Heat Recovery

Air

“Heat Recovery” is the generic term used for removing all heat possible from foul air and foul water before it is discharged from a building, or a process, so as to reduce the use of primary energy for that building / process and to re-use that recovered / recuperated heat elsewhere within that same building or process.

When ensuring that your building is as energy efficient as is possible, one element that now has to be included is to ensure that the structure is air-tight (or as far as this is practically possible to achieve). There are now air-tightness figures that have to be met and there are also techniques available to physically measure it.

When doing this, however, the occupants of that building will still require a fresh air input to prevent suffocation.

In addition, lack of air movement leads to humidity build up and this, in turn, leads to moulds and mildews developing and breeding together with their spores and odours. Overall, poor air quality develops and this produces what is known as “Sick Building Syndrome”.

In order to alleviate this situation, sufficient fresh air is brought in from outside and distributed to the living and working areas of a building to ensure that good air quality to the occupants is maintained.

Bringing air into a space causes the air that is already within it to be displaced out, taking with it the heat that exists within that space that has been expensively produced in both financial and primary energy use terms.

To ensure that this exhausted heat is not just simply discharged to atmosphere and being wasted, meaning that the cold, fresh air coming in has to be heated, expended yet further energy to do so, residual heat is taken out of the stale air stream before it is discharged and transferred back into the fresh air stream before it is distributed around the building.

This recovery process can be accomplished using “cross-over coils”, “cross-over plates”, “HR Wheels”, “run-around coils”, etc.

Each of several different technologies available has its unique raison d'etre and preferred application / range of applications situation suited to different engineering scenarios, but generally, it would be expected to be able to recover around 80% of the heat embedded within the waste air stream for transfer into the fresh air stream.

In order to prevent cross contamination from foul to fresh, clean streams the heat transfer fluids / plates are totally and physically separated from both the foul and clean systems

Atmosphereric Air

The atmosphere that surrounds us, contains heat that is recoverable and can be usefully utilised to reduce our dependence upon fossil fuels to heat the buildings that we live and work in.

Using techniques that, in simple terms, are effectively refrigeration plants operating in reverse, external atmospheric air can be cooled by drawing it across an evaporator coil.

Within that coil is a compressed liquefied vapour which is allowed to heat when the external air is drawn across it. As it does so it expands and returns to its vapourous state.

The laws of thermodynamics indicate the heat transfers from a warm point to a cool one and therefore, with the coil being made from an excellent heat conductor, heat is drawn out of the atmosphere surrounding the coil into the cooler liquid within that coil so as to gain equilibrium across the whole. (energy is simply being transferred from one medium to the other one by application of various parts of Newton's, Boyles and other laws of physics and thermodynamics)

That vapour that is then formed is then moved from the first coil to a second one through a compressor effectively producing a super-heated vapour.

Once in the second, condenser coil, the vapour cools and returns to its liquid state giving up both its sensible and latent heat to whatever medium surrounds that similarly efficient coil conducting material. By carefully managing that surrounding medium, the energy transferred into it, can be utilised to heat either spaces, liquids or both.

These equipments are called AIR-SOURCE Heat Pumps and can have output media for onward energy transmission as either air or liquid.

Output temperatures vary from plant type to plant type, but generally for water output type machines, most units will have a maximum output flow approaching 55°C and a return of 35°C (more normally 48-33°C)

New generations on machines are slowly coming onto the market which, in effect, double refrigerate using two compressors and these are allowing output flows of 75-80°C to be achieved. This will, of course, be a boon for the retrofit market, as existing heating systems, providing that they are still fit for purpose, can be utilised. The units will, however, be expensive.

In addition to their heat acquisition and transfer capabilities, ASHPs do so for an overall energy gain in that, the amount heat energy produced at the output in kW, can be apparently as much as 4 times the amount of electrical energy input, in kW, required for them to operated the compressor and pump motors. (Coefficient of Performance, COP)

For normal design situations always allow no higher than COP3, anything else actually achieved above this is then a bonus.

Water

As with the example of atmospheric air above, heat can be recovered from process cooling waters using similar principles to those indicated above and also, domestically from waste dish-washing water, clothes-washing water, bath, shower waste water and even from sewage.

This particular recovered heat can be transferred into the clean water stream that becomes the hot water provision by pre-heating it and, once again, reducing the fuel required to heat that water to temperatures suitable for storage and legionella prevention. (The heat pump in this instance will be classed as a Ground Source Heat Pump, GSHP)

A ground source heat pump differs from its air-source cousin in that the medium from which heat is extracted is open and free in the sense of the ASHP (fresh-air), but it is totally enclosed and confined in the case of the GHSP (water / other liquids contained in pipes). COPs normally expected in the order of 3 - 3.25.

Additionally, heat can be obtained from naturally based water sources.
 

Surface Solar Energy, SSE: Water-mass, Closed Loop

All externally found volumes of open water absorb energy from the sun. As they do so, they will heat up until this water reservoir evaporates off and dries out altogether.

However, if the mass of water is large enough, whilst some will evaporate off, the heat energy absorbed in the balance remaining, can be transferred for use elsewhere and the resultant cooling by the return water will ensure that the water volume is partially maintained as the evaporation rate will not be so severe.

If a large water mass is available, say a lake or large pond, both with a depth exceeding 1000mm, then interconnected coils can be laid at the bottom of the lake, where the temperature is likely to remain reasonably constant all year round, and be connected by insulated pipes laid at least 900mm below the finished ground surface, again where the temperature is likely to be reasonably constant all year, back to heat-pump equipment at the building being supplied.

The circuit is filled with a water / glycol mix in a closed loop and is subject to being continuously pumped around that loop.

The water in the loop will absorb heat from the water in which it is immersed in the lake and this heat is extracted by the heat-pump before returning to the lake element of the loop.

During the summer, because the water in the lake is likely to be cooler than the air temperature within the building, the heat pump can be reversed and the coil in the water will act as an evaporator cooling the space - not heating it.

This model will require discussions with, and obtaining and extraction license from, the Environment Agency, especially if the lake / pond forms part of, is connected in some way to, an ongoing water course external to the site boundary. The EA are very much disposed to such systems, except where the cooling element might damage eco-systems further down stream, and can be very helpful with their promotion and very careful exploitation.

One word of caution - the over-exploitation of such systems along a continuous length of water course will not be allowed as this could have very obvious deleterious effects.

SSE: Moving Water, Closed Loop

Moving water also absorbs energy from the sun, as well as from the fact that it is moving and therefore has inherent kinetic energy.

If the mass of water is large enough, a continual source of heat energy should be available for exploitation.

Providing the water-course is deep enough to always be at least 600mm deep (except possibly during drought periods) then interconnected coils can be laid at the bottom of the stream, where the temperature is likely to remain reasonably constant all year round. The coils are then connected by insulated pipes laid at least 900mm below the finished ground surface, again where the temperature is likely to be reasonably constant all year, back to heat-pump (GSHP) equipment at the building being supplied.

The circuit is filled with a water / glycol mix in a closed loop and is subject to being continuously pumped around that loop.

The liquid within the loop will absorb heat from the water in which it is immersed in the lake and this heat is extracted by the heat-pump before returning to the lake element of the loop.

As with the previous systems, during the summer because the water in the stream / river / water course is likely to be cooler than the air temperature within the building, the heat pump can be reversed and the coil in the water will act as an evaporator cooling the space - not heating it.

This method can also be used as an open loop system, but this would require filtration along with other plant condition monitoring and protection systems which, in their turn, might not be ecologically friendly to the water source when the water is discharged back into it.

This will require detailed discussions with, and obtaining and extraction license from, the Environment Agency, especially if the stream / river / water course extends beyond the site boundaries. The EA are very much disposed to such systems, except where the cooling element might damage eco-systems further down stream, and can be very helpful with their promotion and very careful exploitation.

One word of caution - the over-exploitation of such systems along a continuous length of water course will not be allowed as this could have very obvious deleterious effects.

SSE: Ground Water, Open Loop

In certain ground conditions, it may be discovered that there is a sufficient volume of sub-terranean water that can be tapped into for the inherent energy that it contains.

By drilling a bore hole down to this reservoir, water can be pumped out to the heat pump set-up, have its available heat giving energy removed and pumped back into the reservoir - only the heat energy is removed not the water content.

Dependent upon the chemical make-up of the subterranean water, precautions may have to be taken to protect the plant being used and this requirement, as indicated above, could also prove to be non-ecologically viable in terms of damaging the original water source.

As with the previous systems, during the summer, because the water in the sub-terranean chamber will be cooler than the air temperature within the building, the heat pump can be reversed and the coil in the water will act as an evaporator cooling the space - not heating it.

In addition, Environment Agency approval would again have to be sought and obtained for any such proposals.

Geo-thermal Energy

Geo-thermal energy is, strictly speaking, that energy derived from the earth's core and that produced by the natural decay of radioactive minerals found in the earths crust naturally - we call this “Deep Geo-thermal Energy”, DGE.

Solar-derived energy absorbed by the surface mass of the earth can also be collected in several ways and each will be provide low grade heat energy that can be transformed for use in buildings of all types and uses as well as for use within processes. We call this “Surface or Solar Geo-thermal Energy”, SGE.

Being systems that utilise the mass of the earth, and essentially have a constant temperature base which is much lower than ambient air temperature in summer, they can also be used in reverse to provide for a degree of cooling at that time.

DGE: Deep Bore, Closed Loop

The earth's core which stands at many thousands of degrees Celsius, disperses its heat mainly by convection through the movement of magma that forms the majority of Earth's structure.

This magma gradually cools as it gets further from the centre to form the extremely thin and very active crust upon which we all live and carry out all of our normal daily activities.

However, the crust still retains some residual heat that is usable and it is possible to tap into this source. Whilst individually more costly to achieve compared with shallow ground coil methods, this cost is fast reducing to become a very viable option (especially where it is geologically viable to do so and it is a tight site).

Deep-bore methods are useful were ground space is restricted, and in theory could be utilised beneath almost all existing buildings and new builds.

Theoretically, anywhere that it is physically possible to drill a deep-bore of approximately 250mm dia., nominally 80 - 180m deep, and to install in it, a closed-loop liquid filled system and back-fill with suitable plug material, can subsequently be connected to a heat pump (GSHP) and provide heating and cooling energy to a structure.

This technique has recently been extended to where the deep poles for foundations are being designed with heat pump loops being an integral element of those pile constructions.

SGE: Ground Mass, Closed Loop.

All areas of the earth's crust will absorb energy from the sun as it passes overhead and as it does so, that portion of the Earth's crust will heat up - even at the poles.

Dependent upon the particular make-up of the crust at any particular location, and in the UK this is found to be between 900mm and 1200mm below the average surface level, the temperature of the ground at that level will be found to be almost always constant at around 12°C. This steady state temperature can vary from site to site by as much as 2°C.

At this relatively shallow depth, in most places across the earth's surface there is little, or no, influence on the earth's crust temperature from earth's core activity.

If the area of available ground is large enough, then the solar heat energy absorbed within it can be transferred for use elsewhere and the resultant cooling by the return water should have little effect upon the local micro-climate.

If a large ground area (say 1/2ac plus) is available, say a large garden or paddock, and that will remain undisturbed by any deep cultivation greater that 600mm deep, the temperature, as stated above is likely to remain reasonably constant all year round. In this situation a snake-like coil of hdpe / pex pipe can be buried in trenches dug across the area in a serpentine format at, say, 900mm min depth, and be connected by insulated pipes again laid at least 900mm below the finished ground surface, back to heat removal equipment at the building being supplied.

Surveys should be carried out to establish the location of the steady state temperature zone, and its average value, before design is undertaken with regard establishing any coil array size and so that dig levels ascertained accurately.

The circuit is filled with a water/ glycol mix in a closed loop and is subject to being continuously pumped around the loop.

The liquid in the loop will absorb heat from the ground in which it is buried and this heat is extracted by the equipment before being returned to the supplying area.

The heat removal equipment will almost certainly be a GSHP designed to operate within parameters and closed loop techniques.

The temperature difference between that collected and that returned to the area should be between 4°C and 6°C and should not over cool the local micro-climate, especially if this starting point is located in an area of non production, say below a path or some similar structure.

During the summer period when cooling may be desired for the space, then theoretically, it should be possible to reverse the system through the heat-pump and with the ground coil acting as the system evaporator. This will in turn re-heat the ground surrounding the coil and restore some balance in replenishing the heat store for the following winter take period.

Buried in ground AIR heating

As stated above, all areas of the earth's crust will absorb energy from the sun as it passes overhead and as it does so, that portion of the Earth's crust will heat up - even at the poles.

And again as stated above, dependent upon the particular make-up of the crust at any particular location, and in the UK this is found to be between 900mm and 1200mm below the average surface level, the temperature of the ground at that level will be found to be almost always constant at around 12°C. This steady state temperature can vary from site to site by as much as 2°C.

We have established that If a large ground area (say 1/2ac plus) is available, say a large garden or paddock, and that will remain undisturbed by any deep cultivation greater that 600mm deep, it is possible to lay pipes that can absorb heat into liquid for onward transmission and further use.

This principle can also be used for air.

Air that is required for ventilation purposes within the building, can be preheated by absorbing heat from the ground that directly surrounds a network of large diameter pipes laid between two headers at the zone of Steady State Temperature.

Surveys should be carried out to establish the location of the steady state temperature zone, and its average value, before design is undertaken with regard establishing any coil array size and so that dig levels ascertained accurately.

The circuit has a suitably engineered intake louvre positioned remotely and which is connected to a header pipe. At the building end of the array, a similar header is located and this is connected via suitably sized fans to the ventilation system.

Between the headers is located a series of pipes laid at the optimum depth to within the SST and directly surrounded by correctly selected and placed materials that will maximise on energy collection and transfer.

The air passing through the pipes will absorb heat from the ground in which it is buried and this heat is directly used as a pre-heat thus reducing energy inputs from other sources to maintain building occupant comfort at all times of the year.

Solar-thermal collectors

All built structures will have elevations facing both energy positive and energy negative directions and dependent upon where the structure is located in the world, these elevational effects, with respect to the particular structure being considered, will vary in terms of their effective strike angle to the mean solar aspect of that structure.

In the UK, the negative face would generally be the north elevation and the sun's mean angle (solar aspect) would be approximately 35° above the southern horizon. This angle, however, varies daily throughout the year and minute-by-minute throughout each day, no matter where the structure is located on the world's surface.

Building elevations in the UK that will exhibit the greatest requirement for energy input, and are the least effective in attracting absorbable free energy, are those north-facing aspects.

Solar-thermal collectors are a mechanical means by which solar radiation can be captured where there is a requirement for heat to be provided within the structure as well for the heating of water and even possibly for cooking.

The simplest and, by today's standards, also the most inefficient of the solar-thermal collectors is the flat-plate collector.

Within this unit, in it's simplest form, a metal plate has a tube(s) welded to it in a serpentine format and this, in turn, has system pipe work connected to each end of the tube(s).

The whole finished article is painted matt black (a black-body is the most effective radiator and absorber of energy according to the laws of physics) and it is located so that the major flat plate surface is pointing at the sun's main axis of travel.

To reduce the effects of wind / breeze cooling on the surface of the metal plate, the unit is covered with a spaced-off glass plate.

The pipes from the collector are connected to a transfer coil situated within a hot water vessel and the system is filled with a water / glycol solution which is then pumped continuously around the closed collector loop.

As the trapped collector solution passes through the loop welded to the back of the plate, it absorbs the heat, transmitted as infra-red energy by the Sun, that has been transferred to the plate and the serpentine pipe. This heated solution, in the sealed pipe loop, then passes through the vessel full of cold water, where this absorbed heat energy is given up and transferred to that colder water in the vessel surrounding the pipe. By suitably sizing of the collector and the circulating pump, this transfer rate can be sufficient to allow the tank to heat up, thus allowing sufficient hot water for washing to be produced.

Modern solar collector technology has advanced considerably and an evacuated tube technology (ETT) is currently considered to be the most efficient at converting solar radiation to usable heat in a practical and efficient manner. Further developments that will raise the efficiency of solar-thermal panel technology still further in both the short and longer terms has to be expected.

In ETT, the battery consists of several single glass tubes each containing a metal tube, which in turn encloses the closed circuit transfer media. Each metal tube is held within its dedicated glass tube that is individually sealed and evacuated of air to achieve a modest vacuum. The glass tubes can also be mirrored on the inside face to the rear of the media tube to act as a concentrator and to ensure that the maximum amount of energy is available to be absorbed by the transfer medium.

A series of such tubes are fixed into a frame with the metal pipes connected to a header on the device and this group (battery) of tubes forms the solar-thermal panel.

The tubes vary in length and the batteries vary in the number of tubes that they comprise. Both variations will depend upon the required duty being needed by the user system.

Some manufacturers will recommend that the tubes are laid horizontally, whilst others will insist that they are vertically laid. The reason for this will be probably determined by the particular manufacturer's header arrangement and proven operational characteristics of his particular panel.

Solar thermal collection panels should, where this is possible and if the architectural arrangement allows, be laid so that they are always situated with their maximum surface area pointing directly at the sun's mean axis for its particular geographic location.

However, the solar elevation above the horizon is dependent upon physical location of the arrangement, the time of the year and to the fact that the sun tracks across the sky during the course of the day.

In the northern hemisphere, and at the latitudes of, say, Devon and Cornwall, providing that the mean axis of the panel array is between South-east and South-west and the panel array is angled at approximately 35° from the horizontal, then maximum possible absorption from a fixed position system should be achievable for that particular geographic location.

It is possible, with today's array technology and with the right budget being available, to continuously track the sun's actual position in the sky, both longitudinally and in elevation, throughout the entire day and at any time of the year. This has the obvious potential to significantly increase the amount of solar energy capture, both thermally and PV, especially if fixed mounting surfaces in the correct direction and elevation are restricted to any significant degree. This additional feature obviously requires motive power to enable it to monitor and track the sun. It also contains moving parts which require maintenance, repair and eventual renewal if it is to maintain the additional energy acquisition that the additional capital expenditure should be able to theoretically achieve and it requires to mounted on a flat surface.

NOTE:

Do not forget that a lack of direct sunlight, clear blue skies, “correct” elevation and “correct” direction of solar collectors - be they thermal or pv, will NOT prevent solar equipments from being successful.
 
These factors WILL affect their collecting efficiency and therefore the amount that can be collected and utilised by them.
 
They will work on NORTH-FACING ROOFS and walls, in the vertical and horizontal plane and even in clear moonlight!!!!

With acknowledgement for use of images to: