An introduction to Heat Exchange Systems

draft February 2007

Bryan Hirst.

This paper is intended to serve as a general guide and introduction for end users, specifiers and others interested in Heat Exchange systems and their applications.

Heat exchanger systems for space heating and cooling have been widely used overseas, however they are relative newcomers in The United Kingdom.

Recently the UK Government has become committed to reducing the carbon footprint or fossil fuel requirement of property of all types especially in new buildings. Oil is too good to burn. There is concern about Global Warming. All the drivers are in place for new technologies to have a rapid expansion in practical applications in buildings. This will be achieved with better insulation and building specifications, ground, air and solar heat exchangers and new technology including the generation of electricity using bio- fuels, wind and direct solar energy.

Ground source heat exchangers are used for heating and cooling of buildings with a low carbon footprint. This will be achieved in conjunction with other technologies such as solar exchangers, heat pumps and under floor heating and cooling technology.

The technology will change and improve with time as it becomes widely used.

Intoduction

After working with solar panel and ground source heat exchange manufacturers and visiting heat pump manufacturers and importers, consultants, system designers and contractors and taking part in installations I feel that I have a grasp of what is involved in the creation of no carbon and low carbon buildings utilising heat exchangers and hope that this simple primer will be of help to others on the same pathway.

As I have been trained as an ecologist and designer rather than an engineer I have attempted to put the information I have gleaned in language that can be understood by laymen. Engineers may grind their teeth!

I have outlined below the different forms of geo source energy available now as well as the benefits and limitations of practical applications from each form.

Forms of Earth heat energy

Geo source heat (or energy) comes in two forms:

1. Geothermal energy from the hot core of the earth.

On Lanzarote in the Canary Islands lunch can be cooked by placing a metal grill over a hole in the earth where heat erupts from the earths core. Lanzarote is one of a chain of volcanic islands in the Atlantic ocean. Most recently In the 19th century Volcanic eruptions devastated the island. Today, at a tourist centre built on a volcano rim there is still enough heat rising from molten material below the earths crust to grill food. This form of energy is also used in Iceland where it is used to provide central heating and hot water for the inhabitants of the capital Reykjavik.

At only a few places on the earths surface is this energy is available; generally it is only accessible by drilling several kilometres into the earth’s crust, in a way that is not cost effective. In the Netherlands, however, where deep wells were drilled for natural gas and all the gas has been extracted, the wells are now used with heat exchangers to extract earth’s core energy to warm greenhouses.

Interestingly, health benefits have been noticed from water that has been passed through open loop deep geothermal systems. Perhaps rare minerals are released which benefit plants and animals.

In the U.K. this form of core energy is rarely of practical use using current technology, though an exception is found at the springs in the City of Bath. Water at 45 degrees C (perfect bathing temperature) issues from springs as it has since pre Roman times.

Another form of core energy can be found in Cornwall where isotopes in the granite rock as they degenerate create heat the which can be used for heat exchangers. Specialists can advise on the rare practical applications.

In the UK it is exceptional if earth core energy has practical applications.

2. Ground source heat energy comes from the daily and annual heating of the earth’s surface by the sun. The surface layers act as a heat store or pool from which energy can be extracted or sunk. At 5 metres below ground level the ground is at a constant temperature (above the air temperature in the winter and below it in the summer.) The transfer of this heat between the earth and our buildings will replace current methods of burning fuel in boilers or power stations. As with wells, care must be taken, since to extract or insert too much energy from around a heat exchanger impairs the efficiency of the system as it will have the effect of temporarily heating or cooling a small area until more energy “trickles” out from or into the surrounding ground. In some cases This “battery” effect can be harnessed to improve efficiencies.

How Heat pumps work.

Essentially ground source heat exchangers, in conjunction with heat pumps, exploit stored solar energy in the ground and make practical use of the potential difference exchanged.

(energy).

A refrigerator works on the same principle. Heat is taken by the evaporator from warm food (the ground ) and expelled by the condenser to the radiator on the back of the fridge (the under floor heating array)

In a heat pump there are essentially 4 parts:

a compressor

a condenser

an evaporator

an expansion valve

These are inter connected and set in a box .

The thermal energy is transferred to (or from) the building.

What matters most is the Coefficency of Performance (COP) which measures the amount of energy required to operate the system. For example, if 1 kw of energy is required to produce 4kw of heating the COP figure is 4.

If 1kw of energy is required to produce 2.5kw of energy the COP figure is 2.5.

The higher the COP figure the more efficient the system. and the less carbon is generated for the work to be done.

The less work the heat pump has to to do raise or lower temperature the greater the COP figure will be.

Heat pumps are expected to last over 25 years. The major limitation is from the wear when the compressor starts and stops.

The less energy needed to heat or cool a house the smaller the electrical bill. Heat pumps work best as part of a complete system and are at their best in new build property or major conversions where all the parts of the system are considered and matched.

As well as lowering the carbon footprint of buildings when correctly used heat pumps and integrated systems offer benefits including:

a) no fire hazard

b) increased comfort in buildings as the temperature is more controllable.

c) no maintenance requirements.

d) a long anticipated life.

e) no wall mounted radiators required.

A system is designed after a series of calculations:

First the heat requirement of the property has to be calculated. This is measured in Watts per square metre (the floor area to be heated) per hour against an exterior temperature of 0 degrees centigrade (in the UK). This is calculated using tables (available on the internet) which put a figure against any material material. For example a 9 inch unplastered brick wall of 1 square meter will have a demand of 2.2 and a well insulated cavity wall a demand of .6 per square metre.

The total heat demand for the area to be heated depends on the temperature differential between the exterior and the interior

The lower the exterior temperature the greater the heat demand. A modern well insulated house will demand about 40 watts per square metre at an exterior temperature of 0 degrees centigrade whilst an old building may have a heat demand of up to 200 watts per square metre.

When the area to be heated and the heat requirement has been assested the total maximum demand is calculated in kilowatts.

When that figure has been calculated it is possible to size the heat pump. A heat pump is either on and producing energy at full capacity or off producing nothing.

There is often an advantage in under sizing a heat pump. For example, suppose a house needs 5 kw to heat it effectively at 0 degrees centigrade and a 5 kw heat pump were to be fitted. most of the time the external temperature will be above 0 degrees. The desired temperature would be quickly achieved and the pump switched off . The temperature would drop and the heat pump would come on again and short;y after off again off and on etc. This wears out the compressor and shortens the life of the system.

This problem can be overcome in 2 principle ways.

1.Use a buffer tank.

A buffer tank is a well insulated tank containing a fluid (either water or petroleum gel) which is heated to the temperature required for the underfloor heating . When the pump is turned off the energy in the tank can be used to maintain temperature for a period, generally over an hour, reducing the frequency of on off (duty) cycles. Rather like an electrical capacitor.

2.Undersize the heat pump.

For example:

If the maximum demand is 5 kW and the average demand 3 kw a 3 kw heat pump will work consistently and efficiently to maintain a temperature. If the demand is greater than the ability of the 3k heat pump to provide it can be supplemented with, for example, an immersion heater. It will be relatively rarely that the supplementary heating is required. This will save on the capital cost of the equipment and the increase in the electrical bill will be small compared to the capital cost of a larger heat pump. However it will reduce the all important COP.

The coefficient of performance is directly related to the change in temperature from input to out put.

Underfloor Heating.

Underfloor water heating has various advantages over traditional radiators, most noticeably in the lower operating temperatures.( approximately 35c)

Other benefits of under floor heating include the lack need for radiators which take valuably space, and send out convection heat which rises to the ceiling . The same degree of comfort requires less energy (between 5% and 20%) from underfloor heating.

The disadvantage of underfloor heating is that is is not possible to supply more that about 100 watts per square metre. A greater demand would require a higher water temperature which would adversely affect the COP

Water for bathing and washing is generally required at about 50 degrees C. this is about 15 degrees C higher than the requirement for underfloor heating There are different approaches to this demand from different schools with the industry, at present the British manufacturer Kensa engineering suggests that its heat pumps are only used to produce space heating as this can be done at a high COP.

Others manufacturers such as IVT install dual systems which allow the water to be heated to higher temperature , either using an immersion heater or at a lower COP.

Add to the formula to include solar heat exchangers which introduce energy which can be stored either in a buffer tank or transferred to the earth via the exterior heat exchangers.

In cities the ambient temperature may be up to 8 degrees warmer than in the country, this is because of the amounts of solar energy absorbed by the glass, concrete and tarmac of cities such as London

In cities the energy needed to cool properties is almost as great as the heating need.

Heat may be transferred to the ground via the external heat exchangers. However underfloor water pipes are not very effective at cooling because cold air sinks and tends therefore to sit on the floor surface providing a blanket to reduce heat exchange, a secondary danger is that the cooling air may reach its dew point depositing water on the floor surface.

For cooling it is much more efficient if air is passed over cold water warming the water. This energy can be transferred to the air or the ground.

In certain instances a heat pump is not needed and energy directly is transferred to the end use (for example: airport runways or roads.) This is generally known as a heat pipe.

Specifying Heat Exchangers.

W hen a heat pump has been sized the next choice is the mediums from which heat is to be exchanged must be considered, options include:

1. Air Conditioner

An “air conditioner” is an air to air heat exchanger which generally runs a a low COP.

2. Air to Water Heat Exchange.

An air to water heat exchanger takes energy from the air and transfers to (ideally) underfloor heating. Fans pass air over the heat exchanger. This system has the advantage that no space for ground source heat exchangers or capital is required for a GSHE.

However, they are at their least efficient when the need is greatest, so if for example, cold air at say 0 degrees has to be raised to 35 degrees the COP drops to close to 1. In effect making no carbon or cash saving on conventional electric heating . They can be noisy and must be placed where air can flow about them which often makes them prominent and little loved by architects and designers.

3 Ground source heat pump.

This is always the system which produces the highest COP when correctly installed and is therefore favoured by planners above all other forms when possible.

The efficiency of the Heat exchangers is affected by the thermal insulation properties of the material in which it is placed. For example, in peat which has a high insulation value a greater area of heat exchanger for the energy to be extracted (measured in Watts) is required than in wet sand which as a low insulation value. Heat exchangers are sized and the particular type of HE specified when the following information has been gained:

Energy requirements (Watts or Kilowatts)

This is calculated based on the size of the area to heated and cooled and the insulation value of the materials of which it is constructed. This has changed over time and new build properties will have to comply with new legislation (Part L 2007) which demands the highest insulation ever specified in the UK. Demand is usually measured in kw at 2000 hours per year.

Available space (Square Meters)

Where high density building is planned the horizontal space available for heat exchangers will be limited and therefore reduce the options available.

Geological factors.

Where solid rock is at or near the surface cone penetration techniques may not be used and drilling may be the only available method of installing Heat exchangers. The availability of water and the ground material in which the heat exchangers are installed have varying heat transferring properties.

With this information the size and type of Geo Source Heat Exchanger is calculated and specified and the various forms of heat exchanger installations considered and costed. This is usually given as a figure in Kilowatt hours per annum that must be achieved and warranted.

The end cost cannot be concluded precisely because of unknown factors that might arise when the ground is actually penetrated, experience will usually enable a reasonably accurate guess

Types of Ground Source Heat Exchangers

There are two basic types of Ground Source Heat Exchangers:

1. Open loop.

2. Closed Loop.

Open loop. in the U.K. the minority of installations use this form

A heat exchanger where the energy is extracted from or inserted into the primary source, for example, water may be extracted from an aquifer; river or stream then passed through a heat exchanger and then back to the aquifer, river or stream at a different location at a different temperature.

Benefits

Large quantities of energy may be sourced or disposed of with minimum construction costs. When water is available this is the most cost effective form of heat exchanger.

Limitations

Raises concerns with environmentalists (and in the UK the Environment Agency) There are dangers of the changes in water temperature affecting the ecosystems of surface water (as happens in the discharge from power stations) or of pollution of aquifers whilst the water is brought to the surface.

Whilst this type of heat exchangers may be considered in a minority of applications, special consideration must be given to these peripheral issues. It may suit cooling only applications well, but it can be difficult to achieve reliable heating from open loop H Es in typical UK conditions.

Closed Loop . The majority of GSHE's in the UK are of this type.

A non-corrosive, biodegradable anti freeze mixture is passed by the primary source and energy extracted or inserted by convection, (heat exchange), into the loops fluid.

The fluid is pumped to a heat pump where the energy is extracted or inserted before repeating the process.

With all plastic pipe based heat exchangers, there is a long expected life (equal to gas and water pipes installed by the utility companies.) Perhaps a hundred years. There is little oxygen in the ground reducing oxidation, no light (Ultra violet) exposure and no extremes of temperature. Any pipes now laid will be expected to exceed the writer’s life span.

Installation of Ground source Heat Exchangers

There are various ways in which exchangers can be installed, principally:

1. Horizontally

2. Vertically.

Horizontal Installations

The ground is excavated to enable coils (often called “slinky” ) of thin walled plastic pipe to be installed, generally at a depth of between 1 or 2 metres below the surface. Where more than one H E pipe is used they are joined using a manifold to make a two ended circuit leading to and from the heat pump. After pressure testing the pipe is “blinded” or covered with sand and the excavation refilled and compacted and reinstated.

As a rule of thumb the area required for the HE is approximately twice the area to be heated or cooled. In 2006, this was the most common method of installing GSHE's. It is important that in installing these exchangers all underground services and obstructions are identified and worked around safely. It is also important that topsoil (if there is any on site) be stockpiled and replaced separately from the subsoil.

Generally, horizontally fitted heat exchangers may be cost effectively used in low density projects, such as full barn conversions where ornamental gardens are not established and space is not at a premium. It is unlikely that they will be practically used in typical modern high density building situations.

Benefits

It requires low technology and may be the least costly method. When the ground is correctly reinstated leaves no permanent visible footprint.

Limitations:

The large surface area required.

Obstruction or damage to existing services.

The disruption made during installation

Limitations on later development.

May be affected by tree roots.

Other Horizontal Applications

HE's may be placed in lakes or streams in the form of mats or coiled plastic pipes.

It is possible be install plastic HE pipes under butyl or other lake lining materials at the time of construction.

Patent Plastic “radiators” can be used to extract energy in a minimum of space . These are controversial at the moment as there are fears that too much energy may be extracted from a small area and that the ground may consequently freeze. When this happens the anti freeze mixture may become slushy and stop the heat pump.

Benefits

No construction costs, (if installed at the time of construction of water features)

Apart from trenching (at about 1 metre depth) to the heat pump.

Limitations

If coils are placed in the bottom of lakes this may well affect the ecosystem of the lake by allowing the water temperature of the lake to be reduced , affecting fish that go to the warmer water at the bottom of the lake during the winter months. If the water temperature is raised during the summer months this will reduce the waters ability to carry oxygen and aversely affect fish.

In time, the lake will require dredging, removing detritus from leaves etc. At that time it will be necessary to place on one side the HE. This will be dirty and difficult and will probably damage the pressurised HE system

Vertical Installations

This may be undertaken either by:

1. Hydraulic pushing technology.

2. Drilling

Heat exchangers are fitted vertically into the ground and joined together at 1 metre or so depth and trenched in to the heat pump.

The heat exchangers vertically inserted into the ground are of two principle types:

1. Coaxial

2. U tube.

Coaxial

A coaxial heat exchanger is made by inserting a slightly shorter narrower pipe into a larger pipe on which the bottom end has been capped. The exchange fluid is pumped down the outside pipe (where it exchanges heat with the surrounding material) and then is forced up the inner pipe. The temperature of the fluid is affected by this journey. The piping used is considerably less expensive than the alternatives however this form of heat exchanger can be less efficient than U tube exchangers.

U tube

Exchange fluid is pumped down one side of the u tube and forced up the other. The temperate of the fluid is affected by this journey. Whilst more expensive to manufacture U tube and double U loop exchangers do not have direct thermal contact between the fluid entering and leaving the exchangers, and are consequently slightly more efficient than coaxial systems.

There are 2 main methods of installing vertical heat exchangers:

1. Hydraulic pushing technology

2. Drilling

Hydraulic Pushing Technology.

A self contained truck is driven on to site. Hollow steel rods are pushed into the ground using hydraulic rams on the truck. A sacrificial cone is used to stop the interior of the rods filling with the material pushed though. A coaxial heat exchanger is inserted into the hollow rods and rods removed. The substructure then pushes the soil back around the exchanger. Trenches are dug between the array of heat exchangers are joined to make a circuit to and from the heat pump using fusion welded manifolds and horizontal pipe work. If this is undertaken at a depth of less than 1.2 metres the horizontal array is insulated be insulated. The system is pressure tested the horizontal array blinded or covered with sand, the trenches are refilled, compacted and the ground reinstated

Benefits

Fast

Low installation costs.

Low site interference

No cuttings or “mud” disposal required

No cross level contamination

Limitations.

Only suitable in sedimentary (non rock) applications

Greater number of HE's may be required because of practical maximum “push depth”

The trenching for the connecting pipe work may cross existing services.

Drilling.

Using either rotary percussion or sonic technology. A rig and appendages is brought to site and a vertical hole drilled and lined with hollow steel rods. The spoil (“arising”) and lubricant “mud” is removed from site. A heat exchanger is inserted into the bored hole, the air gaps filled with a high thermal conductivity grouting and the steel rods withdrawn.

Benefits

Able to achieve any reasonable specified depth what ever the geology underlying the project.

Minimum space requirement after installation because of the minimal numbers of HE's.

“Sonic drilling”, from Canada may be able to operate without drilling “mud “in certain applications.

Limitations

Generally slow

Generally drilling requires lubricants to carry the cuttings from the well head. This and the cuttings must be disposed of.

Rotary percussive drilling sets up vibrations which may affect near by geology and others.

The spoil if contaminated may require special arrangements for its safe disposal.

It may be necessary to line the bore hole. Etc.

Dangers of inter layer contamination.

Summary.

There is major rhetoric from central government about lowering carbon footprint for buildings however the main driver appears to be coming from local Government planning departments particularly following “ The Merton Rule” when Merton council changed to the word “expects” in stating its demand for 10% renewable where planning permission was sought. The baton has been picked up by others not least the mayors office for London. The demand for renewable as a criteria for all planning permission is being legislated. Zero carbon footprint communities are being planned in development areas in London. The Department of Trade and Industry web site outlines the governments position. The grant and tax saving structure is unreliable

Whilst there is many a slip twixt cup and lip the author sees no possibility of the increase in heat exchangers demand decreasing despite promised price cuts from fossil fuel purveyors such as British Gas and the 7 sisters oil companies.

Established heating engineers proficient in the use of underfloor heating as well as established drilling companies are thriving in the race for dominance in the emerging industry.

New consultancies combining renewable energy techniques are, at time of writing rapidly establishing combining and expanding.

We are at the start of a new age in energy control and use the effect of which will be as great as that of the computer age. How necessary and what fun!

Acknowledgements.

In researching this primer I have had the pleasure of help, advice and conversations with many kind people including: Harry Hart of Green Deserts and Sunseed, Andrew Sheldon of Ice Energy. Tony Book of Riomay, Eric Zon, and Rob Gardener of Lankelma Ltd. Steve Grey of Cool Planet. Nick Wincott of Econic., Dr Robin Curtis of Earth Energy.

David Hirst of RL Tec, Ian Dove of British Geological Survey. Rosemary Rawlins of BRE

Mr Wim .... of ......, Robert Harden of CIAT, Hugh Jones of Viessman, Andrew Honey of Microgeneration, David Neame of Neame Design and help and support of my wife Leslie