SolarUK's Solar Heating Glossary
Buffer vessel
A tank storing heated water in anticipation of variable demand requirements.
This term covers any tanks used to store heated water to cope with either variable heat input or variable demand requirements – in the context of solar energy produced by the panels.
The volume of water in the buffer vessel absorbs any heat available but surplus to, or not needed for, the present requirements. Solar heat can be delivered to a buffer vessel where it is collected until the volume temperature has exceeded the temperature of the volume water which it will replace. At this point control components will allow exchange of the solar heated water with cooler water. The objective is to increase the temperature of the replaced volume by solar energy rather than other means. The buffer vessel provides additional solar dedicated volume which can be heated in periods of low hot water demand and high solar gain. The energy can then be stored and used at a later date. In addition, buffer vessels can be used to accumulate solar energy which is available but not at a high enough temperature to directly transfer heat to a desired destination until circumstances are suitable (such as after a peak draw-off period).
Calorifier
A water storage vessel, which has the capability to increase the temperature of the water contained within it.
This is another name for a hot water cylinder/tank. A calorifier is different from a heat store; as the water within a calorifier is the same water which will come out of taps and showers. A heat store works in a similar way except the water in it merely stores the sun’s energy captured by the collector.
SolarUK’s calorifiers and heat stores range in size from 110 to 6000 litres – the size will depend on the scale of the hot water system and its projected water usage.
Also see: Cylinders, Calorifiers & Heat Exchangers
Circulation module / Pump module
Operates the heat transfer system.
It includes a pump which circulates the heat transfer solution through the collectors on the roof to absorb the sun’s light energy and heat (radiation) and then moves it through the heat exchange coil in the hot water cylinder/tank where the energy is transferred to the stored hot water within the ‘tank’; This heated water will come out of the taps or showers when outlets are opened. The circulation module also contains a pressure gauge, flow meter, one way valve and filling/flushing points.
Also see: Solar Thermal Circulation Controls
Collector (Solar Thermal)
A device, also called a ‘panel’, which collects the sun’s energy and converts it into a more usable form.
Typically, a system circulating fluid (water or antifreeze) transfers the heat from the collector to the domestic hot water cylinder (often referred to as a tank).
The two main types of collector are ‘flat plate’ and ‘evacuated tube’. The latter system performs particularly well in colder climates: the vacuum within the tube limits the loss of heat (which, without a vacuum, would occur through convection and conduction) to the surrounding environment, allowing it to reach higher temperatures.
Also see: LaZer2 Solar Thermal Collectors
Controller
The controller compares the different temperatures at specific locations in the solar systems circuit.
When the collector sensor location (panel) is warmer than the coolest part of the stored water calorifier by a preset minimum difference (usually 3°C), the solar pump switches on, circulating the water through the system and transferring the heat from the collectors to the heat exchanger inside the calorifier. If the temperature difference is below this set value, the pump will not be triggered to run. This prevents the water in the calorifier from being cooled by the solar circuit.
Controllers incorporate an LCD display, which in a SolarUK system will show three temperatures: that of the water leaving the collector, the water leaving the coil/heat exchanger and the water in the calorifier. The controller measures temperature difference and behaves according to a programmed logic. This logic enables the system to recognise and make use of energy from the sun whenever it is available. The position of the sensors is vital to the operation of the system.
Also see: Solar Thermal Circulation Controls
Expansion vessel
A trapped volume of pressurised air or gas inside a steel vessel to allow for the expansion that accompanies temperature increases.
Any fluid filled sealed circuit system, which subjects the fluid contain within it to temperature change, needs to have the capacity for dealing with expansion as the temperature increases. The expansion vessel in a solar hot water system has to be able to operate at higher temperatures than those a standard heating vessel are subjected to.
An expansion vessel consists of a trapped volume of pressurised air/gas inside a steel vessel which contains a rubber bag or diaphragm. This rubber bag, referred to as a diaphragm, contains fluid which is inside the sealed circuit system. The vessel is connected by pipes to the sealed system so that when the fluid in the sealed system expands the diaphragm within it stretches and compresses the gas within the vessel. The pressure increase in the gas is disproportional to the pressure increase that would have occurred in the liquid if the provision for expansion were not there. The liquid pressure is maintained, or the change of pressure at least limited, by this expansion vessel as the volume of fluid in the system fluctuates due to changes in temperature.
Unvented cylinders also require provision for expansion as they have non-return, pressure regulation and pressure relief valves. They are not open to the atmosphere and unless a tap is open/on, the water contained in the cylinder and pipe work effectively behaves as a sealed system. Any volume which is not allowed to expand when it goes through temperature change will result in a potentially damaging or dangerous pressure change. If the diaphragm in the vessel has breached then liquid will fill the gas side and result in the expansion provision being eliminated.
As water in a sealed system expands it is pushed into the expansion vessel, compressing the air inside the diaphragm. As the air is compressed its pressure – and also that of the water in the liquid system contents – increases. The larger the expansion vessel, the smaller the increase in pressure. The appropriate size should be fitted, and the correct pre-charge pressure needs to be set. The pre-charge in the expansion vessel is crucial to it operating correctly and an understanding of ‘head’ as well as ‘applied’ pressure in needed to determine what the correct pre-charge should be. The size of an expansion vessel is relative to the volume in the system, working pressure, and temperature change the system is subjected to during its normal duty.
Feed in Tariff
A method to reward the producer of renewable energy (electricity) by paying a fixed amount of money for each unit of electricity generated.
Also see: Feed in Tariffs
Heat exchanger
The heat exchanger is normally inside the hot water cylinder. It transfers heat in the solar circuit (which has been water warmed by the sun) to the water inside the hot water cylinder/ tank.
The liquids are kept separate. The transfer works on the principle that the heat energy in the hotter material (solar heat transfer liquid) will be transferred to the cooler material (stored water inside the cylinder/tank).
The heat is transferred through the metal wall of the coil which is a pipe which runs in a loop inside the hot water cylinder. This loop or coil is surrounded by the water inside the cylinder and achieves effective heat transfer by having a large surface area in contact with both the solar heat transfer solution inside it, and the stored water (to be heated) which surrounds it.
The type of energy transfer occurring is both conduction through the coil to the surrounding water, and convection through the stored water in the cylinder, which moves via thermal currents, carrying the energy to the other areas within the cylinder.
A car radiator is a good example of this passive heat exchange: it transfers heat from the engine’s cooling system to the outside air, by exposing to each other two materials (water circulated around the engine; and the air temperature outside the radiator) of different temperatures while maintaining the separation of materials by an exchange surface - the exchange surface in this example being the external casing of the car radiator.
Also see: Cylinders, Calorifiers & Heat Exchangers
Heat store
Stores the suns energy as heat captured by the collector.
A heat store works in a similar way to the calorifier except the water in it merely stores the sun’s energy as heat captured by the collector: This stored energy is transferred to the water which comes out of the taps via another heat exchange element/coil. Heat stores can have a range of heat input sources including conventional gas, oil and solid fuel boilers. Also, ground source and solar thermal energy can be stored. Each input needs to have a separate heating coil within the store so that the different systems can maintain hydraulic separation; alternatively, heat stores can be directly heated if the fluid contained within them is circulated out of the vessel to the heat source or external heat exchanger and then returned to a different port by a pump or thermosyphon (the movement of particles in liquid or gas through natural convection). In this respect heat stores are very similar to storage cylinders except that a ‘heat store’ stores heat, which is then drawn from the storage medium by heat exchange, whereas stored hot water is stored in cylinders/clarifiers, heated there, and flows out from the cylinder to taps and showers.
Solar storage tanks can be configured in many ways and one advantage of a store over a cylinder is that hydraulic separation is achieved between the volume of the vessel and the water which will be delivered from your taps: this reduces the perceived risks associated with stored hot water at low temperatures due to the volume in the extract coil being considerably less than the cylinder volume. A disadvantage is that for it to work effectively, the store temperature needs to be much higher than the equivalent cylinder volume. The efficiency of the solar thermal system is also improved when it is heating a cylinder as there are less stages of heat transfer and the lower part of the cylinder is replenished with cooler water as draw-off occurs. Some systems will incorporate two-tanks, with the solar energy heating a heat store. The extract coil in the store would pre-heat the domestic hot water before it enters a conventional water heater/calorifier where its temperature is at the desired level before it is delivered to the outlets. This type of system is favoured in large buildings where a recirculation loop is needed and significant risk from legionella is present due to varied and inconstant water demand and the old existing infrastructure.
Also see: Cylinders, calorifiers and heat exchangers
Manifold
Any pipe with several outlets to or from other pipes, or a junction of pipes.
In a vacuum tube collector, the energy absorbed by the collecting surface is trapped by the vacuum inside the collector tube. Energy is extracted by a manifold of pipe work containing the heat transfer fluid. This manifold passes in and out of each collector tube and the flows and returns from all tubes in a single panel (9 tubes) which are in series.
Collectors in each array are collated by a manifold, so that all pipe work from collectors are grouped to a single pair of flow and return pipes. The manifold design inside the LaZer2 collector is particularly efficient because it is a direct flow, serial manifold. This means the heat transfer fluid is directly heated by the collector surface rather than via another exchange stage and that it passes in and out of each tube in succession.
The advantage of a direct flow, ‘serial’ manifold is that the transfer solution passes through each vacuum tube in series rather than in and out of a single tube, making it move faster while prolonging the period of time it spends inside the collector; this improves the heat transfer from collector to the heat transfer fluid.
Pre-feed cylinder
A cylinder placed between the cold water feed and the existing hot water cylinder.
Assuming the existing hot water cylinder is being retained, a separate pre-heat cylinder is placed between the cold water feed and the existing hot water cylinder. This new pre-heat cylinder raises the temperature of the water which enters the existing cylinder, thus reducing the load on the conventional heat source. The existing heat source would then only increase the water temperature further to the desired set point if and when required. In some cases there will be no need for further heating if the desired water temperature has been achieved by the pre-heating alone. Pre-feed/pre-heating is an efficient use of low temperature heat inputs and also a convenient arrangement in many circumstances. As the size constraints of many buildings do not allow for an increased volume (larger vessel) of stored hot water in the desired location, a pre-feed/pre-heat system is often favoured in these situations. Many large buildings will duplicate plant as protection against service/maintenance downtime, which can sometimes be used in a manner that increases the performance of renewable energy sources and lowers the energy consumption of the building.
A solar pre-heat cylinder will normally perform better (that is, be more efficient) than a combined heat source cylinder as the solar pre-heat cylinder will often be at lower temperatures and so heat/energy transfer can occur in less favorable climatic conditions.
Going a step further, sequential heating of different coils/ different volumes of water provides the best service provision as recovery rates through solar heating alone are reduced while efficient use of the energy available is maintained. By sequential switching of solar heat destinations, the concept of pre-heating can still be achieved, as whenever the water surrounding a solar coil becomes saturated with heat (lowering the efficiency), solar heat can be redirected to a coil which is surrounded by pre-feeding water. (I.e. solar energy heats the water surrounding the bottom coil of a primary cylinder first, the top being heated by just a gas-fired heating coil. Next solar energy is directed to heat the water surrounding the top coil of a pre-feeding cylinder, and then redirected to heat the bottom coil of the pre-feeding cylinder).
Primary and secondary systems
This refers to the water travelling round the boiler and the water in the cylinder which is heated by it.
In an indirect system, primary water travels round the boiler and transfers its heat to the secondary water which is in the body of the cylinder. The volume contained within the cylinder is referred to as the hot water secondary side/system. This secondary water comes out of the building’s taps and showers, whereas the primary water is treated with a solution of chemicals (SolarUK uses a synthetic solution which does not pose the threat to aquatic organisms associated with glycol) and is used as a heat transfer medium between the heat source and the boiler coil/heat exchanger in the cylinder.
Recirculation - pumped secondary
Pipework carrying secondary hot water.
Recirculation or pumped secondary systems are a loop of pipework which travels round the building, carrying ‘secondary’ (hot) water. This loop is circulated by a pump and is designed to avoid long dead legs in large buildings which occur due to the water in the pipe cooling between periods of draw-off. These dead legs can lead to bacteria growth in the pipe work and water wastage as water is run away to drains before hot water is delivered to the taps. A ‘recirculation loop’ maintains the temperature of the hot water supply pipe work as if it were an extension of the cylinder volume, and minimises the stretch of pipe between hot water and the taps. The secondary circuit is drawn from the top of the cylinder and returned to a port on the side of the cylinder within the top third. If this port is lower, the recirculation will have a de-stratifying effect on the cylinder volume. All pipes carrying hot water to outlets are drawn from this loop.
‘Drain back’ and ‘sealed’ primary solar circuit principles: definitions
When the temperature drops below zero, sealed solar systems either run the circulation pump (active) to draw heat from the cylinder through the pipe work and collectors to avoid freezing, or are left stationary with anti-freeze inhibitors (passive) providing protection to the fluid in the entire system (including external parts).
Drain back/drain down systems drain the water from the collectors into a tank when the circulating pump is not running; this also avoids freezing.
Solar systems use a temperature differential controller to measure and react to temperature differences, according to a pre-determined logic, which results in relays which power pumps and valves being switched on and off in the appropriate circumstances.
Other systems also use temperature differential controllers to circulate fluid within them to avoid freezing. However it is common to dose all primary systems to protect against corrosion as well as frost damage. The reason secondary systems are not dosed is that the concentration would continually be diminished through draw-off and also some of the dosing chemicals (inhibitors) are potentially harmful to the environment.
Also see: Solar Thermal Differential Controllers
Renewable Heat Incentive (RHI)
A feed in tariff for renewable generation of heat as an alternative to the use of fossil fuels.
Commencing in April 2011, the RHI will guarantee long-term payments for installers of solar thermal, ground source heat pumps, biomass boilers and other renewable technologies. It will apply to heating at all scales, from households to public sector buildings to industrial processes in factories.
Also see: Renewable Heat Incentive
Sequential heating arrangement
The heating that occurs as the solar thermal system heats the water before the conventional heat source provides any necessary additional heating.
This has already been touched on under 'Pre-feed cylinder'. Sequential heating occurs in all solar cylinders as the solar system heats the water to a temperature relative to climatic conditions and rate of draw-off, before the conventional heat source applies heating (if necessary) to ensure a usable temperature has been reached.
When two cylinders are fitted so that one is pre-feeding the other, it is most important to ensure the water leaving the cylinder and going to the taps has reached the minimum desired, usable temperature before heating the pre-feeding cylinder. This means that a sequence of priority applies to different, sometimes stratified volumes of water.
The priority is to achieve a usable temperature and eliminate bacteria growth. Once this has been achieved, the next step is to make best use of the available free energy in order to reduce fuel consumption and pollutants, and to save money. This is done by redirecting the solar energy from the initial priority volume to a “buffer/pre-feeding” volume where exchange can be achieved more efficiently at lower temperatures and also valuable benefits can be had more easily from low light conditions. This also allows available energy to be stored, which would be lost without the storage capacity. This stored hot water in the pre-feed cylinder will fill the primary cylinder when its volume has been drawn off: it could therefore be used, for instance, on the following day which might have a lower level of solar gain than the day before. When high demand is experienced and both the secondary/pre-feed cylinder and the primary/pre-fed cylinder have cooled, the focus of available solar energy is directed to recovering the temperature of the primary cylinder as the priority of service provision to the client’s satisfaction is paramount to an effective hot water system.
Solar Thermal
The collection of the sun's energy as heat rather than converting it to electricity.
Solar thermal systems are usually seen as being solar panels or solar vacuum tubes. Both work in a very similar manner, each presenting a dark surface that collects the energy from the sun. The energy, in the form of heat is then carried using a circulating fluid to a heat store or some kind of heat exchanger.
Solar thermal could also be used to describe passive solar, perhaps a conservatory or greenhouse where the energy from the sun as heat is used directly.
Also see: Domestic Solar Water heating
Stagnation
Refers to any period when the heat transfer fluid is not circulating, even though the solar panel is receiving the suns energy.
In hot weather, a system can receive more heat from the sun than is needed. Usually, in these circumstances a system is allowed to ‘stagnate’. The stagnation temperature that the collector can reach if the fluid is not circulating is governed by the balance of heat gained from the sun and heat lost to the environment. In the case of LaZer2 collectors the temperature is 100 – 120°C.
When we refer to stagnation in the context of boil tolerance, we are assuming that the full temperature range is achievable as this has an effect on the fluid content in the panels. As the temperature passes the 135°C mark the fluid which is at the applied system pressure begins to evaporate. The small amount which is first to change state forces the rest of the fluid (which is still liquid) from the panels where it can no longer receive any energy input. The remaining moisture in the collectors continues to receive heat input. The collectors continue to rise in temperature as they are now no longer filled with fluid. The conditions inside the collectors begin to stabilize as they are now filled completely with water vapor at the same pressure as the liquid which fills the rest of the system. Any water vapor which exceeds the volume of the collectors at applied pressure will condense in adjoining, localized pipe work, as energy can be dispersed in these locations and no further energy can be transferred to it. A state of ‘stagnation’ is achieved when the input energy equals the energy being lost through all forms of possible energy transfer. At this point the pressure is stable and the panels are effectively drained back even though the circuit is a sealed system. As temperatures in the panel drop and input energy diminishes as daylight is lost, the amount of losses begin to exceed the input energy and the fluid re-enters the collector at the same time as the water vapor condenses. The actual pressure inside the system will in real terms fluctuate as the system goes into and out of stagnation, and during stagnation the expansion vessel must be able to allow for expansion of the fluid through the range of temperatures plus the sum of the cold fluid volume in the collectors and manifold: therefore it must be sized carefully for the system to work. Based on the proportion of expansion volume to system volume, the expansion vessel on a solar system will be considerably larger than other systems.
Stratification / de-stratification
The layering of hot water on colder water.
In a solar hot water storage tank, the hot water is layered on top of colder water. As a result of this layering, or stratification, when the hot tap is turned on the water at the TOP of the cylinder, which has been topped up in temperature by gas heating delivered by the upper coil, is allowed to emerge from the taps. Meanwhile water enters the BOTTOM of the storage tank to replace the water which has been drawn off. In a solar hot water cylinder the solar energy is transferred to the cooler water surrounding the solar coil in the lower part of the cylinder which raises its temperature. As the stratification of the water volume stored within the cylinder occurs due to differences in temperature aided by the orientation of inlet and draw-off ports the solar heated volume is pre-feeding the boiler-heated volume/proportion of the cylinder. In a well designed system, for much of the year the solar heat input is sufficient to encourage solar heated water to exchange with water in the boiler-heated proportion of the cylinder due to temperature difference, and thus for the solar heating coil to heat the entire cylinder volume to above the boiler coil control stat temperature. In this case there will be no need for further heating by the boiler and the solar will be providing the entire hot water heating necessary for that cylinder. Furthermore the system may be sized to achieve more water heating than this in these months and so move to heat a second, pre-feed cylinder, which will fill the bottom of the already hot twin coil cylinder with pre-pre-heated water. The traditional single coil cylinder is replaced by a dual coil one, so that heat input can come from a greater number of sources which each have separate systems and separated heat transfer fluid. The increase in the number of coils also allows for greater use of stratification and efficiencies gained through sequential heating: solar cylinders are usually narrower and taller, which allows for better stratification of the water temperature – though this is not always the case. In many circumstances, the water at the top of the cylinder (this volume being referred to is normally between 1/3 to 3/5 of the cylinder volume, from the top down: the remainder is considered solar-dedicated volume.) is enough for immediate needs. However, if a full tank capacity is needed then a circulating pump can be turned on (manually or by timer) which mixes, or de-stratifies, the water in the cylinder, enabling the full volume of water to be heated by the top coil. The operation of de-stratification dramatically reduces the effectiveness and efficiency of a solar system’s heat input capacity fitted to that cylinder. The boiler is also heating a much larger volume of water than it would be subjected to if stratification was allowed to occur.
De-stratification, as well as being used to eliminate the very small potential for bacteria existence in cooler parts of a cylinder, minimises the size of a cylinder that can service the demand it is subjected to or makes up for a cylinder which is much too small to cope with peak demand periods (due to the differences in recovery rates which are about one hour maximum for boiler heat input and a day per cylinder volume for solar input). A cylinder that has been completely de-stratified or is heated completely by the boiler by incorrect use of the cylinder ports will result in a solar system which is seriously incapacitated and will not perform as designed. Also, the fuel costs for hot water provision are likely to be higher than for a standard size cylinder as a solar dedicated volume will now become part of the heating load which the gas fired boiler will have to heat.
Vacuum Tube Solar Collector
This is a technology that uses a glass tube, think of a long thermos flask to insulate the solar collector and ensure that effectively all solar energy is retained.
The LaZer2 collector features a unique aluminium heat sink and a direct-flow, serial-manifold. A direct-flow manifold is more efficient than a heat-pipe manifold because it does not require an in-direct transfer of energy, which will always lead to some loss of energy. In a collector with a serial-manifold, the transfer fluid passes through each vacuum tube in series rather than just in and out of a single tube. This means the fluid moves much faster as it has further to travel (38m in a LaZer2 collector). The increased speed means more turbulence and a distinct improvement in heat transfer.
Also see: /lazer2_solar_thermal_collectors.asp
