Common Questions

What is an Evacuated Tube?

An evacuated-tube collector consists of parallel rows of glass tubes connected to a header pipe. Each tube has the air removed from it to eliminate heat loss through convection and radiation. Evacuated-tube collectors fall into two main groups.

Direct-Flow Evacuated-Tube Collectors

 These consist of a group of glass tubes inside each of which is a flat or curved aluminum fin attached to a metal (usually copper) or glass absorber pipe. The fin is covered with a selective coating that absorbs solar radiation well but inhibits additive heat loss. The heat transfer fluid is water and circulates through the pipes, one for inlet fluid and the other for outlet fluid. Direct-flow evacuated tube collectors come in several varieties distinguished by the arrangement of these pipes.

1. Concentric fluid inlet and outlet (glass-metal). These use a single glass tube. Inside this is a copper heat pipe or water flow pipe with attached fin. This type of construction means that each single pipe can be easily rotated to allow the absorber fin to be at the desired tilt angle even if the collector is mounted horizontally. The glass-metal design is efficient but can suffer reliability problems. The different heat expansion rates of the glass and metal tubes can cause the seal between them to weaken and fail, resulting in a loss of vacuum. Without a vacuum, the efficiency of an evacuated-tube collector is no better, and may be worse than, that of a flat-plate collector.
2. Separated inlet and outlet pipes (glass-metal). This is the traditional type of evacuated-tube collector. The absorber may be flat or curved. As in the case of the concentric tube design, the efficiency can be very high, especially at relatively low working temperatures. The weakness again is the potential loss of vacuum after a few years of operation.
3. Two glass tubes fused together at one end (glass-glass). The inner tube is coated with an integrated cylindrical metal absorber. Glass-glass tubes are not generally as efficient as glass-metal tubes but are cheaper and tend to be more reliable. For very high temperature applications, glass-glass tubes can actually be more efficient than their glass-metal counterparts

Heat Pipe Evacuated-Tube Collectors

 These consist of a metal (copper) heat pipe, to which is attached a black copper absorber plate, inside a vacuum-sealed solar tube. The heat pipe is hollow and the space inside, like that of the solar tube, is evacuated. The reason for evacuating the heat pipe, however, is not insulation but to promote a change of state of the liquid it contains. Inside the heat pipe is a small quantity of liquid, such as alcohol or purified water plus special additives. The vacuum enables the liquid to boil (i.e. turn from liquid to vapor) at a much lower temperature than it would at normal atmospheric pressure. When solar radiation lowers the surface of the absorber, the liquid within the heat tube quickly turns to hot vapor rises to the top of the pipe. Water, or glycol, flows through a manifold and picks up the heat, while the fluid in the heat pipe condenses and flows back down the tube for the process to be repeated.

An advantage of heat pipes over direct-flow evacuated-tubes is the "dry" connection between the absorber and the header, which makes installation easier and also means that individual tubes can be exchanged without emptying the entire system of its fluid.

Some heat pipe collectors are also supplied with a built in overheat protection – when a programmed temperature has been reached, a "memory metal" spring expands and pushes a plug against the neck of the heat pipe. This blocks the return of the condensed fluid and stops the heat transfer.

What is a Heat Pipe?

A heat pipe is a heat transfer mechanism that can transport large quantities of heat with a very small difference in temperature between the hotter and colder inter surfaces.

Inside a heat pipe, at the hot interface a fluid turns to vapor and the gas naturally flows and condenses on the cold interface. The liquid falls or is moved by capillary action back to the hot interface to evaporate again and
repeat the cycle.

Heat pipes contain no mechanical moving parts and typically require no maintenance, though non-condensing gases (that diffuse through the pipe's walls, result from breakdown of the working fluid, or exist as impurities in the materials) may eventually reduce the pipe's effectiveness at transferring heat. This is significant when the working fluid's vapor pressure is low. Our water heating solar systems come with a solar planned maintenance, so you don’t have to worry about the maintenance.

What are Solar Collectors?

A solar thermal collector that stores heat energy is called a "batch" type system. Other types of solar thermal collectors do not store energy but instead use fluid circulation (usually water or an antifreeze solution) to transfer the heat for direct use or storage in an insulated reservoir. Water/glycol has a high thermal capacity and is therefore convenient to handle. The direct radiation is captured using a dark colored surface which absorbs the radiation as heat and conducts it to the transfer fluid. Metal makes a good thermal conductor, especially copper and aluminum. In high performance collectors, a "selective surface" is used in which the collector surface is coated with a material having properties of high-absorption and lowermost. The selective surface reduces heat-loss caused by infrared radiant emission from the collector to ambient. Another method of reducing radiant heat-loss employs a transparent window such as clear UV stabilized plastic or Lowermost glass plate. Again, Low-E materials are the most effective, particularly the type optimized for solar gain. Borosilicate glass or "Pyrex" (tm) has lowermost properties, which may be useful, particularly for solar cooking applications.

As it heats up, thermal losses from the collector it will reduce its efficiency, resulting in increased radiation, primarily infrared. This is countered in two ways. First, a glass plate is placed above the collector plate which will trap the radiated heat within the airspace below it. This exploits the so-called greenhouse effect, which is in this case a property of the glass: it readily transmits solar radiation in the visible and ultraviolet spectrum, but does not transmit the lower frequency infrared re-radiation very well. The glass plate also traps air in the space, thus reducing heat losses by convection. The collector housing is also insulated below and laterally to reduce its heat loss. The second way efficiency is improved is by cooling the absorber plate. This is done by ensuring that the coldest available heat transfer fluid is circulated through the absorber, and with a sufficient flow rate. The fluid carries away the absorbed heat, thus cooling the absorber. The warmed fluid leaving the collector is either directly stored, or else passes through a heat exchanger to warm another tank of water, or is used to heat a building directly. The temperature differential across an efficient solar collector is usually only 10 or 20°C. While a large differential may seem impressive, it is in fact an indication of a less efficient design.

What is Solar Electric?

Solar Electric is also know as Photovoltaic's (PV) or solar cells as they are often called are semiconductor devices that convert sunlight into direct current (DC) electricity. Groups of PV cells are electrically configured into modules and arrays, which can be used to charge batteries, operate motors, and to power any number of electrical loads. With the appropriate power conversion equipment, PV systems can produce alternating current (AC) compatible with any conventional appliances, and can operate in parallel with, and interconnected to, the utility grid.

What is Solar Isolation?
The amount of electromagnetic energy (solar radiation) incident on the surface of the earth. This means how much sunlight is shining down on us.

Why is knowing the isolation level useful?
By knowing the isolation levels of a particular region we can determine the size of solar collector that is required. An area with poor isolation levels will need a larger collector than an area with high isolation levels. Once you know your region's isolation level you can more accurately calculate collector size and energy output.

What measurement of units are used to express isolation levels?
The values are generally expressed in kWh/m2/day. This is the amount of solar energy that strikes a square meter of the earth's surface in a single day. Of course this value is averaged to account for differences in the days' length. There are several units that are used throughout the world.

The conversions based on surface area as follows:
1 kWh/m2/day = 317.1 btu/ft2/day = 3.6MJ/m2/day

The raw energy conversions are:
1kWh = 3412 Btu = 3.6MJ = 859.8kcal

Is my region's isolation level low, moderate or high?
The following scale is a basic guide for isolation levels. Although a value of 5 is not considered very high during the summer months, as an average annual value this is very high. You will see that in central Australia, which is a hot, sunny place, the annual average isolation is 5.89. You may compare your location to the following
two extreme locations.

Average annual isolation levels:
Central Australia = 5.89 kWh/m2/day - Very High
Harrisburg,, PA = 3.84 kWh/m2/day - Average
Helsinki, Finland = 2.41 kWh/m2/day - Very Low

How long will it take to recoup my investment?
Solar collectors are much more affordable than many other solar hot water heaters. The price of a full system may not be too much more than a new electric or gas system. Depending on your location (solar levels) and current hot water usage the annual electricity or gas saving will differ. However, in a normal building that spends 25% of its electricity bill on hot water heating, the full cost of the purchase may be recouped as quickly as 4-5 years in reduced bills. You will definitely make considerable savings during the life of the solar hot water heater.

Can solar collectors be used in cold conditions?
Yes. Collectors can be used in temperatures as low as -30C, although performance is greatly reduced in such extreme conditions. Good heat output is still achieved in mild sub-zero conditions.

What happens if one of the solar tubes is broken?
Firstly, tubes are very strong and not easily broken, but if the worst should happen, solar tubes can be replaced very easily. They are inexpensive and available though your Local Harrisburg PA Installer. The solar collectors can operate with several broken tubes, but the efficiency will be reduced, so we recommend that broken tubes be replaced immediately.

Will water be heated on a cloudy day?
Yes, although the heat output of the solar collector is reduced on overcast days it will still be able to provide heating. If it is a heavily clouded day or raining, then more gas or electric boosting may be required to maintain water at the required temperature. This system will be automated so you don't have to worry about running out of hot water on a rainy day.

Can I use a solar collector with my existing hot water system?
Normally yes. Simple retrofit valves can often be used to allow solar to connect to your existing cold water inlet. If your tank cannot accept the solar input directly an additional storage tank can be installed to pre-heat the cold water prior to entering the existing tank.

Are the solar collectors noticeable on the roof?
If only the collector is mounted on the roof it should blend into the roof design quite well. Solar collectors are very thin and can be flush mounted on a roof. From a distance they look somewhat like a skylight. You may have to check with your local council regarding building restrictions when installing your solar collector.

Can solar collectors be mounted on a flat surface?
Yes, they may be mounted on a flat roof or on the ground by using a stainless steel flat roof frame. The collector should be installed at a minimum of 20 degrees angle to ensure optimal heat pipe operation.

How do I protect my solar system during subzero temperatures?
If you have a system that is operating in areas with subzero temperatures then freeze protection must be implemented. The easiest means of preventing freezing is to use a controller with a low temperatures setting, so when the manifold temperature drops below a certain pre-set temperature (5C/40F), the pump will circulate, warming the collector with water from the bottom of the storage tank. The pump will not run continually, just periodically, the frequency of which will depend on the outside temperature. In extremely cold areas, a closed loop using a glycol/water mix may be appropriate.

Will the solar collector be a fire hazard during hot, dry weather?
No, the solar collector's components are all high temperature rated and non-flammable so even during strong sunlight with the circulation pump turned off (stagnation), the system will not catch alight or give off any sparks. The majority of the solar collector's components are stainless steel, aluminum, glass or glass wool. The manifold outlet should be fitted with a temperature relief valve, which will prevent the manifold temperature from
exceeding 99C / 212F.

Can the solar collector heat water to a high enough temperature?
Yes, in good weather the solar collector can bring water to boiling point. Generally, this is not necessary and so the system should be designed to provide a daily temperature rise of around 25-30C (45-54F) in the summer. Sizing your system that can bring the cold water up to 60C/141F in a single day is not logical, because if hot water is not used for one day, the following day the system will be boiling and dumping hot water via the temperature relief valve. This is both a waste of energy and water! Please, sensibly size solar water heating system to ensure optimal performance and minimal wastage of water.

What maintenance of the solar collector is required?
Under normal circumstances no maintenance of the system is required. Due to the shape of the tubes regular rainfall and wind should keep the tubes clean. This, however, is an inexpensive and easy job. Any "handy" person can install a new tube (while adhering to local health and safety regulations). Solar collectors can operate with several broken tubes; however the efficiency will be reduced slightly.

Can solar collectors be used for a large scale hot water production?
Yes, solar collectors can be connected in series or parallel to provide large scale hot water production for commercial settings such as a school, hotel or office building. There is really no limit to the size of the system, however collectors must be installed in banks of no more than 150 tubes (in series), otherwise the water may boil.

Can I heat my swimming pool or spa using a solar collector?
Collectors are high temperature collectors, and are therefore ideal for spas, as the volume of water is small and temperature requirements high. For swimming pools, however, the volume of water is large and the temperature rise required is only several degrees. The cost of heating a pool using solar tube collectors (for domestic purposes) may be prohibitively high. For large scale swimming pools, however, collectors may be an extremely viable supplement to gas or electricity.

Are solar tube collectors more efficient than flat plate collectors?
When comparing peak efficiency levels it may seem that there is little difference between flat plate and evacuated tubes, in fact flat plate may actually be higher, but this is during minimal heat loss conditions. When averaged over a year evacuated tube collector have a clear advantage. The key points are:

1. Due to the cylindrical shape of the evacuated tube, the solar tubes are able to passively track the sun throughout the day. Flat plate collector only provides peak energy output at midday when the sun is perpendicular to the collector's surface.

2. Air is evacuated from the solar tube to form a vacuum. This greatly reduces conductive and convective heat loss from the interior of the tube. As a result wind and cold temperatures have less effect on the efficiency of the evacuated tube collector.

3. Solar collectors can often be used in subzero temperatures without the system sustaining damage. Flat plate systems often require expensive and complicated "antifreeze" systems to be installed.

4. Evacuated tubes are strong, long lasting, and should one be broken, inexpensive and easy to replace. If a flat plate collector panel is damaged the whole panel must be replaced.

5. Due to the high efficiency absorption of solar radiation even during overcast conditions, combined with excellent isolate properties of the solar tube, solar tube collectors can heat water all year round (backup from gas and electricity is still required).

6. Due to the various advantages of evacuated tube collector over flat plate collectors, a smaller collector can be used to provide the same heating performance. For example, a standard household of 4-5 people would usually require a 250-300L water storage tank. Depending on your location, only 30 evacuated tubes would be required to provide all summer hot water needs and a large percentage in other seasons.

7. Flat plate solar collectors can produce similar heat output to evacuated tube collectors, but generally only during hot, sunny conditions. When averaged over an entire year, evacuated tube collector heat output per net m2 of absorber area, is 25% to 40% greater that a flat plate collector.

Which collector is the best value for money?
Rather than looking at just peak efficiency levels when comparing solar collectors, cost per unit of energy produced is much more logical. For example: Although Collector A may be 20% more efficient than Collector B, if Collector A is 30% more expensive, then in fact Collector B may be a better choice, as per kWh of energy produced per day it is cheaper. When payback time is of concern, not only price per kWh of the product is important. In this regard solar collectors provide a further advantage as solar collectors are very easy to install, and that can make a huge difference in terms of total install costs.

Where can I purchase a solar collector?
Solar collectors are available through an official distribution network.

Can solar collectors be used in a drain back configuration?
Yes, the end port version of the solar collector is well suited to drain back use. The question is often asked if the solar collector will be damaged when the pump turns off and the system stagnates in good sun - no it won't, as the collectors are designed to withstand stagnation. What must be considered though, is the insulation used on the piping close to the collectors, as this must be able to withstand stagnation temperatures.

Is solar water heating a viable alternative to gas or electricity?
Solar should not be seen as an alternative to gas or electricity, but rather a supplement. Solar water heating cannot totally replace the need for gas or electric heating as there are sometimes days when there is little sunlight. When averaged over a year, a correctly sized solar system can provide 60%-70% of a household's hot water needs. Providing more than this is unadvisable, as too much heat will be produced in the summer. The hot water system can easily be automated so hot water is guaranteed regardless of sunlight levels.