Air ground and water source energy

Heat is not the same thing as temperature. The water in a stream or well, the soil a few metres below ground level, even the ambient air, might be significantly cooler than the desirable temperature for domestic hot water or space heating use - but they still contain heat energy. If the heat from a large volume of lower temperature water, soil or air can be extracted then transferred to a much smaller volume of water or air, the temperature of the receiving water or air will be raised, even if it started out warmer than the source. One device that can achieve this desirable end is the rather misleadingly named heat pump, a technology that has attracted a lot of attention and development over recent years.

In fact, heat pumps have been virtually ubiquitous in the developed world for many decades. Small heat pumps lurk behind refrigerators and freezers, extracting the heat from inside the heavily insulated cabinets and transferring it to the air in the room. They are quiet, reliable and unobtrusive. Efficiency is high. At first glance, a heat pump might appear to defy the laws of thermodynamics; indeed, to be a virtual perpetual motion machine. It is often claimed that a typical heat pump installation can produce 3kW of space heating (or even more) from only 1 kW of energy input, giving it an ' efficiency of 300%' (or even more).

This is a misconception. What is happening is that the heat pump is using 1 kW of energy to transfer 3kW of energy from the ground, water or air outside the building to the interior. In its most basic form, the energy in the outside source comes mainly from the sun. However, in more sophisticated installations the ground or the water might have been topped up with surplus heat from other processes (see Chapter 15). Unlike other solar energy systems, heat pumps drawing energy from the ground and water are not weather-dependent, and so work equally well night and day. Air source heat pumps obviously are more sensitive to the weather, but ambient temperatures fluctuate much less than sunlight.

Most heat pumps are based on the same vapour compression cycle used for refrigeration and air conditioning, and use a compressor driven by an electric motor. This type of heat pump is sometimes said to 'replace' the energy lost in the generation and transmission of electricity over national grids. Thus the 3 kW of heat transferred to the building by the 1 kW of power consumed by the heat pump equates to the 3 kW of energy - mainly chemical energy from fossil fuels - that is needed to ensure 1kW of electrical energy arrives at the point of use. The missing 2 kW is dissipated as waste heat and transmission loss. From the consumer's point of view, a heat pump should cost only one-third or less to run than a basic electric resistance heating system.

For those wishing to minimise their dependency on fossil fuels, an increasingly popular alternative is the absorption heat pump. This runs on heat rather than electricity, has fewer moving parts and usually utilises a safer refrigerant than the vapour compression alternative. Heat can be supplied from a number of sources, of which the most energy efficient is normally waste heat from an industrial process. Solar-heated water can also be utilised, especially if it comes from evacuated tube collectors. Again, there is an apparent multiplication process. One unit of heat put into the heat pump will transfer around one and a half units of heat into the building's interior. This represents a coefficient of performance (COP) of 1.5, as against a vapour compression heat pump's COP of 3 to 4 or more, and may seem comparatively ineffective. However, the potential of an absorption heat pump to provide both heating and cooling from one heat source and - with a little help from solar PV panels to drive the essential fans and pumps - to be independent of energy grids, can make it an attractive solution for some projects.

The first generation of heat pumps designed for space heating and cooling used external air as the energy source. Air source heat pumps are relatively cheap and simple. They are particularly effective where winters are mild and long periods of freezing weather rare or unknown. In its simplest form, an air source heat pump consists of two air-to-refrigerant heat exchangers - one outside, one inside - an expansion valve, and a compressor. The outside heat exchanger acts as an evaporator in the heating mode, absorbing heat from the air into the refrigerant. This then flows into the internal heat exchanger where it condenses, releasing heat into the inside air. Ducts and fans then distribute the heated

A split-system heat pump heating cycle

A split-system heat pump heating cycle

Underfloor Ducted Air Conditioning
Water loop system using external heat pump

air throughout the building. In effect, this is air conditioning in reverse (see Chapter 13), and in many installations the air source heat pump is expected to perform both functions. More versatile units can offer both space heating and domestic hot water via refrigerant to water heat exchangers. Many of these are packaged systems, and are installed entirely outside the building.

Drawbacks include aesthetics - the external elements can be visually obtrusive -compressor noise, and a high risk of the external coils icing up in cold weather. Defrosting is needed, a function normally performed by switching the heat pump into reverse. However, this cools the inside of the building while it occurs. Prolonged low ambient air temperatures can be a real problem. As the temperature falls the heat pump must do more work in order to move the same amount of heat into the building - which will be losing more heat anyway. Ultimately the pump will be using one unit of electricity for every unit of heat that it moves into the building, and will be highly stressed. To cope with such extremes, many small packaged air source heat pumps have back-up electrical resistance heaters built in.

Ground Coupled Heat Pump
Air source heat pumps are well developed and widely available (Reproduced with permission from Nu-Heat)

A much less obtrusive and reliable, if significantly more expensive, option is the ground source heat pump, also known as the geothermal, geoexchange, ground-coupled, earth coupled or earth energy heat pump. (Geothermal energy more properly refers to the tapping of the energy of hot rocks up to 10 km below the surface.) The ground used as an energy source in this application can be as little as 1 m below the surface where there is little ground freezing in winter. At such depths soil temperature will be influenced by solar gain, rainwater evaporation and atmospheric conditions. Experience suggests that as a rule of thumb, annual average shallow soil temperature can be taken as average annual air temperature. This will generally lie in the range of 7°C to 21°C, but will vary significantly between winter and summer. At greater depths - in excess of 10 m - soil temperature is

Water Heater Desuperheater Piping

Cooler -antifreeze out

Warm antifreeze in

Typical ground source heat pump providing warm air space heating

1.

Cold air return

2.

Hot refrigerant out

3.

Desuperheater

4.

Domestic hot water

heater

5.

Refrigerant piping

6.

Reversing valve

7.

Compressor

8.

Warm air to house

9.

Blower

10.

Secondary heat

exchanger

11.

Expansion device

12.

Primary heat

exchanger

Cooler -antifreeze out

Warm antifreeze in much more constant, but again it can be assumed that it will be approximately the same as average annual air temperature.

How much energy can be extracted from the ground is a function of its thermal properties. Solid rock has much greater thermal conductivity than soil. Dry, loose soil has a significantly lower thermal conductivity than wet, compacted soil. Groundwater movement across the site can also increase the potential rate of heat transfer. Before any decisions on what type and performance of ground heat exchanger - or ground loop - will be needed for a particular project, a proper geotechnical survey is essential, including trial boreholes.

A trial borehole will enable the size of ground loop needed to be calculated. The final choice is between a horizontal or vertical design. Horizontal ground loops used to be the most common, especially for individual dwellings, and were significantly cheaper than the vertical alternative. They do depend on enough land being available, land where extensive trenching work is acceptable. Trenches have to be at least 1,200 mm deep and up to 600 mm wide. The ground loop itself was originally a single pipe, which ran out, and back again along the network of trenches. The so-called 'Slinky' layout is quite popular. Shorter, deeper trenches are used, with the pipework installed in a series of overlapping vertical loops. This usually cuts installation costs - it can also make a horizontal ground loop a practical option when site space is limited.

Slinky Loop System
Horizontal closed loop system with vertical 'Slinky' layout
Slinky Vertical
Horizontal closed loop system with horizontal 'Slinky' layout (Reproduced with permission from ICE Energy Heat Pumps)

A more recent development from Sweden is the 'compact collector', basically 2 m x 2 m assemblies of eighteen 40 mm diameter high density polyethylene (HDPE) pipes that act like radiators in reverse. These can be installed horizontally or vertically, depending on which is the priority: minimising the depth of excavation or its extent.

Vertical ground loops used to be considered only for larger projects, or where there was no room for a horizontal ground loop, or where there was little soil above bedrock. It was usually a much more expensive alternative. However, recent advances in small-scale drilling technology make the vertical option competitive with horizontal designs, even on the domestic scale. As housing plots grow even smaller, this development is timely. It can also make ground source heat pumps a much more attractive retrofit option, as there is minimal disruption to mature gardens.

Horizontal 'Compact Collectors' from ICE Energy (Reproduced with permission from ICE Energy Heat Pumps)

Another option being tried in Europe lies somewhere between straightforward solar water heating and ground energy capture. In its simplest form, a horizontal ground loop is installed a short distance below the surface of car-parks or access roads; these absorb solar energy quite effectively. Areas with dark surfacing are particularly suitable. A central car-park may also be a convenient location for an underground thermal store (see Chapter 15) from which warm water might be pumped back through the ground loop in winter to de-ice the car-park and access roads.

Most ground loops form part of a closed loop indirect system . A pump circulates a mixture of water and antifreeze though the loop then through a heat exchanger at the heat pump. High-density polyethylene tubing between 20 and 40 mm diameter is commonly used, with all joints heat fused. Closed loop direct systems dispense with the heat exchanger, and circulate the heat pump's refrigerant fluid through copper pipes in the ground. These are falling out of favour, largely due to planning authorities' reluctance to allow potentially toxic refrigerants to circulate close to aquifers. Trenches for horizontal ground loops are usually kept at least 3 m apart to avoid thermal interference. Multiple pipes within individual trenches should be separated by at least 300 mm. Pipes are normally laid on a sand bed and protected by at least 150 mm of sand before being carefully backfilled. Proper compaction is important.

Vertical ground loops typically will use boreholes up to 150 mm in diameter and 120 m deep, spaced 5 m apart. After the pipes are installed they are commonly backfilled with a high conductivity grout. Getting the size of the ground loop right is crucial - too large and capital costs could be uncompetitive; too small and the loop will not be able to collect enough energy to meet the building's needs. Perhaps the most difficult calculation, even with modern software, is the determination of exactly how much energy will be needed and when (see Chapter 16). In many cases the most economic installation will depend on other forms of back-up heating for peak demand, and this is usually supplied by natural gas or mains electricity.

Much the same comments apply to water source energy installations. These are only possible when there is a suitable water source on or close to the site, or even below it. Lakes, ponds, rivers and streams, flooded quarries and even old wells can be used, as can the right type and depth of aquifer or flooded mine workings. The earliest form of water source energy installations used an open loop design . Water was pumped directly from the source, through a heat exchanger on the heat pump, and either discharged into a convenient stream or river or pumped back into the source again. Usually, systems that return the water back to the source are reversible; heat is dumped into the source during the warmer months, some of which will be recovered during the cold weather (see Chapter 15). Problems with biological growth inside the pipe networks, corrosion and planning limitations make closed loop systems more popular these days. These usually consist of the same heat fused HDPE pipework as used in ground source installations, which has to be installed deep enough to avoid any risk of freezing.

Open system using well water
Using The Deep Well Cooling Tower

Confusion can be caused by the misuse of the term 'water source heat pump systems' for installations; which is more correctly described as water loop heat pump systems . These are alternatives to conventional heating and air-conditioning systems for larger buildings. They use water to air heat pumps in each zone connected to a separate pipe loop which usually passes through both a cooling tower and some type of boiler. Together the boiler and the cooling tower maintain the water in the loop at a temperature somewhere between 15°C and 30°C as the heat pumps heat or cool the air in the individual zones. Where there is a mixed use building, such as retail/residential, there is often a simultaneous need for heating and cooling in different zones, so the loop will need little energy input or cooling as the heat is transferred from one zone to another.

In some cases the heat pumps on the northern side of a building could be drawing heat from the water loop at the same time as the units on the sunny southern side are adding heat to it. Although such a system can use simpler, cheaper heat pumps as they have to operate over a much smaller range of temperatures, there are the usual caveats regarding the risks of legionella in the cooling tower. And in most cases the overall energy demand is greater than with other forms of heat pump system. Running costs can be significantly reduced, of course, if the heat in the loop can be supplied by renewable energy. Domestic scale water loop heat pump systems are available in the US. One variant of the system uses an external air source heat pump to both heat and cool the water in the loop.

Water source energy systems gain efficiency if the water is used as a thermal store, effectively cooled down in winter ready for the summer and vice versa. Other forms of heat pump systems can benefit significantly from the inclusion of a more conventional thermal store or buffer tank in the circuit. At the very least, this allows the heat pump to operate mainly on off-peak electricity; heating or cooling the tank cheaply in advance of peak demand. It also makes heat pump systems powered by solar or wind energy more practical. Coupled to an air source system, the thermal store provides the heat to defrost the external heat exchanger when necessary, eliminating the unpopular routine of putting the pump into reverse to achieve the same effect (see above). It can also store the heat collected by the desuperheater, a secondary refrigerant to water heat exchanger that can be

Thermal store

Heat pump cooling mode ht

13°C Cold air

Heat pump heating mode

30°C warm air

Typical air source heat pump in action installed at the compressor outlet, and which normally yields about 10% of the total heat pump capacity.

The real benefit of a desuperheater, however, is that it has an output temperature of up to 70°C, which is very suitable for DHW. Maximum output occurs during the summer months, and will usually be almost negligible at some times of the year. Heat pumps are not otherwise normally capable of producing hot water above 55°C. This is only barely adequate for DHW, and many commercially available heat pumps use supplementary electrical resistance heaters to boost the temperature to a more useful 65°C. This also eliminates the theoretical risk represented by legionella bacteria (see Chapter 2). Regular 'pasteurisation' - heating the water to more than 60°C to kill the bacteria - is required by the regulatory authorities in most developed countries. In the absence of a desuperheater, this can be achieved with an immersion heater in the storage tank running on off-peak electricity if no renewable energy source is available.

Another potential problem is that conventional space heating radiators are only efficient at temperatures between 60°C and 90°C. For space heating, especially on new build projects, hydronic underfloor radiant heating is the sensible choice, as it will work efficiently with water temperatures no higher than 30°C to 45°C. Hydronic wall radiant heating is another possibility. (Hydronic = water based rather than electric.) These could be described as large low temperature versions of conventional radiators, operating at 35°C or less, and warming occupants through direct radiation rather than indirectly by convection currents in the internal air. Radiant ceiling panels are another option.

Radiant energy was utilised for centuries by the Romans, whose villas and bathhouses were heated by underfloor ducts known as hypocausts which connected into vertical flues in the walls. Slaves kept wood fires burning at the entrance to the hypocaust, and the hot gases were sucked along under the floors and up through the flues, creating a much more comfortable internal environment than the open hearths used by the Barbarians at the time. In Korea it was not just the rich who enjoyed the benefits of underfloor heating. Houses with ondol - literally warm stone - floors had their kitchens positioned about 1m below the rest of the house. Heat and smoke generated by the kitchen stove flowed under the main rooms and out through a conventional chimney at the far end of the house. The famous American architect Frank Lloyd Wright is said to have invented hydronic underfloor heating after experiencing an ondol floor in Japan in the early 1900s. Using hot water rather than hot combustion gases eliminates one of the weaknesses of both the hypocaust and the ondol floor - the constant risk of carbon monoxide poisoning.

Hypocaust Layout
Roman underfloor heating was more sophisticated than the Korean equivalent shown below (Reproduced with permission from Akajune)
Modern Hypocaust Floor Heating
But Korean houses were the inspiration for more modern housing
Modern Hypocaust Floor Heating
An example of a modern Korean home

First generation hydronic underfloor heating systems used copper pipes cast into concrete floors, and had a mixed record. Leaks were common, and difficult and expensive to repair. These days fusion welded high technology polymer piping is used, and the pipes may be cast into the structural floor proper, cast into a screed on top of insulation supported by the ground floor slab, or hung between the joists below a timber floor. Leaks are rare, and the extra space and safety achieved by the removal of hot wall-mounted radiators from the rooms above can easily justify the retrofitting of such a system. Underfloor

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