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SOLAR HEATING
EVACUATED TUBE SOLAR HOT WATER PANEL (20 tube)
An assembled Navitron 20 tube standard panel
A 10 tube high efficiency panel
These top-of-the-range solar panel heat collectors are suitable for heating domestic hot water, swimming pools etc - even in the winter! One unit is adequate for an average household (3-4people), and it is modular, so you can add more if required. A single panel is sufficient for a 200litre cylinder, but you can fit 2 or more for high water usage, or for heating swimming pools or underfloor heating. And with an overall efficiency of almost 80%, they are much more efficient than electric photovoltaic solar panels (efficiency of 7-15%). And if we compare them on a like-for-like basis, the price of a Navitron Solar Water Heater is around 30p/watt (compared to £6+/watt for solar photovoltaics!)
This picture shows a more basic design - which is commonly employed in hot countries such as Turkey, Greece, Mediterranean, Africa etc. This type of system does not employ heatpipes; instead the water fills the tubes, which in turn heat the header tank (which can vary between 85 and 220litres) by convection. This system is far more basic, but has obvious advantages: no requirement for electric power, no controller, pipe insulation etc required and is very low-cost. However, it is less functional, too large to appear roof-integrated, cannot accept pressure system kits, cannot be used with a standard domestic hot water tank.
Key Benefits of Vacuum Tube Solar Panels
- High temperature output even in cold weather
- Warms water even during winter, as long as the sun is shining
- Produces heat even in overcast weather
- Easy to fit
- Higher efficiency and greater installation flexibility with heat to many of the more expensive brands on the market - as the tubes and headers are all manufactured in the same factory by an OEM supplier.
This high tech unit concentrates the sun's energy at the tip of the copper 'heat tube' - like a solid state magnifying glass!
The vacuum tube solar panel has been around for over 10years, and has proved to be reliable and dependable. The vacuum tubes consist of a double wall glass tube (made from strong borosilicate glass ie pyrex) with a space in the centre which contains the heat pipe. The sun's radiation is absorbed by the selective coating on the inner glass surface, but prevented from re-radiating by the silvered innermost lining. This is in effect like a one-way mirror which has been optimised for infra-red radiation. In fact it is very efficient, of the sun light's energy hitting the tube's surface, 93% is absorbed, whereas only 7% is lost through reflection and re-emission. The presence of the vacuum wall prevents any losses by conduction or convection - just like a thermos flask. Because of this, the system will work even in very low temperatures, unlike traditional flat plate collectors. This is why our system can be used to heat up water at the South Pole Antarctic Science Base - where ambient air temperatures can drop below -40°C.
The heat transferred to the tip of the heat pipe is in turn transferred to a copper manifold in which water circulates to heat the domestic hot water tank. If a tube is placed in direct sunlight on a summer day, the tip temperature can reach 250°C - so the system easily heats domestic hot water cylinders to 60°C even in cooler weather! The manifold is heavily insulated with a 2" thickness of pre-formed rockwool to keep the heat in. Unlike flat plates, these headers are so well insulated that they should not require antifreeze in normal operation - the temperature of the header is unlikely to fall below 10°C even in very cold weather. The more advanced solar controllers include a low-temperature facility- should the temperature of the collector fall below a defined level, the pump will operate to allow the the water at the bottom of the tank to heat the collector slightly. In normal conditions, this would never be necessary - but it acts as a good safety margin.
How Much Heat Will I Gain?
Average Daily Irradiation (Insolation) Figures for Each Month of the Year:
(Solar energy reaching each 1m2 of the earth's surface at UK latitudes)
|
|
Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
|
MJ/m2 day |
2.3 |
4.2 |
7.0 |
11.6 |
15.0 |
18.0 |
16.0 |
13.0 |
10.0 |
6.0 |
2.8 |
1.7 |
|
kWh/day per m2 |
0.64 |
1.17 |
1.94 |
3.22 |
4.17 |
5.00 |
4.44 |
3.61 |
2.78 |
1.67 |
0.78 |
0.47 |
|
Heat Gain from 1 Navitron panel per
day |
1.15kWh |
2.11kWh |
3.49kWh |
5.80kWh |
7.51kWh |
9.00kWh |
7.99kWh |
6.50kWh |
5.00kWh |
3.01kWh |
1.41kWh |
0.85kWh |
How Do I Connect the Panel?
- Navitron 20 Solar Panel
- Pressure Gauge
- Air Vent (optional - these have a habit of failing - and the cheaper ones will melt on hot days!)
- Drain Cock
- Expansion Vessel
- Tap
- Non-Return Valve
- Double Check Valve
- Filling
- Solar Circulation Pump
- Pressure Relief Valve
- Primary Coil
- Secondary Coil
- Overflow Pipe
- Overflow Outlet (exterior)
Pressurised Systems
The drawing is one example of how to connect the panel. This is a typical installation that would be carried out by a professional installer, utlising a twin coil solar cylinder. These cylinders are far superior to a standard domestic hot water cylinder - and I can supply these up to around 500litres capacity! Typical solar cylinders are 175 - 260litres. The additional water capacity allows the user to store heat for several days, to even out variations in day-to-day insolation. A typical 210litre cylinder is £320. The diagram above shows how the solar loop can be operated as a pressurised system - this is almost universal for various reasons:
- Ease of fitting
- Not limited by headroom in the loft area
- Most cost effective solution in terms of components and fitting time
- Can allow operation above 100°C (eg if you run your system at 1bar, the water will not boil until it reaches 120°C)
What else will I need to go with this panel?
It depends on your application and the method you employ. The simplest system would be based on thermo-siphon (convection or 'gravity feed'). The panel would be mounted lower than the hot water cylinder, when the sun shines, the hot water will rise, and displace the colder water which returns to the panel. This system requires no additional energy, no control system and is self-regulating, simple, cheap and effective - but not very convenient for most situations. In this case, you will only require the header tank or pressurised system kit plus copper tube and insulation.
A typical system would normally include:
- Solar panel
- Solar Hot Water Cylinder (eg 210litre £320, 175litre £290)
- Pressurised System Kit (£65)
- Controller and temperature sensors (Basic Resol B1 £65, or Resol DeltaSol B with display and additional control features £130) Pump and Isolation Valves £42
- Non Return Valve to prevent siphoning (not absolutely necessary with evacuated tube systems, but will increase efficiency slightly) £3.50
- Copper pipework, fittings and pipe insulation (obtain from any good DIY or Builders' Merchant)
A Word on Flat Plates
Flat plate collectors only provide a useful amount of heat during the summer months in the UK. due to their high heat losses during cold or windy weather. When the ambient temperature drops, the heat loss can easily exceed the heat gain - turning your flat plate solar collector into a radiator! The reason for the popularity of flat plate collectors is their cheap construction - you can build one yourself for a few pounds worth of scrap material, or you can buy one for £300-500. This is typically a lot cheaper than evacuated tube panels, which typically cost £800-1200.......until now! Don't fit a dinosaur!
Vacuum tube collectors will provide useful heat for 12months of the year, as long as the sun is shining, and can even produce heat in overcast weather. (In April, an overcast day without any sunshine is still able to heat a 210litre hot water cylinder to 40°C)
A Word on Plastic Pipework
There are some flat plate systems for sale in the UK, such as the Solar Twin system, which use plastic, silicone or rubber pipework, which makes installations easier and to reduce costs. Whereas this pipework may be ideal for flat plates, it is definitely not adequate for evacuated tubes. On a hot sunny day, flat plates may heat water up to a temperature of 60°C. Evacuated tube systems are capable of heating the water beyond 140°C. Most plastic pipework has a working temperature of no more than 65°C, and is able to stand no more than 90°C - so you should always use copper for solar installations.
These are the basic features of the kit:
- 20 vacuum heat tubes with copper heat pipe and internal high efficiency heat collector
- 1.5m tubes, 47mm diameter
- Frame area of 2.25m2
- Heat Reflectors which mount in the gap between tubes (to increase efficiency further)
- Full instructions (downloadable above)
- You can buy the other components from me, or from elsewhere - you chose!