Water Source Heat Pump Systems

Water Source Heat Pumps use the heat in the water and mud at the bottom of a pond, lake, or river. For maximum benefit, to avoid any risk of freezing, and to be out of the way of recreational water use, the piping should be at a depth of 2m or more. The piping could be placed at 1+m depth in the muds under Reeds or water plants without risking damage. The area needed within the house will be the same as for the Ground Source Heat Pump.

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How might it work for you

While these systems are normally used for heating houses, they could also be used to heat a Swimming Pool or even an Aviary/Pigeon loft etc.

NB although the water in a pond may get warmer in the summer, the flowing water in a river will keep the river muds warmer in winter as it crosses the floodplain in its lower reaches. Therefore, for maximum heat abstraction with minimum electrical usage, river muds are best with the proviso that the depth of flowing water above the muds is great enough to prevent them freezing.

The first step is to assess how many kWh you spend on heating your property, as this will give the supplier an idea of the size of installation that is needed.

If you are lucky enough to have a large pond or small water body on your premises, then this is the system for you. One can utilize the fact that water is densest at 4°C to place a useful piping system in the muddy bottom. The water will need to be deep enough to avoid freezing. In the UK a depth of 1.5 - 2m should be deep enough to avoid all problems.

The actual Heat Extractors for both: the Air, Soil and Water heat sources will require housing. This could be a lean to shed, or under-stairs cupboard, or cellar, or a small room dedicated to housing the Heat Pump itself, plus accumulator tanks and associated electronic controls for the house's heating system. The volume of space needed will depend on the size of the house and the system installed, but a rough estimate would be for a minimum area of 3 x 2m that is tall enough to stand/operate in.

There are two techniques to avoid an hour of a cold house in the early morning, or hours of cold offices on a Monday morning.

  • The Heat Pump can be left running all the time, preferable to avoid condensation forming in offices; and usually recommended by the manufacturers using the most efficient modern systems. However, this does mean that the installation is also using electricity all the time; and most people sleep better at a lower air temperature.
  • The Heat Pump can be turned off at night, but an insulated Accumulator/Bulk tank can be plumbed into the system to store extra hot water from the day. This hot water can then be used to prime the system first thing in the morning until the cooled water in the piping can be returned to the pump and heated up from the fluid in the outside piping network/loop. This means that the house will warm up quickly as soon as the Heat Pump is turned on again.
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Facts - Energy Produced and Costs of Production

When working well, with a well-designed pipe layout, a Water Source Heat Pump can have a CoP (Coefficient of Performance) of 1: 4 i.e. 1kW of electricity used to drive the system will result in 4kW of heat energy abstracted. However, the lower the temperature of the incoming fluid, the more electricity will be needed to abstract the same amount of heat; in this case the CoP can fall to 2.5 or below.

Ideally the system installed should be reversible; so that in summer, excess heat can be returned to the water, thus cooling the house. This is an especially attractive feature in warmer climates. Though not so useful for the muds of a pond that are already being warmed by the heat of the summer sun, this can usefully help to increase the restoration of heat to the soils a couple of metres below the lawn, or to the bedrock surrounding a Ground Loop system.

Currently no payments equivalent to FiTs (Feed in Tariffs) are payable for non-electric heating systems. However, from June 2011 there will be RHI (Renewable Heat Incentive) payments for any installations completed after 15th July 2009. They will equate to the FiTs available for electricity generation. These are to be calculated to bridge the gap between the cost of heating by Conventional and by use of Renewables. A component will be towards the establishment of the system and the RHI will also offer a Rate of Return of 12% on the additional cost of Renewables, or 6% for Solar Thermal systems - the input from the sun being free - whereas Ground source has an element of electrical input to extract the heat and Biomass etc. requires growing and payment for the crop.

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Pros and Cons

Pros

Pros

  • The systems can be run with under-floor distribution or with radiators. In the first instance - suitable for new builds or conversions - a thermostat controls the temperature of the whole room or house. However, if radiators are to heat a whole room to an even temperature, then the radiators would have to be kept very hot. Inevitably there will be a temperature gradient across the room from the site of the radiator, and to deliver the heat to where it is required most will require careful thought. In this scenario the electrical input is often greater, depending on where the thermostat is situated.
  • With all types of ground source heating, the distribution system does not have to be fitted throughout, but can be limited to specific rooms in the house. It should be possible to switch the flow to specific rooms off and on as required e.g. for a guest bedroom.
  • It should also be possible to arrange the plumbing so that the house heating could be boosted by use of a wood fuel boiler or a solar thermal system; instead of using extra electricity to provide point sources of heat.

Pro's of the Water Source system

  • The Water body provides a more stable winter temperature than the air, but is likely to be less stable than the Ground Source Slinky or Ground Loop
  • The CoP will hold good throughout the coldest winter. This means that
  • The electrical energy needed to drive the system can be predicted accurately.
  • The area required for the installation will be as large as the available pond, but not less than 10sqm
  • No excavations, other than for the piping from the pond to the house, will be needed

Con's of the Water Source system

  • It will be necessary to get wet and muddy in order to place the Slinky, or other piping network in the muds at the bottom of the pond.
  • As for the other Ground Source installations the cost of the inside work has to be added on to the cost of the installation in the pond; and, as for all systems, the actual cost will depend on the size of the house and type of heating (under-floor/radiator).
  • This will require Planning Permission which is likely to require knowledge of any pond species that would be disturbed; and the precautions to be taken to preserve such species, and to restore the habitat after the siting of the piping network.
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How and Why it works - Horizontal Pipe arrays in loops; Slinkies; and Borehole Loop systems

For a detailed explanation see en.wikipedia.org/wiki/Heat_pump

The 35% of the infrared radiation that reaches the earth causes the behaviour of our winds, tides, water cycle (rain and evaporation) through the action of heating and cooling; as well as powering all of the flora and Fauna on the planet. A 1sq.m area of plant leaf intercepting 0.8kw is able to make use of ca. 0.1% of this to produce the chemical energy needed to power photosynthesis. A lot of the infrared radiation is reflected and much of the ?stray? energy causes the soil to heat up from the surface downwards. The top 3m are also heated from the earth?s crust, but the temperature in the top 1m is influenced more by the solar radiation reaching the surface and the season. The deeper one goes, the warmer the temperature becomes and the less it is affected by events on the surface; after all the molten core of the earth (centre at 6,500km below the surface) has a temperature of ca. 5,500°C. As cavers will know: in summer, caves always seem cool and refreshing when one enters from the heat outside; but on a frosty winters day, the same cave seems warm and cosy by comparison. In fact cave entrances are often found in winter by looking for patches of vegetation that, warmed by the air from below ground, are not white with frost/snow.

In most maritime temperate climates, at a depth of approximately 0.6096m (2 feet), the soil does not freeze. All Ground Source systems make use of the fact that the temperature from this depth downwards gradually increases. The temperature of the top 2 feet (0.6096m) varies widely with the ambient air temperature at the surface. But below this the minimum temperature will be >0°C (-32°F) and the max. can be as high as 10°C at 3m depth. This provides a varying level of heat throughout even the winter months. Obviously if the 2 feet above is frozen solid, then the temperature below will be nearer to 0°C than 10°C. In order to maximize the available temperature without going into mining, it is normal for the top of a ground source, piping network system, to be buried at a depth of 1 - 1.5m. Therefore, this is the depth of the whole of a horizontal network, whilst the Slinky - a favourite with many plumbers - having a diameter of approx. 1m can be set in the ground either horizontally at 1.5m or in a vertical slot with the top of the pipe at 1m and the bottom at 2m; or pulled out to form a spiral. The latter arrangement obviously gives the best heat extraction.

A simple heat exchanger is just like your refrigerator, in that the fridge takes heat out of the food put in to it and pumps it out into the kitchen. The heat exchanger takes heat out of the air or fluid that is pumped into it from outside, bulks it up using a compressor, and passes the final amount of heat on to the fluid circulating in your heating pipes. Electricity is used to power the compressor, as in a fridge, and to pump the circulating liquids around the pipework; though systems can be set up - and the early ones always were - that use the difference in density between hot and cold water to move hot water around the system. However, with the electric pump, the energy used works out at approximately 1 unit in for 2.5 - 4 units of heat out; so over all, it saves energy. The electricity needed to needed to run the heat pump can be supplied from the Grid, or it it could be generated by Wind or Solar installations, and stored in modern, highly efficient accumulators (batteries).

The Heat Pump itself consists of a closed loop of piping containing a condensable gas, or a gas such as carbon dioxide that remains in its gaseous state at normal atmospheric pressure until cooled to -78°C. When it is allowed to expand, so that its molecules are well spaced out, the gas becomes cold - or liquid - at which point it can absorb heat from the incoming air. This heating makes its molecules move faster and if a liquid it will be converted back to the gaseous state in the 'evaporator' chamber. The warmed gas is now passed through a compressor causing the molecules to knock into each other in the smaller space available. This procedure increases the heat of the gas, which is then transfered to the fluid in the central heating pipes. The molecules of the cooling gas in the 'condenser' chamber move more slowly until they either change back to the liquid state, or are so spaced out that they no-longer interact, this stage ends by passing the gas/liquid from the 'condenser' through an expansion valve into the 'evaporator' where the cycle begins again.

NB in carbon dioxide (CO2): the C = an atom of carbon and the O2 = 2 atoms of oxygen. Together these make up 1 molecule of carbon dioxide.

Basic chemistry explains this phenomenon by noting that in a substance in its gaseous state the individual atoms/molecules are far apart and rarely come into contact. However, as they are compressed into a smaller volume, increasing numbers of the molecules collide; the individual atoms are also jostled and the result is that the electrons surrounding the atoms lose energy with each collision and move from an higher to a lower energy level releasing heat in the process. A more pictorial way of thinking of it, is to consider the electrons as toddlers and the nucleus as their teacher. Whilst their teacher stops to talk they are are told to stay at her side. Stationary, they cool down, but their metabolism continues to store energy rich compounds in their muscles, where it remains as a potential source of energy. However, as soon as the teacher moves on they are free to run around again and the stored energy is released becoming kinetic energy. This energy is expended in mechanical activity and the heat produced.

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