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How do Vacuum Tube Solar Collectors Work? The vacuum tube is a key component of the solar collector. Each vacuum tube consists of two borosilicate glass tubes that have high chemical and shock resistance characteristics. During the manufacturing process the outer side of the inner tube is coated with a heat absorbent material. The outer tube and the inner tube are then sealed together. During the sealing process all gas is evacuated between the outer tube and inner tube. This vacuum is maintained by a barium gas getter which absorbs any residual gas. The vacuum virtually eliminated any heat loss from the inner tube by either conduction or convection. For improve performance the outer surface of the inner tube is coated with three different materials consisting of a copper based reflecting layer followed by an absorbance layer that absorbs heat followed by a remittance reducing layer that helps retain the absorbed heat. The combination of these three layers help to solve the typical problem associated with heat remittance performance; the higher the temperature the higher the remittance factor. The heat absorbance efficiency of the tube has been increased by 14% while the remittance has been reduced by 30% – 40%. These gains in efficiency translate into a higher stagnation temperature of 270°C.
This heat, which builds up on the surfaces of the inner glass tube, gets transferred to the heat pipe by a series of aluminum heat absorption fins. As indicated in the cross section below these aluminum heat fins are in contact with the inner surface of the inner tube and as the fin heats up the heat gets transferred to the heat pipe.
The heat pipe consists of a refined, stable copper tube that will not react to the continual heating and cooling. The interior of the heat pipe is hollow and this hollow space is filled with a small quantity of purified water mixed with special additives. Before sealing the hollow space the air inside the heat tube is evacuated so as to create a vacuum inside the heat tube. The reason for creating the vacuum is to alter the state of the liquid in the hollow space. At sea level water boils at 100°C, but if you climb to the top of a mountain the boiling temperature will be less that 100°C. The decrease in boiling temperature is as a result of the decrease in air pressure as one move to a higher altitude. This principle forms the basis of the heat pipe. By creating a vacuum in the interior of the heat pipe the air pressure in the inside of the heat pipe is minimized. This low pressure environment lowers the boiling point of the water in the heat tube to + 25°C so that when the the heat pipe is heated above 25°C the water vapourizes. As illustrated in the schematic below this vapour rapidly rises to the top of the heat pipe transferring heat to the condenser bulb at the top end of the heat pipe. The condenser bulb has a much larger diameter than the heat pipe shaft; this is to provide a larger surface area over which the heat transfer can occur. The condenser bulb fits into the manifold’s heat conductor bushing. As the condenser bulb heats up it transfers heat, via the bushing, to the water flowing through the manifold. This transfer of heat causes the vapour to condense into a liquid state (water) and the water returns to the bottom of the heat pipe where it heats up again and the process is continually repeated. Prior to being approved for use each heat pipe is tested for heat transfer performance and exposed to temperatures of 250°C. It is for this reason that the copper heat pipes are relatively soft.
Even though the boiling point of the water has been reduced to + 25°C the freezing point still remains at 0°C. Because the heat pipe is located within the evacuated glass tube, overnight temperatures as low as -20°C will not cause the heat pipe to freeze.
System Layout Generally solar water-heating systems, whether thermosipho or pump assisted, can be divided into two categories, open circuit / direct systems or closed circuit / indirect systems:
There are two ways in which water circulates takes place in solar water heating systems; pump assisted circulation or natural thermosiphon circulation. The choice one uses is dependent on various factors, including roof structure, water quality and winter temperatures. An important difference between the two circulation methods regards the way the heating in the geyser occurs. In thermosiphon circulation as the water is heated in the collector manifold it rises automatically through the hot water return pipes and settles as a layer of hot water at the top end of the geyser. As more hot water is heated and rises into the geyser the layer of hot water deepens until by the end of the day one has a full tank of hot water. In pump assisted heating the pump may circulate the water in the tank various times during the course of the day. By circulating the water no layer of hot water develops, instead the heat is distributed more uniformly through the water in the tank. It is important that the client be made aware of these differences since pump assisted systems experience gradual heating over the course of the day. This may give the incorrect impression that the system is not working. This is unlike thermosiphon based systems where a layer of hot water develops at the top of the tank. However it takes a full day for this layer to displace all the cold water in the geyser. Thermo siphon heating
Thermo siphon solar heating relies on the natural movement of water; as the water in the collector is heated it rises naturally through the pipe work and into the geyser forcing the cooler, heavier water in the geyser to flow down into the collector causing a natural circulation of water through the system. In order to achieve this ongoing circulation the geyser must be positioned above the collector and to limit reverse circulation a non return valve is fitted to the manifold.
Thermosiphon systems are installed using either a close-couple or a split configuration:
Thermosiphon circulation is both reliable and cost effective eliminating the need for pumps to circulate the water while maximising your energy savings
Pump assisted heating Where thermosiphonic action can’t be utilized one requires a small dedicated pump to circulate the water from the tank through the collector and back into the tank. The pumps in forced circulation systems have low power requirements and there are two options available electric driven pumps or solar driven pumps. Electrically driven Pump The operation of the electric pump is controlled by a temperature differential (delta T) controller. This controller comprises a control panel complete with display that connects to two temperature probes. One temperature probe measures the water temperature inside the geyser while a second temperature probe measures the temperature inside the exit port of the solar collector. When the temperature difference between the two probes exceeds a pre-programmed value the panel automatically activates the circulation pump. This means that if the Delta T controller has been programmed to activate on a 5°C temperature difference it will activate the pump when the temperature measured by the probe in the solar collector manifold exceeds the temperature measured by the probe in the geyser by 5°C. As the water circulates through the collector manifold it heats up and subsequently heats the water in the geyser to the point that the difference in temperature between the collector manifold and the water in the geyser is less than 5°C. At this point the controller deactivates the pump. In areas affected by freezing night time temperatures the controller also has an anti freeze function by monitoring if the temperature in the manifold drops below 5°C. As soon as the temperature drops below this point the controller activates the circulation pump forcing hot water from the geyser to circulate through the system thereby preventing the pipes from freezing. Solar driven Pumps Direct current (DC) pumps can be powered by small photovoltaic (PV) panels which convert sunlight directly into DC electricity. As the photovoltaic panel receives sunlight it converts this light into electricity which activates the pump.
The problem associated with photovoltaic driven pumps is that while there may be sufficient sunlight to activate the pump there may be insufficient solar radiation to generate heating in the solar collector. This can be true for early morning or late afternoon and such a situation can result in the cooling of the hot water in the geyser. To prevent this situation from happening a differential temperature controller, similar to what is utilized with the electric pump, must be included.
Maximize the efficiency of your installation Orientation Even though the performance of evacuated tube collectors is less dependant on orientation than flat-plate collectors the correct orientation and inclination of your solar collector ensures maximum efficiency from your installation through out the year. However if your home does not allow for this orientation or the pitch of the roof is less than the optimal value the system will still work so long as it receives direct sunlight. Your efficiency and temperature of the water will however be reduced. In the southern hemisphere collectors should face true north. In the event that one does not have a north facing roof a west deviation is preferable to an east deviation due to the higher afternoon ambient temperatures. If the deviation from the true north exceeds 45° the installation will require extra square meters of collector area to compensate for the loss in solar energy. Inclination The inclination of the solar collector is another aspect that influences the efficiency of the installation. The general rule for pitching the collectors is latitude +10°. For Johannesburg (latitude 25°) this equates to a pitch of 35°. Most roof pitches vary from 17° to 25°, in such instances mounting the collector directly onto the roof will result in a marginal loss in heating efficiency. Shade Shade is another important factor when deciding where to position your collector. Where possible the collector should receive uninterrupted sunlight between the hours of 09:00 and 15:30, alternately another position should be found. Partial shading caused by chimneys or TV antennas during sunshine hours is acceptable provided it does not exceed 10% of the area. In Johannesburg and surrounding areas a simple rule to keep in mind is to position the solar collector at a distance from any object that will cast a shadow. Ideally the distance must equal one and a half times the height of the object. Frost There are two main factors why vacuum tube solar collectors are less susceptible to the effects of frost than flat-plate collectors. First it is important to point out that the heat exchanger in the manifold is well insulated. This is unlike flat plate collectors which have a large area that radiates heat outwards during cold nights, making the flat plate collector more susceptible to freezing. The second factor relates to the protective effect of the vacuum tube. In the same way that the vacuum between the two glass tubes does not allow for heat loss through conduction or convection the same is true for the transmission of cold from the outer surface of the outer tube to the inner tube. As a result of these two factors the heat tube is not affected by wind chill factor and this allows the fluid in the heat tube to withstand ambient temperatures as low as -20°C. All piping connecting the vacuum tube solar panel collector to the geyser storage tank must be insulated with a suitable lagging. In areas that regularly experience severe frosting indirect freeze tolerant systems is the way to go. Water quality Calcification is a serious problem in areas with hard water as heat induces the precipitation of dissolved minerals. This results in a hard calcite deposit forming in the inside of the copper piping. Not only does this calcite deposit reduce the heat transfer efficiency of the collector but can, over time, result in blockages occurring in the piping. In such areas the collector needs to be fitted with a water softener or a de-scaling device such as the Electro Pulse descaler that is supplied by IES. Corrosion is another problem that can occur, particularly in rural areas where bore hole water is utilized. In such instances a water analysis needs to be undertaken to identify the nature of the problem. Dust The external surface of the vacuum tube has to be clean in order to be as effective as possible. In extremely dusty areas or areas subject to dust or sand storms the glass tubes of the collectors should be washed with clean water once a month to ensure optimum absorption. Adequate rainfall may clean the glass panel naturally. Timer A solar water heater with an element that works on an thermostat cannot save electricity to its full potential. If a substantial part of the overall volume is consumed in the early morning or the late evening, the element will want to heat the water up immediately, even though there is sufficient solar radiation available to do so. Maximum savings will only be achieved by either switching off the element during day time or timing it out automatically. By setting the timer to operate from 15:30 to 19:00 and from 04:00 to 06:00 the user maximises the benefit attained from the solar panel installation and ensures hot water during morning and evening periods even during days without sunshine. Test results from pilot studies have shown that energy savings without a timer or load management device averaged around 44%. The introduction of a timer improves your energy savings by up to 50% - a significant benefit from a small additional expenditure. See case study for more information. Setting your element Depending on the daily volume of hot water required additional electrical savings can be achieved by lowering the thermostat setting to + 60°C. Insulation In addition to insulating the pipes connecting your geyser to the solar collector insulate the geyser and the hot water outlet pipe (2 m). Tests have shown that such insulation can reduce the rate at which hot water in the geyser cools thereby requiring less electricity to reheat the water. Hail resistance In order to withstand hail with a diameter < 35mm the outer glass tube of the vacuum tube is made from a 2mm thick hardened borosilicate glass tubes that has high chemical and shock resistance characteristics. In the event of a severe hail storm that manages to smash a vacuum tube there is no risk of leakage from the manifold. The replacing of a vacuum tube is easily achieved by sliding out the damaged tube and replacing it with a new tube.
In certain instances the force of the hailstone will only be strong enough crack the outer tube and thereby affect the performance of the vacuum tube in question. This only affects the heating capacity of the damaged tube and has no impact whatsoever on the heating capacity of the remaining vacuum tubes. This is easily diagnosed in two ways; firstly the outer tube will be hotter than usual as heat is now conducted from the inner tube to the outer tube. Secondly and more importantly as indicated in the photo below, the barium gas getter at the base of the tube becomes transparent and looses its shine. In the event of a combination of these two factors the vacuum tube needs to be replaced.
izing your system The factors that influence the sizing of the panel are dependent on the performance of the panel and whether one is installing a new system or converting an existing electric geyser to a solar/electric geyser. System performance The average annual insolation level for Johannesburg (latitude -26 longitude 28) is 5.39kW/m2/day (winter minimum is 3.79 kW/m2/day, summer maximum is 6.88 kW/m2/day.)
One 58/1800 vacuum tube has an absorption area = 0.08m2 per tube with an efficiency of 84%. Therefore the heating for a collector panel can be calculated by utilizing the following formula; (absorption area of vacuum tube * quantity of vacuum tubes * insolation level * efficiency). This gives us the following performance values for 18 and 24 vacuum tube collectors:
If one utilizes a cold water temperature of 20°C the amount of energy in kW/hr required to heat 150lt and 200Lt to a temperature of 65°C the respective amount of energy required is 7.7kW and 10.2kW. In order to minimise heat loss it is important to install the solar collector as close as possible to the geyser. In instances where this is not possible additional heating capacity must be added to compensate for heat loss over long pipe runs. The general rule is 1 extra heat pipe for every extra 5m of piping between the collector and the geyser. The amount of sunshine during the day is also a factor that can affect system performance. While we all know that during summer the days are longer than during winter the typical highveld summer afternoon thunderstorm affect the amount of sunlight hours we experience. As a result of the highveld summer weather patter we experience more sunlight hours during winter than during summer. New installations - selecting the right size solar water heating system Just like you choose a 150 or 200litre conventional geyser, in any new solar water heating system installation you need to determine the right volume of solar water heating capacity to install. Sizing a solar water heating system involves determining the total collector area and the storage volume required to provide 100% of the household’s hot water needs. It is extremely important to select the right size geyser and collectors to install. When sizing a solar water heater system, a rule of thumb is to allow at least 50 litres of water at a minimum of 55°C and preferably 60 ºC per person per day. However, the household's specific consumption patterns should be taken into account. The following example shows how to size a solar water heating system:
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