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solarsense.co.uk solarcylinder.co.uk B9nrg.co.uk Since 1990

System designers and purveyors of renewable energy systems since 1990

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Sizing your Coils

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Boiler and Solar

Basic energy principles without being over technical

The coil is a heat exchanger. The efficiency or effectiveness is largely a function of;

1. Material conductivity (copper is superior to stainless)
2. Surface area.

The rate of heat transfer is also a function of temperature difference and flow rate.
Turbulant flow is ideal

Our coils are copper and our solar coils 'finned' to greatly increase the surface area.
The most common sized solar coil would measure 1.65 square meters if hammered flat.

The inner bore is ribbed to promote turbulance. In this case, fluid at the tube inner exchange surface, will be much the same as the average content temperature.
The opposite is smooth bore where the fluids at the exchanger surface will be not much different, to the surface itself at a given moment and with a temperature graduation to the core; "Laminar Flow" = less exchange/ heat transfer.

SOLAR & HEAT PUMP COILS

Unlike boilers which can supply constant heat at 80-85'C all day long, the efficiency or effectiveness of the coil is not so important. Heating the cylinder is just a matter of time.

The solar coil efficiency however is crucial as we do not have the same average temperature differences to play with and often very intermittant energy supply peaks. To comprimise here is crazy.
With solar we have a limited or fixed energy supply and have to maximise heat transfer efficiency and storage
For this reason, our 110kw Solar coils are around 100 x more efficient than a typical boiler coil.

SOLAR COILS -
.
Solar coil - DO NOT use less than .25M2 of solar coil per 7 large/ 10 small tube or M2 of plate solar collector -
or your solar gains will cycle back up to roof. (Aim for 27-30M2)

Our solar coils are also 'finned' - Imagine 100's of washers over a thick copper tube, spaced at appx 3mm.
This achieves the design objective of maximising surface area, minimising offset volume.
As the 'surface area' is effectively concentrated, this also allows the coil to be positioned as low as possible within the coolest water - increasing heat transfer rate at the coil and therefore reducing the running temperature at the collector.

Our finned coils also have a ribbed inner surface thus eliminating 'laminar flow' of the heat transfer fluid and
creating 'turbulant flow' which significantly increases heat transfer.
Beware of (solar) non finned coils. Look for solar storage volume. - Cylinder capacity/ volume minus boiler heated volume.
More is better, there is no point generating solar heat if you dont transfer the heat (coil) and store when freely available for later use when needed. (evening/ next morning)

A well specified quality cylinder can make a significant impact on the overall system efficiency!
Solar coil M2 surface area needs to relate to your solar collector area - see ......
Solar coil sizing (table below) is based on 0.25 - 0.30M2 solar coil per M2 collector ('Clear Skies')

Sizing Your Cylinder

Solar Coil M2	Person	Collector
1.21 M2	up to 4	up to 3.5 M2
1.65 M2	5.5 - 6.6	3.5 - 5.0 M2
2.43 M2	8.0 - 9.0	5.0 - 7.5 M2
3.24 M2	11 - 13	8.0 - 11 M2

BOILER COILS

Your Gas/Oil/Wood 'Fuel' is (solar) energy that has allready been effectively stored.
This energy is not degrading whilst in it's present 'fuel' state. (unlike heat)
There is therefore no need to burn this fuel and store as heat/ hot water untill you actually need it -
Unless the imminent rate of demand for hot water will exceed the rate at which your fuel/boiler and heat exchanger can deliver.
This is the inherent limitation with a standard 'combi' and the reason for wanting a 'buffer' or cylinder storage volume.
Top of the range 'combi's' do include 'heatbank' buffer volumes for exactly this reason.
Either way, we require a cylinder to store daily solar energy and so a cylinder system is preferable to a 'combi' boiler system.
One cylinder if possible, is also allways better than two

Your boiler can heat or 'lift' water 1'C - At a rate of 14.3 Litres per minite per Kw
If your boiler is 30kw then you can 'Lift' water from 10-40'C (30 'Lift') at a constant rate of 14.3 L'min.
(Log burner Kw ratings need to be typically halved)
This may be considered adequate for a reasonable shower but would be the limit of an instantaneous water heating 'combi'

Note that;
If were to double the Flow 'Rate' - The Temperature 'Lift' would be halved.
If were to double the Temperature 'Lift' - The flow 'Rate' would be halved.

SIZING YOUR COIL/ BOILER HEATED VOLUME

The Kw Boiler Coils in the table above relate to the heat your boiler can effectively exchange/ deliver.
A boiler or coil with a rating of 1kw is a coil that can transfer heat or 'lift' water 1'C at a rate of 14.3 litres per minuite

15kw is the current industry standard known as the 'half hour' or 'high efficiency' coil.
In accordance with BS1566, this 15Kw coil will heat 117 Litres from 10'c-65'c (30 min pumped/ 1Hr Gravity)

Our "Standard Solar Cylinder" now includes a boiler coil of 18kw and is typically positioned to heat 93 Litres
If your boiler is - or may one day be larger than 18kw, we would reccomend you choose a larger coil.

Ifyou had a 30kw boiler and were happy with a maximum 30 'Lift' at 14.3 L'min then you could specify a 30kw coil and in theory have no (boiler heated) water storage requirement at all. (you would of course need A volume to submerse the coil)

Think of the size, volume and principles behind overnight electrically heated cylinders - so called 'Economy 7'
Many competitor cylinder designs are 2/3 boiler heated and only 1/3 solar!
SO DO calculate the solar storage available below the boiler heated volume.
MORE solar volume is allways better (Only solid fuel users should see notes within 'laymans tech'
Fossil fuels and electricity is (solar) energy that has allready been stored. You do not have to convert to heat and then store again.
What you need is an effective boiler and heat exchange coil efficient enough to convert to hot water at a rate equal to demand

Example

If you wanted to run 2 showers at once, (for 10 minites) you would need a buffer storage volume of 143 Litres at 40'C. (or 72 Litres at 80'C)
This volume would then take 10 minites to fully 'Recover'
If each person were to wait 5 mins between showers, you could half the buffer volume to 72 Litres at 40'C (or 36 Litres at 80'C) and run continusly.
Note that if you wish to supply say 100 Litres at 40'C* (4000)
This will be winter cold - say 4'c (10% of temp and 10% of volume - 4 x 10 Litres = 40) and therefore Store 90%*(3960) = 90 Litres at 60'c
The Ideal is to Maximise Kw boiler coil/ Minimise store temperature/ volume.
Optimum system is a boiler coil Kw equal to the boiler kw rating and comprimise the trade off between store temperature/ heated volume.

Note that:
1. There is little to gain by sizing the coil kw greater than the boiler kw
2. Boiler temperatures can often be increased (for short term needs- i.e. Christmas)
3. Power shower will be the single major influence on coil sizing
4. Losses from higher temperatures can be addressed with additional insulation (at the cylinder top)
5. Bath will be the major influence on volume (depending on speed of fill)
6. Coil position/ heated volume cannot be changed once installed.
7. Best allow + 20% to your calculations.

END.

Recovery	10kw Ref	18Kw Coil	27 Kw Coil	33 Kw Coil
30 Litres	10.49	5.84	3.89	2.92
50 Litres	17.48	9.73	6.49	4.86
80 Litres	27.97	15.56	10.38	7.78
100 Litres	34.97	19.46	12.97	9.73
120 Litres	41.96	23.35	15.56	11.67

Table calculations are based on a typical pumped
system heating water 50'C from say 15-55'C