What is Solar - why is there so much interest in new
forms of energy
Solar energy is produced from the sun, it's source is free and endless, it is also the cleanest form of all
known energy sources. Solar radiation includes heat, light, and other radiation emitted from the sun, is
absorbed in the form of heat by anything it strikes driving almost all natural processes on planet earth.
This natural resource delivers energy we need with little impact on the surrounding environment.
Using solar and other forms of renewable energy reduces our appetite for fossil fuels used to produce energy, this directly reduces CO2 emissions which are the main contributor to global warming, an environmental issue which is increasingly in the global spotlight.
Solar thermal - what is it and how is it useful
Solar thermal creates usable heat / energy from the suns intense heat. This simple underutilised
process is 5 times more efficient than photovoltaics (PV) at converting sunlight into usable energy which makes it much more cost-effective per unit of energy produced reducing payback 250% faster than photo-voltaic electric cells.
Can be used in open or closed loop solar systems including space heating to heat water in domestic or
commercial buildings these applications require connections to supplement gas or electric hot water
production greatly reducing energy costs.
Thermal energy absorbed from the sun can be used in a wide range of applications producing hot water
for pool heating, industrial heating and even cooling in conjunction with the use of absorption chilling technology.
Elements of a solar system for hot water -how solar hot water works
Automatic solar controller.
Temperature sensor on the collector.
Temperature sensors on the storage tank x 2.
Solar circulation pump (230 volt).
Cold water inflow.
Hot water expansion valve (temperature pressure relief).
Charged glycol circuit- solar circulation pump.
The electric booster element guarantees that enough warm water will be available even when the solar
collector supplies little or no heat. Solar hot water circulating systems can be integrated into buildings
although some are more complex than others.
Modern solar heating setups have at least twenty years life expectancy which exceeds that of most
household appliances, and ideally works hand in hand with conventional heating technology such as a
boiler, gas or electricity.
Thermal solar energy systems operate using a solar automatic controller. As soon as the temperature
on the collector exceeds the preset temperature in the storage tank by a few degrees, the regulator
switches on the solar circulation pump and the heat conducting liquid shifts the heat received from the
solar collector to the solar storage tank.
How does a solar heating system work
The action center of the solar heater is the collector. The evacuated tube solar collector has gained
popularity over flat plate systems because of improved efficiency specially in colder climates.
The absorber is made up of selective layered paint consisting of graded A1/N/A1 which dramatically increases the temperature, this high level of heat being transferred from solar radiation to the heat pipe through alluminium fins is contained within 2 layers of tough borosilicate glass of which a vacuum is trapped reducing thermal heat loss. Solar tubes used by solardirect.co.nz measure 1800mm x 58mm have a weight of 2.2 kilograms, insolation temperature of 200 degrees, startup temperature of 25 degrees, resist hailstones to 25mm plus if left in the sun can reach temperatures of 250 degrees although these specifications are impressive high performance solar tubes are capable of significantly higher temperatures.
What is an Evacuated Tube
Solar tubes which in this case refers to evacuated tubes absorb solar energy which in return converts the
heat for use in water heating. Several different types of evacuated tubes are used in solar thermal collectors.
Solardirect.co.nz collectors use the most common "twin glass tube". This type of tube is chosen for its
proven reliability, performance and low production cost.
Each tube consists of two glass tubes constructed from extremely strong borosilicate glass.The outer
tube is transparent and allows sunlight to pass through with almost no reflection. The inner tube is coated with (Al-N/Al) an aluminum nitride coating. Referred to as a selective surface is fantastic at absorbing solar radiation with minimal reflection loss. When being manufactured the air contained in the space between the two layers of glass is sucked out while the top of the tubes are exposed to extremely high temperatures.
This process fuses the two tubes together into a single evacuated tube. This removal of the gasses creates the vacuum, which creates the most important part of the evacuated tubes.
Why use a vacuum
The vacuum is an excellent insulator as you would already know a thermos flask holds heat or cold because of the vacuum. The same principle applies in the solar tube the vacuum removes any physical connection between the two glass layers of the tube which means there is no transfer of collected thermal energy, so once the heat has been captured there is no escape passage.
This is the most important aspect of the evacuated tube, once the radiation from the sun has been collected and converted to heat, we don't want to lose it! The vacuum is important because now the insulation properties are so good that the inside of the tube may be 200 degrees or more, the outer tube remains cool to touch, maybe a few degrees difference with the ambient air temperature. With these facts in mind evacuated tube solar water heaters are more efficient than flat plate collectors with less heat loss but are of similar performance in summer.
To maintain the vacuum between the two glass layers, a barium getter is used and placed at the bottom
of the tube (not unlike those used in television tubes). While being manufactured the getter is put in place then exposed to high temperatures causing the bottom of the evacuated tube to be coated with a layer of pure barium looking rather like a mirror finish. This barium layer actually extends the life of the vacuum absorbing any random CO, CO2, N2, O2, H2 and H2O out-gassed from the evacuated tube during it's lifetime.
This barium layer also shows as a clear visual indicator of the vacuum status; the chrome mirror looking barium layer turns white should the vacuum be lost making visual failure easily identified.
How big should your solar panel be
To form a complete collector bank individual evacuated tubes are inserted into a manifold using heat
sink paste, collectors are usually manufactured in banks of, 20 24 or 30 tubes making larger panels
possible when matched to your hot water cylinder water volume, rule of thumb is 7.5 to 9 litres per
evacuated solar tube to achieve maximum efficiency. Should you move away from this suggested
evacuated tube efficiency calculation you will encounter either poor performance or overheating.
Solar panel direction - should we cut down any trees
When setting up your collector orientation for New Zealand it should be true North, due to the round absorption surface a passive tracking effect is achieved in the collector, meaning the collector is absorbing the sun's radiant energy throughout the day providing there is no shade from buildings or trees and not just when the sun is directly overhead. In a North South orientation, a similar effect is achieved as the sun rises and falls across the sky over the course of the year.To set these collector orientation angles to more suit your location see solar orientation.
Solar efficiency - how much insolation does my solar collector need
The efficiency of an evacuated tube heating system is strongly related to the intensity of the solar insolation level ( radiant heat) at your site. To learn more about insolation levels and the average values in New Zealand visit-
Drawbacks - how do you know which heating systems to review or consider
Here is a solar heater link which is worth a look
Solar Heating Auckland | Solar Water Heating Systems | Diy Solar Installers | Solar Hot Water Cylinder
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Wilo Star 3 speed secondary circulating pump
Manufactured for longtivity using brass and other non-ferrous metals
Suitable for use in secondary hot water systems