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4.1 Summary table of the LowEx Technologies
4.4 Strategies for design of low exergy systems of buildings
For future buildings, a minimum of energy at a very low level of temperature difference between the system and the room should be used for thermal conditioning. In this way a maximum of high quality energy (exergy) could be saved.
The big efforts made in the field of energy saving in buildings by constructing well-insulated and tight envelopes, sufficient window shading and the use of thermal storage result to a much better usage of the energy. But there is still a big saving potential left. To make the energy use in buildings even more efficient, new low temperature heating and cooling systems are required. The components and systems presented in this chapter show a step further in this direction.
4.1 Summary table of the LowEx Technologies On the following pages, a summary table of the LowEx technologies is presented, with some key information of the components. At the CD-ROM there is a link to the case, where the technology has been used. There is also a search function, with which the technologies can be sorted by different parameters. There are links from the summary table to the data sheets.S.1.1 |
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For every presented system component, a data sheet is given and sorted in main and sub groups. To improve the understanding of the different systems a small sketch and some main data are given.
It is important to note that the figures given and explained below, are figures that apply to the technical solutions in general and what has been believed to be practically obtainable at present. However, different products available in the shown category diverge and many of these solutions are still in a developing state. We hope that with time, researchers, developers and manufacturers will be able to provide new information to this database and it can develop into a tool for benchmarking new products. In many cases it has been found impossible to give relevant information and the fields are then simply left empty.The information fields are as follows:
Application: Mostly if the system can be used for heating or cooling or both.
State of the art: Is the system available on the market or is it still just an innovative concept, or is it somewhere in between.
Minimum heating temperature: The estimated minimum practically applicable inlet temperature to the system is given. A typical range can be given as well.
Maximum cooling temperature: The estimated practically applicable inlet temperature to the system is given. A typical range can be given as well.
Relative mechanical energy consumption: Some systems require pumps or fans to move the heat transport medium through the system. This requires mechanical power, which is related to the nominal heating/cooling power output.
Relative exergy consumption: There are two parts on the consumed exergy. One for the mechanical work, as it mentioned above and one for the temperature drop in the system. Both are added and related to the nominal heating/cooling power output.
Estimated investment costs: The additional costs for the system including measures necessary for the operation of such a system. The total costs are related to the nominal heating/cooling power output. A typical range can be given. The investment costs are roughly estimated according to information from manufacturers.
In recent years system solutions have appeared where heating and cooling is carried out in a holistic system solution where the energy use is planned in a wider and more general perspective.
One example of a LowEx system is the use of boreholes to provide cooling in summer. This was apparently a very promising method but in some cases the borehole would gradually over the years become warmer and above the temperatures that could be used for direct cooling in a rational way. By also extracting heat in winter with a heat pump the heat balance of the hole is restored and the system solution becomes sustainable in time. In most cases where exergy is being consumed at different temperature levels in the same system a thorough system study in an exergy perspective can lead to substantial savings.
4.3.1 ThermoNet system concept for heating and cooling
ThermoNet system can be applied to a variety of building types including hospitals, swimming halls, offices, industrial buildings, residental high-rise buildings etc. An application of ThermoNet system for grocery stores is presented here (Figure 32).
The exploitation of condensation heat, waste heat, and excess energy in a ThermoNet system is based on two factors: an air heating system that utilises low temperature technology, and efficient energy recovery. By applying ThermoNet technology, the consumption of purchased energy may be cut by more than one half when compared to conventional solutions, and electrical consumption may be reduced to one third. The ThermoNet low temperature system is able to utilise district heating return water from other properties, reducing peak loads by 60-70 %.
Figure 32. A Low Exergy system for a market.
Energy systems:
Heating
In grocery stores, the relative sise of grocery departments is large. Consequently, a large volume of condensation energy is abundantly available, and its temperature is well suited for utilisation by the ThermoNet air heating system. Primarily for this reason, shop spaces are warmed by air through the Dirivent impulse system. The air heating system is also able to control the rapid load fluctuations typically found in grocery stores. Vestibules are fitted with air circulation equipment connected to the ThermoNet network.
Heat Sources and Heat Recovery
Heat sources include heat recovered from exhaust air, condensation heat from refrigerated grocery department display cases, and supplementary energy sources such as district heating, electricity, natural gas, or oil. With the ThermoNet system, the connection to district heating can be made in an exceptionally economical fashion: ThermoNet is able to take advantage of return water flowing from other properties to the combined heat and power plant. This water contains sufficient energy that can be utilised by the ThermoNet system.
Cooling
The cooling efficiency given off by refrigeration equipment, together with indirect evaporative cooling, is usually sufficient as a source for cooling energy.
Air systems:
Air heating is implemented using the Dirivent impulse system that improves the operation efficiency of warmed air. Heated air, because it may be quickly adjusted and controlled, is the most effective alternative.
Air conditioning is implemented using a single technology centre.
If necessary, air conditioning operates during the day either using return air or fresh air and during the night using return air. The air flow and temperture are controlled so that the total benefit derived from the recovery of condensation heat, as well as from the savings in fan energy will be as large as possible.
Air distribution
Efficient air distribution can be used to achieve an economical heat balance. Air distribution is implemented to maintain temperature layers in cooling situations. In heating situations, the formation of temperature layers is blocked using Dirivent impulse system. (vertical distribution for indoor air)
Control system and remote supervision
The ThermoNet system’s ability to control the indoor climate while effectively utilising heating energy created in grocery stores is based on an energy utilisation and control system designed for that purpose. Additionally, trough the programme, the system can be controlled and supervised remotely in real time, and necesary adjustments and alternations to the ThermoNet system may be made by experts from remote locations.
4.3.2 Heating and cooling with Are Sensus
The use of Sensusâ building services system in an office building is described here (Figure 33). The exergy consumption of the Sensusâ system is lower than in comparable high-standard systems, which also decreases environmental impact during use.
Office ventilation employs a Sensusâ ventilation unit connected to the Sensus panels with a three-pipe network. The ventilation units utilise surplus heat collected from the rooms with the cooling water system for the heating of intake air whenever heating is needed for the intake air. This conserves heating exergy. The ventilation machine also has an efficient rotating heat collector for the exhaust air (over 70% heat efficiency).
The Sensusâ ventilation unit utilises outdoor air for cooling the cooling water for the rooms when outdoor temperature is sufficiently low (under +12–14 °C). This free cooling carried out with ventilation units operates alongside mechanical cooling when necessary. It has a considerably longer annual period of utilisation (over half of the year’s working hours) than conventional free cooling. This lowers the electricity consumption of cooling unit in the Sensusâ system in comparison with conventional solutions.
Figure 33. Main components of the ARE Sensus system.
Heating and cooling
The office space is heated and cooled with Sensus panels installed on the ceiling. Hot or cold water circulates according to need to heat or cool the room. Heating and cooling are transferred from the panel to the room mostly via thermal radiation. In ordinary solutions heat is transferred by conducting the air of the room through heating and cooling room units. Sensusâ is draught-free, because it does not circulate air. This has been noted in practice and verified with measurements carried out by the Finnish Institute of Occupational Health and Safety in premises where the Sensusâ system has been installed.
There is thermal radiation from the panel on both people in the room and the surfaces of the room (floor, furniture, windows), which changes into heat upon contact with the surface. Therefore the panel produces a more even distribution of heat in the room than radiator heating. This has also been noted in studies by the Finnish Institute of Occupational Health and Safety comparing the Sensusâ system with radiator heating. International studies have showed that ceiling cooling is the most pleasant way to cool office and work space. Sensusâ follows the same principle as ceiling cooling. People in the space will feel coolness but no draught.
The panels are installed at a distance of approximately 150 mm from the ceiling or intermediate floor. The panels are placed modularly in the office rooms.
Ventilation
Supply air is conducted into the rooms through supply air diffusers . The diffusers are installed beneath a panel on the side of the suspended housings and the air is conducted along the surface of the panel. Ventilation is draught-free, because the air keeps well to the smooth panel and will not "fall down". At the same time the surface of the panel will heat or cool the intake air precisely according to the requirements of the room. The air flow is dimensioned to maintain high air quality in the room.
Lighting
Sensusâ lighting fixtures together with the Sensusâ panels provide very high quality indirect lighting. The lighting fixture is installed under the panel. The lighting fixtures use energy-saving T5 fluorescent tubes. Indirect lighting is a highly flexible solution for modern offices which are mainly intended for computer monitor work. The lighting is suited to different types of furnishing and relocations of partitions without alteration work. The light is not reflected from the computer monitors. Although the panels have a high reflectance of light, they are not shiny or glossy. Therefore, the required electrical power for the lighting is low and electricity consumption is also low.
Temperature control
Room temperature control functions considerably better than in conventional designs. A single electronic temperature control unit regulates heating and cooling in series. There is no simultaneous heating and cooling in the same space, as in solutions with thermostat-valve radiators and separate controls for cooling. This improves interior conditions and lowers exergy consumption. Control is also improved by the fact that both heating and cooling are mainly provided through thermal radiation. Heating or cooling is focused directly where it is needed, i.e. on people and the surfaces in rooms. Adjustment is quick, because the radiated temperature of the panel is felt immediately. In traditional designs heat and cooling are conducted through the air with slow effect on room temperature.
Temperature control in office spaces is carried out with electronic room controls regulating the valves for heating and cooling. The controls have dials with which the office workers can set the desired temperature. Each room has a separate control unit. In the open office, temperature is regulated independently in sections (4 spatial modules 2.7 metres wide). Each panel has its own regulator valve (thermomotor valves) and piping, permitting alterations to the walls without changes to the piping.
Electricity
Electrical and computer connections in the office space are realised by distribution from above employing socket columns. The connection cables of the work stations are placed freely above the Sensus panels. The socket columns can be placed or moved where desired in the work station area.
4.3.3 Ceiling cooling with well water
This concept is a ceiling radiant cooling system using well water, and with outdoor sun shading on the South-facing window. The ceiling radiant cooling system is installed in the living room (Figure 34, right hand side) of a two-storey wooden house, which has well-insulated exterior walls and double-glazing windows. The living room window (W: 3.6 m x H: 2.2 m) faces south.
Figure 34. The house equipped with ceiling radiant cooling system using well water.
Figure 35 schematically shows major components and well water flow in the ceiling radiant cooling system: deep well (depth: 66 m), well water pump, three-way valve, circulation pump, ceiling radiant panel (aluminum panel with embedded pipes), and percolation well. Water from the deep well is pumped up and mixed with return water from the ceiling radiant panel by the three-way valve to maintain the temperature at the set point. The circulation pump enables this mixed water to run within the panel and cool it. A portion of the return water is sent to the three-way valve and the rest is discarded via the percolation well.
Figure 35. Components and well water flow
Indoor Thermal Environment
Figure 36 shows temperature [°C] time histories in the room equipped with the ceiling radiant panel, with and without outdoor sun shading on the south-facing window (dark blue and light blue line, respectively).
During the first hour of cooling system operation, temperature in the room was ca. 28.5 șC, with and without sun shading. Thereafter, the effect of outdoor sun shading on temperature became more noticeable. In the case with outdoor sun shading (dark blue), the temperature remained relatively constant until the cooling ceiling was turned off and the sun shading removed at 16:00 hours.
In the case without shading (blue), the temperature gradually rose and reached 30.5 șC at 16:00, in spite of the cooling provided by the ceiling radiant cooling system. These results show how the room indoor thermal environment benefits from outdoor sun shading.
The indoor thermal environment discussed in Figure 36 was realised by means of supplying and consuming thermal exergy of well water and electricity at the ceiling radiant cooling system. In the following items, we show how the ceiling radiant cooling system consumed exergy from well water and electric power at each subsystem.
Outdoor air
temperature was 27 șC to 31 șC and solar radiation at |
Figure 36. Temperature time history in the room, with and without outdoor sun shading
Exergy flows with sun shading.
Figure 37 shows exergy input, consumption, and output for the whole ceiling radiant cooling system, with sun shading. Exergy consumption [W] is shown in the rectangles for the subsystems radiant ceiling, three-way valve, circulation pump and well pump. The numbers shown beside the marks indicate the cool exergy content of the well water within the pipes. Blank arrows show cool exergy transfers associated with radiation (straight) and convection (curved). At the subsystem well, 435 W of electric power was supplied to the well water pump, and 398 W was consumed to draw well water from the bottom of the deep well. Well water pump exergy consumption was about 56 times lager than that of the ceiling radiant panel (=398/7).
At the subsystem three-way valve, the cool thermal exergy input was 62 W (well water pumped up from the well) plus 126 W (return water from the ceiling radiant panel). The exergy output at this subsystem was the cool thermal exergy content of the mixed water coming out from the valve: 179 W. The difference, 9 W (=62+126-179) was consumed by mixing two water flows at different temperatures. This cool exergy of 179 W was the input to the radiant panel subsystem. 7 W of this cool exergy was consumed within the panel, 10 W were emitted as cool radiation exergy and another 10 W by convection. The radiant panel also received 2 W of cool radiation exergy emitted by surrounding walls and floor. These cool radiant exergy transfers also contributed to radiant cooling. The output from the radiant panel subsystem was the cool exergy content of the well water coming out of the radiant panel: 154 W (=179+2-7-10-10).
Figure 37. Exergy input, output, and consumption with outdoors shading
4.3.4 New innovative heating concepts
The energy source for this system concept is ground heat. This heat is extracted through a heat pump. The emission system is, instead of a conventional floor heating system, a floor heating with phase change. This floor heating system has a phase change material (PCM) that can be utilised to store the energy from a solar collector during the day and during the night the energy is released and warms up the room. The same floor heating system could also be used the other way around, "charging itself" during the night and releasing the heat during the day. This could be of use when using a heat pump.
The heat pump uses electricity, about one third of its supplied energy amount. Looking at the electricity distribution system, there is a "overload" in the network during the night. The system is designed to cover the peaks that occur during the day. The best thing for the energy production point of view would be to have a even consumption curve. By using electricity during night time we contribute to making this curve more even, and we get economically compensated for this with the right kind of electricity contract.
The exergy analysis of such a system looks just like the analysis of a system with a traditional floor heating system (Figure 38).
Figure 38. Exergy analysis of a floor heating system using a heat pump as energy generator.
The design process is or at least should be an iterative process. There will always be a need to go back to earlier step and revise the choices made. The design process is described in figure 39. The terms in the boxes are more explained in the following.
Figure 39. The design process of a building as a system.
4.4.1 Target setting
Thermal comfort
Besides the demand on operative indoor temperature, the specifications on thermal comfort can include radiation asymmetry, temperature gradients, maximum surface temperatures, minimum surface temperature for floors etc. Thermal comfort should also include specifications on maximum air velocity in the living zone. Also relative humidity affects the human sensations about thermal comfort.
IAQ
Indoor Air Quality includes aspects like CO2 concentration, particles, house dust mites, air temperature, annoyance and dust. The annoyance from all kinds of emissions (TVOC etc) is correlated to air temperature. A correlation is also found for Sick Building Syndrome and air temperature. The relative humidity and air velocity also have a great influence on IAQ.
Day lighting
Since artificial lighting has to be provided with high quality energy, the access to daylight is an important factor in the planning of a low exergy building. Fenestration, interior planning and the coloring of surfaces are factors that are of importance. Daylight access can be in conflict with the effort to reduce the need for cooling in summertime.
Acoustics
The choice of the heat and cold emitting system has also influence on the acoustic quality of the building (Figure 40). E.g. the surface integrated systems are usually very quiet.
Safety issues in connection with heating and cooling systems include for instance surface temperatures of the heat emitters and toxicity of the refrigeants or heat transfer fluids. Also possibility to legionella growth in DHW system must be prevented.
Usability
If people don’t know how to use their building, they can be disappointed to the performance of the building and besides cause unneccessary growth in the exergy consumption. Therefore it is important, that also the heating and cooling systems are easy-to-use. Studies have also shown, that people are more satisfyied with systems they (or at least think they) can control.
Adabtability
The buildings are planned for a service life of at least 50 years, but even 100 years should be considered normal. Future buildings should be planned to use or to be suited to use sustainable energy sources for heating and cooling. One characteristic of these energy sources is that only a relatively moderate temperature level can be reached, if reasonably efficient systems are desired. For this purpose the buildings and installations should be designed for low temperature difference. Appropriate distribution systems, like floor and wall heating, have a life cycle of 40 to 50 years. So to implement sustainable energy sources within the next half of a century, heat distribution systems should be designed for lower temperatures as soon as possible. If a high-exergy emission system is chosen, this affects the possibility to choose among different energy sources for a long time. The low-exergy emission system, instead, can be used with a high-exergy source, if desired.
LCC - Cost effective solutions
Life Cycle Costs of the building include the investment cost, maintenance and operation costs, but also the cost of dismounting and waste treatment. If a building integrated heating and cooling system is chosen, the investment cost is included in the cost of the building structure. This is one example, which shows the neccessity to consider the building as a whole.
From a consumers point of view, the incitement to pay more for low exergy solutions is limited and the design therefore has to follow general economic terms. However we know of added value factors that real estate owners generally consider worth money.
In the system design it is important to include all the mechanical energy to run the system in the energy and exergy analyses. This applies to fans and pumps, frost protection of heat exchangers etc. Correct (over)sizing of ducts and pipes and demand controlled operation can lead to a substantial reduction in energy costs.
Environmental impact – LCA
Usually the impact of building materials is relatively small compared to the environmental impact of the operational phase. Specifications for low energy use and low exergy consumption therefore account for a dominating part of the total impact. Other specifications that should be regarded is the overall material efficiency with minimum waste and the long term durability of the building constructions and materials and the impact of chemicals needed for cleaning and maintenance.
4.4.2 Preliminary estimations for energy, exergy and power
At the early stages of desing it is important to identify key numbers based on earlier experience that can be used as a first goal for the design and for benchmarking the results.
Peek power loads
The peek power loads basically can be based on the following parameters
The predicted internal loads are a precondition for the design of systems for heating and cooling. They can for dwellings offices be specified as a constant load for dwellings or or as a fixed time varying load profile on a daily or weekly basis.
Annual energy use for heating
The maximum annual energy use for heating is in most national building codes specified directly or indirectly. Even if we use the exergy approach for design of buildings this does not have to be in conflict with an effort to reduce the energy consumption. Theoretically we can create cases where it can be shown that the low exergy approach can lead to economical solutions with a relatively high energy use. In most practical cases however, the final exergy consumption for the final solution will be strongly depending on the energy use. A low value for space heating can be expressed as 1 kWh per square meter of living area and 1000 degree hours. In badly insulated buildings the figure can be three times that.
Annual exergy consumption
Since our aim is to produce low exergy systems for heating and cooling in buildings, the exergy consumption is obviously a parameter we consider to be a major output of the design work. It is however difficult to set a limit for the annual exergy consumption since we in a real project are depending on the energy sources which are available at present. If we exclude the energy sources and only include storage, distribution, heat emission and loss to the environment the limit for annual exergy loss can be expressed as the sum of the following:
Quality factors for some energy sources 20°C are presented in Table 5.
A practical choice of reference temperatures can be:
| Source | Quality factor q |
| Mechanical energy | 1.00 |
| Electrical energy | 1.00 |
| Solar radiation | 0.95 |
| Nuclear fuel | 1.00 |
| Fossil fuels | 0.90 |
| Thermal at 100°C | 0.21 |
| Thermal at 40°C | 0.06 |
| Thermal at 20°C | 0.00 |
4.4.3 Options
As low energy use is generally a precondition for low exergy it can be recommended to start the design process by creating an effective building shell and reduce the heating and cooling loads as much as possible within the economical limits.
Building shell – Thermal insulation
In the design of the thermal insulation the following items should be regarded:
Building shell - Air tightness
Good air tightness is always a precondition for effective energy use and low exergy solutions. For mechanically ventilated buildings, the ventilation rate can be kept at a predetermined level and the heat recovery from exhaust air is enhanced. Even for naturally ventilated buildings it is important to be able to control the air flows to minimise infiltration losses, to prevent uncontrolled flow of pollutants in buildings and to prevent damages due to high moisture levels in the exterior constructions.
Building shell - Fenestration and solar shading
It is generally beneficial to reduce the heat transmission through the glazing as long as the light transmission properties are within limits.
The frame construction stands for a significant part of the window heat loss and the frame construction should not add to the U value at the centre of the glazing.
The light transmission is reduced with the number of glazing, heat reflective coatings, solar reflective coating and with the use of solar absorbent glazing. The light transmission is normally not influenced by the use of gas in the glazing cavities. The glazing should be specified with the light transmission properties with respect to the reduction and the distortion of the transmitted light.
As windows are generally the weak part of the envelope their total area should be kept at minimum considering other functions such as aesthetics, visibility and daylight. Windows facing the south with good thermal properties may, due to solar radiation at the fringes of the heating season, be a positive factor in the annual energy balance but will increase the maximum heating and cooling loads which will not benefit low exergy solutions for local heaters and coolers.
Building shell - Integrated building components
Integrated building components are no modern invention. Archeological remains in northern Sweden show that in prehistoric times smoke from fire places were lead out through ducts in the floor and the Romans used similar techniques to heat their palaces. Floor and ceiling heating have been popular in the latter half of the last century even though the reason has very seldom been to develop low exergy solutions. Such solutions have primarily been selected because of the positive effect they can have on thermal comfort and to bring down temporary high moisture levels such as on entrance hall- and bathroom floors.
The choice for heat and cold emission system
The low exergy components for emission of heat and cold are described in Chapter 3. The choice of heat and cold emission system depends strongly on the design of the building structure and affects the availble options for energy sources. If the building structure is not energy efficient, the low exergy heating and cooling systems may have difficulties in meeting the peek loads in the building. Then again, if a high-exergy emission sytem is chosen, then some possible energy sources are dropped out or their operation can not be realised in most efficient way.
The choice of energy sources
From a thermodynamical point of view, a system solution with the lowest exergy consumption is the most favorable in the overall global future perspective. There are however several practical constraints we have to take into consideration. Since we do not want to include the whole Universe within our system boundaries we have to make a distinction between renewable sources of energy such as solar radiation and the energy sources that exist in limited and finite quantities such as fossil fuels. In following, some things to consider in the choice of energy sources are presented.
Electricity
Electricity from the grid is a high quality energy that should be a major target for effective utilization.
Fossil fuels
Fossil fuels like oil, gas and coal are high quality energy from a thermodynamical point of view. The pollution generated by these fuels could be described as entropy production that partly has to be reversed and some researchers would maintain that the overall exergy consumption due to burning fossil fuels is bigger than the energy content of the fuels as such.
Biomass
Biomass is a form of renewable energy with high exergy content.
District heating
In the same way as electricity, district heating can have many origins from solar energy to fossil fuels. The exergy content is relatively low and a district heating system gives the possibility to seek optimum energy solutions on a community level.
Waste heat
Waste heat in larger quantities is available from industrial processes that to a large extent are at a longer distance from the residential areas where they could be utilised. In Frankfurt, Germany there is a commercial project where waste heat is transported in salt containers from industries to buildings where the energy is used for heating.
Solar (heat)
The exergy of solar radiation that hits the earth is almost the same as its energy content. The sero process, that is when we do not build a house implies that the sun is partly involved in photosynthesis generating biomass. Some is reflected, but the largest part is absorbed in surfaces generating heat at temperature levels close to the local outdoor air temperature. It can be estimated that for these processes about 95 % of the solar energy is lost.
Solar ( PV)
In many building projects with ecological profile, photovoltaic solar panels have been utilised for electricity generation. As for solar heating, this is harvesting exergy, which otherwise would be lost. But it still raises difficult questions since the economy for this generation does not even come close to what can be obtained with other methods. From the individual standpoint exergy should be bought at lowest possible cost. From a global standpoint the question is to which extent spent money reflects the use of exergy. From a more pragmatic view, the question is if the money is going to be spent anyhow, if there are more economic means to generate useable exergy. There is also the argument that investment in photovoltaic energy will lead to a development with decaying production cost and a competitive future for this technology.
Wind
A similar reasoning as for solar energy applies to the wind energy, which of course is generated due to temperature differences caused by solar radiation, where the exergy content is consumed when the mechanical energy of moving air is converted into heat in the friction processes at temperatures close to the ambient temperature.
Snow and Ice
In the past, ice was harvested from lakes and kept in insulated storage heaps for summertime cooling. In modern times, large quantities of snow are removed from the roads. This could be utilised in the same way as in the past with a relatively small extra cost.
Natural water systems
Lake water can be used as a source for cooling and as a source for heat generation by a heat pump. Small scale hydropower generation can also be an issue on a small community level.
Sky radiation
The radiant temperature of the sky can during clear nights be 10 to 20 C under the air temperature. This can be an important source for cooling.
Wave energy
Similarly to wind energy this can be regarded as a secondary form of solar energy. Wave energy is seldom an option within the boundaries of a building or small community project.
Geothermal
As a result of earlier geophysical phenomena, such as volcanic activities, locations with higher temperature can be present deep down under the earth’s surface with accessible water or in a dry state.
Ground heat
Even the earth at normal temperature can be a source for heating with a heat pump, or cooling. Heat from the earth can be harvested by horizontal coils in the ground, from vertical boreholes or from aquifers. Heat generation systems using ground heat are described in chapter 3.
The exergy content is larger for cooling than for heating, but even if cooling is the main issue it can be necessary to extract heat wintertime so that the temperatures around a borehole do not drift upwards with time. Storing heat in a single borehole is only meaningful at low temperature levels. With larger systems of boreholes a strategy for heat storage at higher temperatures can be developed.
Exhaust air
The sensible and latent energy content of exhaust air can be utilised in an air to air heat exchanger, in an evaporator to a heat pump or for heating external parts of insulated constructions as for instant in dynamic insulation and exhaust air ventilated crawl spaces. These systems are described in chapter 3.
Outdoor air, sensible and latent heat
Outdoor air has been used to heat to heat the evaporative element of commonly used outdoor air heat pumps. In cooling towers the moisture deficit of outdoor air is used to generate evaporative cooling of sprinkled water.
Internal gains
Internal heat gains in buildings that are not generated by the heating system are passive solar energy through windows, heat from lighting and equipment, human metabolism etc. The most effective use of this energy is to provide thermal inertia and good control so that excessive temperatures can be avoided as much as possible.
Sewage
Measurements have indicated that the average sewage temperature can be about 27°C, which makes it a significant heat source. The heat exchange, however, includes some technical implications that are not easily overcome.
Energy transformatio
This is usually a subject, that is considered more in planning of the energy chain of a small community, but if for instance natural gas is an option for the energy source of the building, then different transformation systems can be considered, like microturbine, fuel cell or gas boiler. These options are more described in chapter 3, part Generation/conversion of heat and cold.
For the selection of processes for energy transformation primarily the following factors should be considered:
The choice of energy storage
Storage systems are presented in more detail in Chapter 3. Some things to consider in the choice of storage system are presented below.
Building structure
The thermal inertia of the building structure can increase the utilization of heat from solar radiation through windows and internal heat gains. For this to happen, the overall dynamics of the system and the regulation and control of the heat emission system have to be synchronised. In special cases such as for intermittent heating a high thermal inertia of the building can lead to enlarged variations in power load and thereby increased exergy consumption.
Water tanks
When heat is harvested from solar collectors, a water tank can, in case of excessive heat, be heated to around 100 OC. The lower the inlet temperatures required for the heat emission system, the larger the temperature span that can be utilised for the tank and therefore the amount of energy that can be recovered from the storage is increased. Low exergy systems are therefore a strategic means to improve the economic efficiency of water storages both on a single house and small community level.
Ice storage
In its oldest form ice and snow were harvested during the winter period and insulated by sawdust. In that way ice for cooling was available during summertime. This method is gradually coming back, in some cases combined with removing snow and ice from roads and air fields. In modern ice storages a heat pump freeses a water storage down below the freezing point ant thereby the latent heat liberated the freezing process is utilised. This is not a low exergy solution but has interesting potentials in cases where the ice can be utilised for cooling in summertime.
PCM storage
Phase chance storage materials that have been applied directly on building surfaces to harvest excessive heat in buildings have had limited success since relatively high temperature differences are needed for a substantial heat transfer to take place. As an annual storage the price is still too high to be paid by one energy cycle per annum. An interesting commercial application has been developed in Frankfurt Germany where waste heat from industries is harvested to heat a container with a PCM material which in turn is then moved to a building where the energy is used for heating.
Ground storage
Low exergy systems for heating and cooling can substantially enhance the annual coverage of the total system and improve the total economy.
Control systems
A low exergy component such as a heated floor usually includes more thermal mass than a high exergy component such as an electrical radiator. The important issue here is that if the inlet temperature of the floor heating system can be brought down to 30°C, the heat emission from the floor will drop rapidly when the room temperature goes up. In this way the integrated building components can be self regulating. Floor heating systems with inlet temperature of up to 60°C can cause problems with overheating if not properly regulated. Before the system design the dynamic properties of the integrated building components should be studied with proper software.
An interesting issue for control is the choice of input variables. The air temperature is obviously not a suitable variable since we are trying to reach a certain operative temperature with relatively low air temperature and to this strategy to hold we need to keep the surface temperature rather constant. A possible strategy is to control the surface temperature as a function of the outdoor air temperature or a combination of the outdoor air temperature and the indoor air temperature.
4.4.4 Checking towards targets
The quality of the choices made should be confirmed by general system analysis, which should include at least:
