comfort levels for occupants, better control of energy use for building owners and lower seasonal operating costs. Systems are commonly applied to office buildings, hotels, health care facilities, banks, schools, condominiums and apartments. Features include: lower utility bills, less maintenance, no visible outdoor plant, reduction in emissions, and versatility of system. Ground as a heat source for a heat pump coupled with low temperature heating system can fulfil most of requirements of small-scale applications.
The seasonal temperature fluctuations are small and decrease very quickly with depth. With the increase of ground depth down to 10 meters the temperature becomes constant and equal to 10 to 11oC. In Europe the freeze zone in the soil is one meter and in some regions one and half metre deep. This makes it preferable to use vertical ground heat exchangers rather than horizontal.
Pumps and Underground Thermal Energy Storage (UTES)
The general principle of a heat pump operation is to extract heat from low temperature heat source and to give it off at a higher temperature level. The useful energy output must be significantly greater than additional energy required to drive a heat pump to achieve a real reduction in primary energy use. Heat pumps can use renewable energy or waste heat as a heat source. Energy extracted from these sources is converted into useful heat in the low temperature range. This low temperature useful heat can be applied with good efficiency for example for space heating. Generally, when heat pumps are considered for effective use, the following characteristic features of a heat source will be taken into account:
- Availability
- Steady and relatively high temperature
- Coherency between source and user
- Thermal capacity
- Heat extraction cannot disturb natural energy balance of the environment
- Cost of heat extraction and transmission
- Ecological issues.
In a ground heat pump system heat is extracted from the ground by means ground heat exchangers – and is used as a heat source for a heat pump evaporator. During the heat extraction ground is cooled down. When heating system does not operate, ground body should recover to the initial thermal balance (in order not to disturb its natural state), due to natural heat and mass transfer processes (influence of ambient environment and geothermal energy). However, when heat demand is high, it is recommended to apply artificial charging of the soil, e.g., by solar energy or waste heat. This is called long term or seasonal Underground Thermal Energy Storage. UTES improves efficiency (COP) of a heat pump, due to better thermal performance (higher temperature) of a heat source and allows ground medium to return easily and quickly to initial thermal balance. Additionally, in combination with seasonal energy storage, solar energy and waste heat can make a major contribution to heating of buildings.
The seasonal storage facility can be designed in many different ways. Heat can be stored in the ground (clay, sand), un-fractured rocks and in water. Four fundamental options of long-term thermal energy storage can
be considered:
- Water tanks and solar ponds
- Rocks (boreholes in rocks, rock cavern, pit)
- Soil storage (boreholes, ducts in earth, earth coils)
- Aquifers- systems that use ground water flow.
Actually, according to the energy conservation through Energy Storage (UTES) water tanks, solar ponds, rock caverns and pits are not classified as UTES systems. Rocks can be used as a storage medium by drilling in rocks to make a number of boreholes, which are filled with plastic tubes in which water flows.
Geothermal aquifers
Geothermal aquifers exist where heat from the earth’s crust is absorbed by groundwater that collects naturally in the deep porous rocks of certain geological structures. To exploit these aquifers as a source of energy, it is necessary to drill two boreholes: a production borehole to extract the naturally heated water and an injection borehole to dispose of the water once that heat has been removed by surface use. It is also possible to use a single-hole configuration if the used water is discharged elsewhere – to the sea or to some other convenient sink. Because of the poor thermal conductivity of rock and the low rates of natural fluid recharge, heat is usually extracted at a greater rate than it is replenished from the surrounding rock mass.
Geothermal aquifers are not, therefore, ‘renewable’ resources in the strict sense of the word, although they are usually placed in the renewables category. The exploitation of geothermal energy has, to date, concentrated on these hydrothermal resources, most economically from aquifers in countries where seismic activity is high.
Water heating and comfort
The ground-source machine had lower demand (summer and winter) and lower heating energy use than either of the air heat pumps. Comparisons with natural gas must be based on cost since the units for natural gas (therm = 100,000 Btu) are different than electrical energy units (kWh). The development can also be seen in individual regions. In Figure 2, the number of installations realised within an incentive programme of the German utility RWE is depicted.
The high seasonal efficiency of the GSHP systems reduces the demand for purchased electricity and the associated emissions of CO2 and other pollutants. Figure 3 shows the relationship between utilisation efficiency and CO2 emissions for different domestic fuels. For example, it can be seen that, assuming an average CO2 emission factor for electricity of 0.414 kg/kWh, the use of a GSHP with a seasonal efficiency of 350% would result in the emission of 0.12 kg CO2 for every kWh of useful heat provided.
By comparison, a condensing gas boiler (assuming a CO2 emission factor for gas of 0.194 kg/kWh) operating at a seasonal efficiency of 85% would result in 0.23 kg CO2 for every kWh of useful heat supplied i.e., the CO2 emissions would be almost double those from the GSHP. In practice the environmental impact of a heat pump will depend not only on the amount of electricity used but also on the demand profile. In periods of peak demand some electricity will have to be provided by less efficient power stations with emission factors as high as 0.8 kg CO2 / kWh. As well as reducing purchased energy consumption and resulting in low CO2 emissions, the GSHP have a number of other environmental and operational advantages:
- High reliability (few moving parts, no exposure to weather)
- High security (no visible external components to be damaged or vandalised)
- Long life expectancy (typically 20 to 25 years and up to 50 years for the ground coil)
- Low noise
- Low maintenance costs (no regular servicing requirements)
- No boiler or fuel tank
- No combustion or explosive gases within the building
- No flue or ventilation requirements
- No local pollution.
Assumed CO2 emission factors:
Electricity = 0.414 kg/kWh delivered
8.1 Gas = 0.194 kg/kWh
Oil = 0.271 kg/kWh
Conclusions
Underground Thermal Energy Storage is a new idea. Some UTES systems have been already constructed worldwide. Sometimes the users are not aware, that they have used an UTES system. Most of the ground heating systems use heat accumulated in the ground medium in a natural way and they are typically ground coupled heat pump systems. Different types of horizontal and vertical ground heat exchangers are being applied. Due to a lack of tradition and sometimes knowledge of heat pumps and ground systems, some mistakes in planning, designing and construction have been made. Therefore, there is a great need to develop good demonstration project to show advantages of UTES and its role in energy conservation. The building sector is a major consumer of both energy and materials worldwide, and the consumption is increasing. Most industrialised countries are in addition becoming more and more dependent on external supplies of conventional energy carriers, i.e., fossil fuels. Energy for heating and cooling can be replaced by new renewable energy sources. New renewable energy sources, however, are usually not economically feasible compared with the traditional carriers. In order to achieve the major changes needed to alleviate the environmental impacts of the building sector, it is necessary to change and develop both the processes in the industry itself, and to build a favourable framework to overcome the present economic, regulatory and institutional barriers. Today, buildings are largest consumers of energy. Air conditioning and heating consume about 40% of the power in the buildings. Demand to conserve energy has become necessity as there has been rising costs of energy consistently, and this makes us think go green and innovate the greener concept for buildings.
A green building uses less water, optimises energy efficiency, conserves natural resources, generates less waste and provides healthier spaces for occupants. And, a green home can have benefits, such as reduction in water and operating energy costs of the building. This may also mean refrigerant-based chillers and compressors to be shut off or to be operated at reduced capacity. With the environmental protection posing as the number one global problem, man has no choice but reducing his energy consumption, one way to accomplish this is to resort to passive and low-energy systems to maintain thermal comfort in buildings.
Naturally, it would be preferred, for comfort reasons that this index would be small, preferably nil. It may be seen that the variable is directly related to temperature discomfort: the larger the value of the index, farther will inside conditions be from expected wellbeing. Also, the use of electricity operated air conditioning systems will be more expensive the higher this variable is. Hence, energy expenditure to offset discomfort will be higher when comparing two index values; the ratio of them is proportional to the expected energy savings. When the external shade blocks the windowpane completely, the excessive heat gains belong to the lowest values in the set, and the dimensionless index will be constant with orientation. For the climate conditions of the locality, it can be seen that a naked window can produce undesirable heat gains if the orientation is especially unfavourable, when the index can have an increase of up to 0.3 with respect to the totally shaded window.
The European experience with GSHP systems so far is excellent. It is expected that the market will further expand, in the leading countries like Sweden and Switzerland as well as in other countries to follow. The growth can be exponential as the Swiss example. An important factor, related to the further development of electric heat pump systems in general and the GSHPs in particular, is the current process of deregulation in Europe. The energy sector and, especially the electric utility companies, are currently under deregulation and Privatisation. This affects not only the producers but also the customers. The deregulation process may affect the heat pump market in two ways: 1) heat pump economy might be influenced by changes in the energy price structure, and 2) the heat pump market might be stimulated or hindered, depending on changing utility market strategies. So far, in the regulated market, some utilities have clearly supported heat pumps, in line with governmental energy-efficiency Programmes (e.g., by offering grants or special electricity tariffs). However, in a deregulated energy market, the market strategies of utilities will change. Only when the market matures and energy prices drop to a stable level will utilities offer incentives such as products/bonuses or energy efficiency services. Nevertheless, the ecological incentives like avoiding GHGs emissions will further support GSHP development. The CO2 tax in sight is a further (financial) incentive. Of course, there will be considerable differences in this respect from country to country.
