Active cooling options

Passive cooling systems, like passive space heating systems, have many attractive features, not least simplicity, cost and reliability. They also have serious limitations. Windcatchers, solar chimneys and the stack effect on their own can never ensure that stale warm air is always extracted from every nook and cranny of every building and that cool fresh air always replaces it at a rate which significantly improves occupant comfort. Larger buildings with complex floor plans that depend entirely on unforced natural circulation have a mixed record in practice. Odours can be a problem. Occupant expectations can be higher than the building design can satisfy. Where there is a high density of occupation and a multiplicity of heat-generating office equipment the case for active cooling could be unanswerable.

However, this does not mean that traditional air conditioning is the only alternative. Buildings can be cooled very effectively without resorting to conventional refrigeration technology or ozone layer-depleting chemicals. Even small energy inputs can make a significant difference to occupant comfort. Windcatchers with remotely operated vents and dampers are much more efficient when used for night-time cooling; for example, cool towers work better with a pump to keep the cooling pads consistently moist (see Chapter 8). A 'zero carbon' option is simply to add fans to the basic passive system, fans that can be effectively powered by solar PV or other forms of alternative zero carbon energy, such as mini hydro or wind turbines.

Relatively small fans can radically improve air circulation when used as a supplement or a back-up to natural extraction via a windcatcher or solar chimney. Forced circulation usually needs smaller ducts and vents, extraction and cooling can be targeted to high priority areas. Evaporative cooling can be accomplished in a much smaller installation than a cool tower. Even when mains electricity has to be used to power the fans, alternative active cooling systems will usually consume significantly less energy than traditional air conditioning. At the very least, alternative cooling technologies can be used to significantly reduce the need for conventional air conditioning, minimising both capital and running costs.

Night-time cooling can be accomplished more effectively if mechanical ventilation is chosen. There will be no need to open windows at night, reducing the otherwise inevitable security and noise pollution risks, and cooling airflows can be directed more precisely at

Mechanical Ventilation Attic Space
Solar PV-powered fan-boosted windcatchers (Reproduced with permission from Monodraught)

the selected thermal store. Exposed concrete soffits will no longer be strictly necessary. Cool air can be circulated easily through ducts in precast concrete floors, or directed above false ceiling panels to cool the soffits of conventional floors, which are usually much cheaper to construct. In the UK-developed CoolDeck system the air is persuaded to flow through a narrow gap between the soffit and a steel duct plate bonded to it. Turbulent flow is deliberately produced to enhance the heat transfer rate - CoolDeck is claimed to be up to 100% more efficient at cooling the slab than straightforward ventilation, and its performance can be enhanced even further by the inclusion of suitable phase change materials within the ducts (see Chapter 15). Most active night-time cooling systems to date have been powered by mains electricity, as solar PV is obviously no use at night. Care must be taken to avoid overcooling the thermal store; otherwise daytime condensation is a real risk. And there can be no fine control of temperature or humidity with such a system.

Forced circulation can make closed loop earth tubes an attractive option. These are a form of ground-coupled heat exchanger (see Chapter 15). Internal air is blown or sucked in to and out of large diameter smooth plastic or metal tubes buried at least 2 m below ground level close to the building, where soil temperatures are well below ambient air temperatures during summer months. Both ends of the tubes connect to the building; no outside air enters the system. Open loop earth tubes, where outside air flows through the tubes before entering the building, is another option, but require rugged external ground level air intakes which need regular maintenance, and can suffer from condensation and mould growth (see Chapter 8). A closed loop system recirculates internal air, and generally cools it further than an open loop system can, but it does nothing for air quality. On the other hand, simply venting stale air to atmosphere when it is still cooler than ambient is a waste of energy.

Pdec Tower

Fan-driven evaporative coolers, also known as desert coolers or swamp coolers, work on the same principle as cool towers, but are much more compact. External air is drawn through wetted fibre pads, causing the moisture to evaporate and take up heat from the air. The air gains humidity and loses heat, a desirable outcome in many environments. All evaporative cooling systems suffer from the same limitations: they work best when ambient humidity is low and they require an abundant and reliable supply of high quality

Interior Swamp Cooler Picture
Commercial size evaporative cooler (Reproduced with permission from JS Humidifiers)

water. Some swamp coolers are small and efficient enough to be powered economically by solar PV panels, and units with integral PV panels are available for mounting on the exterior walls of buildings. These can be a very effective solution in hot dry climates, as the PV panel will provide most power when the sun is hottest.

Despite its name, all forms of passive downdraught evaporative cooling (PDEC) work better with some forms of energy input, either in the form of pumps, fans or remotely controlled vents and dampers. Swamp coolers, sprinklers or sprays cool and humidify incoming air at high level, either in a separate cool tower or at roof level in one or more atria. This denser air then sinks to ground level, displacing lighter, warmer air as it does so. The cooled air can be directed into intermediate floors on the way. Perimeter vents allow the warm air to escape. All over the world a number of PDEC installations have demonstrated the concept's efficacy and practicality, and the system is said to be popular with building occupants.

Four

Solar chimney

Four

Solar chimney

Solar Chimney

Water tank with toilet float valve assembly

Pressurised water line

Vent

The most common cool tower design

Water tank with toilet float valve assembly

Pressurised water line

Vent

The most common cool tower design

Sometimes the extra humidity produced by direct evaporative cooling can be undesirable. The alternative is an indirect evaporative cooling system, in which the air that is cooled by evaporation passes through a heat exchanger and, in turn, cools the air that actually enters the building. An indirect system can also form the first stage of a two-stage evaporative cooling system, feeding pre-cooled air at ambient humidity into a direct evaporative stage. Evaporative systems have the advantage that their effectiveness increases as ambient temperatures rise - and in most climates humidity generally reduces on the hottest days, another bonus. Direct comparisons with conventional air conditioning are slightly unfair as air conditioning works in humid climates as well and has an inherent dehumidification effect. However, it is suggested that a well-designed evaporative cooling system in the right location will cost much less than half the cost of refrigerated air conditioning to install, and demand less than 25% of the energy to run.

Other active evaporative cooling systems have been tried. One system designed for residential buildings is based on the use of a metal pitched roof. During the night, water is sprinkled on the side opposite the direction of the prevailing winds and flows down to the gutter. Fans draw the air from inside the attic space and distribute it through the building. For non-residential buildings where daytime cooling is more important a flat roof is normally used. Water is cooled by spraying and then captured in roof top drains, stored, then circulated through pipes cast into floor slabs.

Spray cooling usually works best when the water supply is pressurised well above mains pressure, so that the water can be forced through nozzles that produce as small a droplet size as possible. The finer the spray or mist, the more energy is absorbed in the evaporation process - and the lower the water consumption needed for a specific reduction in temperature. At 15% relative humidity a well-designed evaporative cooling system could reduce the temperature of ambient air entering the building by around 50% - e.g. 40°C to 21°C. Even with ambient air at 50% relative humidity the potential reduction is worthwhile; 32°C to 24°C, for example.

Soft potable mains water is usually preferred for evaporative cooling systems, as water from other sources could contain high levels of dissolved salts that would precipitate out and eventually clog up the system. Unfortunately, the periods of highest demand from evaporative cooling systems can inevitably coincide with periods of low rainfall and mains water shortages. Many of the areas of highest demand will have naturally hard mains water supplies. Grey water is not a realistic alternative, and collecting enough rainwater to feed evaporative coolers throughout the summer is likely to be impractical. Suitable water might be available on site from boreholes or artesian wells, but even if the water is too hard or official permission to extract it not forthcoming, water from deep aquifers can still be used for cooling if it is recirculated back into the ground after use.

Aquifer cooling is usually accomplished via a dual-purpose aquifer thermal energy store (ATES) (see Chapter 15). When ground conditions are suitable, cold water from an aquifer below the building is circulated through one or more heat exchangers, where it cools incoming ventilation air and/or water in a closed loop, which can then be circulated through some form of cooling element inside the building. Typically, the aquifer water might leave the ground at 7°C and have its temperature raised to 15°C in the heat exchanger before being pumped back down to the aquifer at least 100 m away from where it was extracted. In winter the system is reversed and used for space heating. An ATES is ineffective as a thermal store where there is significant water movement along the aquifer: it would still function as a source of cold water, however. Ground water that is particularly brackish or otherwise aggressive has to be handled with care, but has been used successfully, usually by circulating it in largely plastic pipe networks. Experience to date suggests that where the right ground conditions exist, aquifer cooling can be remarkably efficient and cost-effective (see Chapter 18).

At some locations cooling water may be available from other sources, such as deep ponds, lakes or even the sea. The source must be deep enough for the bottom water to stay cool right through the summer. In deep lakes thermal stratification begins to take place as spring advances into summer. Below 15 m or more the water temperature can be 20°C below the surface layers, where water temperature varies little with depth due to the mixing effects of wind and convection. A shallow layer with a steep temperature gradient separates the warmer and colder waters. This is known as the thermocline or metalim-nion, and cooling water intakes are normally sited to be below all normal levels of the thermocline to assure reliable supplies. Flooded quarries are often deep enough and sheltered enough to develop a stable and predictable thermocline, and disused underground mine workings often contain water at a constant low temperature.

A recent example is the Enwave Energy Corporation's use of water at 4°C from a depth of 83 m in Lake Ontario, Canada, to cool buildings in downtown Toronto. The water is filtered and processed and eventually passes to the city's potable water supply. On the way it flows through heat exchangers in an , energy transfer station,, the key element in the Deep Lake Water Cooling System.

Summertime thermoclines can establish themselves in the sea as well, but there is no guarantee this will occur at any specific location. In deeper seas there will be a permanent thermocline, which can be as deep as 1,000 m or as shallow as 50 m. Sea bottom temperatures above the permanent thermocline and close to shore will vary widely from location to location, depending mainly on depth, tidal range and exposure. However, in higher latitudes, even the seas at shallower depths take a long time to warm up. Water below 20 m or so, which may be only 2°C in May, could remain below the 10°C mark until late September. These temperatures are perfectly acceptable as the basis of a space cooling system, so buildings close to sea water deeper than 20 m have a very economic cooling option available. Points to consider include the need for corrosion resistance in the system and the prevention of organic growth internally - this is perhaps best accomplished by cathodic protection. A debris screen must protect the intake itself.

Wherever the cool external water comes from, it will normally be passed through a heat exchanger where it will take up heat from either air or water before being returned to the source. The cooled air or water can be utilised in a number of ways. Water is often passed through a second heat exchanger to cool the building's ventilation air. Less energy is needed if the cool water is circulated through a radiant cooling system, which ideally utilises the ceiling as the cooling element. Cooling pipes can be incorporated into ceiling panels, hidden behind false ceilings or cast into the slab above. Of the heat removed from the room below, up to 50% will be lost by radiation and the rest by convection. This type of system can operate at cooling water temperatures as high as 15°C: indeed, temperatures much lower than this should be avoided, because of the risk of condensation.

Chilled beams are another option. Basically a mirror image of a space heating radiator, chilled beams are air to water heat exchangers mounted at ceiling level. They come in many designs, and these days often manifest themselves as multi-service chilled beams: units which can incorporate lighting, fire suppression sprinklers and even public address systems alongside the heat exchangers. Again, the water circulating through the chilled beams is at relatively moderate temperatures. Both radiant cooling and chilled beams

Deep lake water cooling system

Three intake pipes draw 4°C water from Lake Ontario at a depth of 83 meters. The water is then filtered and treated for the City's potable water supply.

At the ETS, the icy cold water is used to cool Enwave's closed chilled water supply loop through 36 heat exchangers. The ETS is adjacent to the City of Toronto's John Street Pumping Station.

Chilled water can bypass the cooling plant and continue to the customer building. If necessary, water can be further chilled by two 4,700 ton steam-driven centrifugal chillers. ^^ C

Heat exchangers at the customer building cool the internal building loop, providing chilled water for the building cooling system.

Enwave chilled water loop extends to other buildings.

Chilled water is returned to the Enwave Energy Transfer Station to repeat the cycle.

Q ENWAVE CLOSED COOLING LOOP

Heat exchangers at the customer building cool the internal building loop, providing chilled water for the building cooling system.

Enwave chilled water loop extends to other buildings.

Chilled water is returned to the Enwave Energy Transfer Station to repeat the cycle.

Enwave Coolers

CHILLED WATER SUPPLY TO OTHER CUSTOMERS

Q ENWAVE CLOSED COOLING LOOP

ENWAVE SIMCOE STREET COOLING PLANT

CHILLED WATER SUPPLY TO OTHER CUSTOMERS

ENWAVE SIMCOE STREET COOLING PLANT

Benefits:

• Uses 90% less electricity

• Reduces thermal discharge from power plants to the lake

• Reduces air pollution

Eliminates ozone depleting CFCs

Eliminated cooling towers and improves water efficiency

Legend #

Chiller

*

Direction of water flow

*

Heat exchanger

Toronto takes advantage of permanently cold water below the thermocline in Lake Ontario (Reproduced with permission from Enwave Energy Corporation)

3"

Once famous for the energy saving gold coating to its glazing - more than $1 million worth of gold was used - Toronto's Royal Bank Plaza is now cooled via the waters of Lake Ontario

are said to be more comfortable for building occupants than traditional air conditioning, although there will be no dehumidification of the internal air.

In buildings where cooling loads are high and alternative options are limited, some form of refrigeration technology may have to be considered, to meet part or all of the cooling needs. Two different approaches are currently used: mechanical vapour compression and heat driven absorption. The first is based around an electrically powered compressor and a heat transfer fluid that these days is a safer form of chlorofluorocarbon (CFC) than the original Freon patented in 1928. Freon and its close relatives are now banned because of the damage they cause to the ozone layer. Other, safer, forms of CFC are now used, but still the potential for ozone depletion exists. Compressive chillers could be powered by electricity from mini hydro, for example, or even from a photovoltaic array, but it is still questionable that a PV-powered compressive chiller is a realistic option.

Absorption chillers are a different matter. They can utilise heat from solar panels, waste heat from industrial processes, or the heat generated as electricity is produced in a trigen-eration installation (see Chapter 14). Solar-heated water has to be at temperatures typically between 80°C and 120°C, which normally imply the use of either high performance

Chaudiere Chaffoteau Ulta

Absorption chillers are well developed and widely available (Reproduced with permission from Ener-G, www.energ.co.uk)

Parabolic Trough Plant
A large-scale solar cooling installation using parabolic trough collectors

evacuated tube collectors or concentrating collectors (see Chapter 9). Some more modern versions are said to be capable of operating at significantly lower temperatures compatible with cheaper collectors. Absorption chillers use water as the heat transfer fluid and ammonia or non-toxic lithium bromide solution as an absorbent. Water evaporates in a low pressure chamber, causing the temperature to fall to around 7°C. More water flows through a heat exchanger inside the evaporator, is cooled, and is pumped to another heat exchanger where it cools the air entering the building. The water vapour passes to an absorber, where it is taken up by the ammonia or lithium bromide and the heat released in the absorption process is exchanged with cooling water, which then circulates through a cooling tower.

Pdec Cooling Tower

2. Thermal store

3. Absorption chiller

4. Cooling tower

5. Air to water heat exchanger

2. Thermal store

3. Absorption chiller

4. Cooling tower

5. Air to water heat exchanger

Absorption coolers can also be powered by highperformance evacuated tube collectors

Absorbent leaving the absorber is dilute and needs to be concentrated again before it can be re-used. Solar-heated water is used to drive off the water in the absorbent as vapour; the freshly concentrated absorber is returned to the absorber while the water vapour passes to a condenser and then back to the evaporation chamber. Gas-heated water is often used as a back-up for when solar power is inadequate. Using surplus heat from a cogeneration installation for cooling to achieve trigeneration of electricity, heat and 'coolth' can be very effective as, unless a seasonal thermal store is available, disposing of surplus heat from CHP units during the summer can be a real headache and may even limit the amount of electricity that can be generated (see Chapter 14). Waste heat from industrial processes is another option, as is the use of solar PV power to drive the essential pumps.

A single-stage absorption chiller as described above is relatively inefficient in terms of energy conversion, but when the energy is heat which would otherwise go to waste the overall package can be very attractive; especially as the incoming air is dehumidified as well. A two-stage absorption chiller re-uses some of the heat normally lost to the cooling tower and is significantly more efficient, although capital costs are inevitably higher. It normally needs water or steam at temperatures of up to 300°C to be really effective, which actually rules out solar power, as temperatures like these can only be achieved by tracking parabolic trough collectors in lower latitudes.

The latest generation of desiccant chillers are able to operate with either solar-heated water or air, and are said to be compatible with solar tiles and other forms of building envelope integrated solar collectors. Earlier designs were based on a simple process. Incoming air is dehumidified by a solid desiccant such as silica gel then cooled in a heat exchanger by the cooler outgoing air. This fresh, dry, pre-cooled air then passes through a direct evaporative cooler before entering the building. Heat absorbed by the outgoing air is used to regenerate the desiccant, which is usually mounted on a wheel that rotates through both the incoming and outgoing air streams. This regeneration needs extra heat, which can be supplied by renewables or back-up gas heating. Installations like this were commonly used to feed precooled air into a conventional mechanical vapour compression air conditioning system: the combined installations are said to be more cost-effective than stand-alone conventional air conditioning.

Developments like the US-developed NovelAire system offer a more integrated option. In it a proportionally larger volume of ambient air is dehumidified and then split into two streams. One passes through a direct evaporative cooler then is used to cool the second air stream before being exhausted back to atmosphere. The second stream is cooled without any increase in humidity, and then passes through a second direct evaporative cooler where final temperature and humidity levels can be determined. Solar and/or recovered heat backed up by gas heating is used to regenerate the desiccant; waste heat can be transferred to the main diurnal thermal store for domestic hot water heating or to a seasonal thermal store (see Chapter 15). Solar-heated air is the simplest option for desiccant regeneration in practice, and the most efficient, as no heat exchanger would be needed as it would with solar-heated water.

A number of other ways to utilise the sun's energy to cool buildings are under development. Most have the same basic advantage: solar air or water heating arrays are sized to perform well in spring and autumn, so that in the summer there is excess energy available. This could be diverted into a seasonal thermal store - or it can drive the cooling system. Peak demand will match peak output. Adding some form of ' coolth' store, which could be anything from a rock bin to a store based on phase change materials, will extend the cooling period into the night. Gas is the obvious back-up energy source.

Heat pumps (see Chapter 12) are also becoming popular as a space heating technology -but heat pumps are essentially reversible. In principle, a heat pump is a device that uses energy to move heat from one location to another. Thus in the space heating mode of most heat pump installations heat is extracted from the air, from the ground or from a convenient water source, and transferred into the interior of the building. The source may be at low temperature, but the heat pump will concentrate the heat collected into a much smaller mass of air or water, raising its temperature to useful levels. Switching the heat pump into reverse will cool the air inside the building and transfer the heat into the outside air, the ground, or the water source, from whence some of it may be recovered in colder weather. Heat pumps are inherently less efficient as space coolers than as space heaters unless the heat removed from inside the building is transferred to a thermal store for later use.

Hot outside air temperatures

Relatively cool ground

Room air returns to air handler

Cooled air is distributed through the house via ductwork

Room air returns to air handler

Cooled air is distributed through the house via ductwork

—1

Room air

returns to

air handler

Cooled air

is distributed

through the

house via

ductwork ^^

In cold zone, refrigerant absorbs heat from circulating interior air

In cold zone, refrigerant absorbs heat from circulating interior air

^ Ground loop releases heat to cool earth

Hot refrigerant flows through coils, releasing heat to cooler water in ground loop

Compressor

Heat pumps can be run in reverse to provide summertime cooling (Reproduced with permission from Geothermal Heat Pump Consortium Inc.)

One practical problem is that it is difficult to balance both heating and cooling needs with one heat pump, as the heat from the compressor motor and any internal fans adds to the heat transferred from the outside source when the pump is used for space heating. When used for cooling, the pump has to transfer both the heat from the building and the heat from the motor and fans. Add to this the fact that many modern buildings, especially offices, have a significantly greater cooling demand than heating, and in practice a heat pump sized for heating might well be capable of dealing with only half the building's cooling needs. Another problem is that if the space heating is based on underfloor pipe networks, reversing the system and cooling the floor slab may cause condensation to form. One way round this is to install an additional small air source heat pump, usually in the loft or above a suspended ceiling, whose sole function is to extract heat from internal air. This heat could then be transferred to the ground loop of a ground source heat pump installation or the water source for a water source heat pump and stored for winter use.

Heat pumps, of course, depend on electrical power. A typical dual-function heat pump installation for a medium-sized building may require up to 100kW.This would imply either a solar PV array of close to 1,000 m2, a horizontal axis wind turbine with a diameter approaching 30 m, or a sizeable mini hydro installation, if alternative energy is to be used. Other routes include the use of various forms of biomass to generate the necessary electricity (see Chapters 7 and 11). Alternatively, an absorption heat pump might be specified. This works on much the same principles as the absorption chiller described above, deriving its power from any heat source available. Its drawback is that it is not yet a fully developed technology, and that it suffers from the same sizing problem as all other dual-function heat pumps.

Using intermittent energy supplies from wind turbines and the like is rarely the ideal solution for cooling purposes, especially where the building has limited thermal mass. The alternative is to use the heat pump to cool a thermal store when energy supplies are available. A heat exchanger within the thermal store supplies the actual cooling function within the building.

For most building projects in the foreseeable future the provision of space cooling will be a major priority. Conventional air conditioning will continue to have some role to play, but that role will have to become much less important if the energy consumption of buildings is to fall significantly. Luckily, the technology exists for this change to happen without major disruption to current lifestyles or architectural fashion.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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Responses

  • EMAAN
    How desert cooler works?
    8 years ago
  • Jenni
    How does the petrodollar system work?
    8 years ago
  • daniel faust
    How solar chimney works?
    8 years ago
  • Makda
    How to side mount a roof mounted swamp cooler?
    7 years ago
  • henna puikkonen
    Does it matter which way vents go on evap cooler?
    7 years ago

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