Principles of solar thermal vacuum technology
Light energy delivered from the sun in the form of radiation, passes through the cold environment of space, through our atmosphere, and onto the objects in our built environment here on earth’s surface.
On its way here through space the only form of energy transfer possible is radiation (direct transfer
through waves) because there are not enough particles (solids) or gasses (fluids) for this energy to be
conveyed by, either; conduction (transfer between two objects of different temperature touching
together), Or convection (the transfer of heat through a fluid via movement or current).
Once this energy enters Earth’s atmosphere it begins to be affected by the environment. Some of the energy is reflected and some is absorbed. The temperature of objects and particles the radiation lands on is raised by this energy input. This heat, being energy can be conducted through contacting materials and moved around by convection through air and liquids in our environment
This diagram (Left) depicts how much of the energy coming from the sun is ‘lost’ in various ways before we have the opportunity to harvest what is left. The LaZer2 solar thermal collector has been developed to the point where it can convert around 93% of radiation that is available to something that is colder! at the earth’s surface, in the location and surface area the collector is positioned, to usable retained heat energy.
This product is at the forefront of this technology, as extensive research and development has discovered we are at the point of diminishing returns in collector performance. In order to understand how a solar thermal collector works we need to consider some of the physics.
To start off, heat is measured by temperature and only flows one way. You can’t make something hotter by placing it next There are three ways in which heat energy can be transferred :-
Conduction: The heat energy moves through the material. A good thermal conductor such as metal allows heat energy to pass quickly and easily through it. For example place one end of an iron bar in a fire and the other end ( the bit you’re holding!) get hot quickly because metal is a good conductor of heat. The end of a wooden pole in the same situation can be held for a considerable period of time because wood is a poor conductor of heat. All materials conduct heat in varying degrees and surprisingly enough water is a poor conductor of heat which allows hot water in a cylinder to stay hot even with cold water below it.
Convection; here the heat energy is carried by moving the material with its heat energy somewhere else. For example, a radiator in a room will heat the air next to it, that air then rises and carries its heat around the room warming the whole room. This is natural convection. Forced convection is where the material say for example the water in a boiler is physically moved by the pump to the radiator where the radiator then gets hot. Because of its very high heat capacity water is a very good medium for convecting heat from one place to another.
Radiation: this is the direct transfer of heat by the electromagnetic spectrum (Solar radiation). You can feel radiated heat coming off a fire.
In the case of a solar thermal panel we are trying to heat above the ambient temperature so conduction and convection will work against us by taking heat from the panel to the outside world. In a vacuum there is no material so no conduction and no convection! The inside of a vacuum tube has no physical connection to the ambient and therefore can not see what the ambient temperature is.
The only way you can get a higher temperature than ambient is by radiation therefore all solar collectors rely on Solar radiation!
The energy delivered by the sun’s rays is sometimes evident, as can be seen by this man cooking an egg on a car bonnet in Bournemouth, England.
On an average day with typically low ambient air temperatures, caused by our geography and maritime climate, cool air flow passing over the bonnet of a car takes away the energy that is delivered by radiation from the sun, faster than it is absorbed (delivered and collected in the form of heat or temperature rise).
A reason cooking the egg is only possible during fair weather conditions is because the heat losses through conduction and convection are not as dramatic under these weather conditions.
In the past before collectors incorporate vacuum technology (and still with some flat plate collectors today) a good performance from a solar collector was only possible on fair days similar to this one, due to the same influencing factors and lack of modern technology.
In our northern European climate the amount of energy delivered from the suns as direct radiation is very similar to hotter climates elsewhere, and so by using vacuum technology in the design and construction of our solar collectors, we are able to utilise this energy with little variation in performance on colder days or high winds. In addition, due to the higher efficiencies that are available by using vacuum technology, our systems are also capable of utilising defused or secondary radiation energy, allowing the system to harvest solar energy even during overcast or cloudy conditions.
The ambient air temperature is negligible due the vacuum space created in the collector tube construction, both isolating and insulating the absorber area from the climatic environment. In the same way that radiation from the sun passes through the vacuum of space, it passes through the artificial vacuum between the layers in the tube and is captured by the absorbers selective coating. This energy is trapped in the collector and so increases its absorber surface temperature significantly. This trapped energy is Conducted to the panel’s internal manifold arrangement and transported away to be deposited elsewhere by forced convection, through our insulated and sealed circuit pipe work. This capture of light energy (radiation from the sun) is proportional to the light quality the panel is exposed to and can work with virtually no loss in efficiency to the collector in cold weather conditions.
Whilst being a top ten contender in the collector performance table, the LaZer2 collector also has additional technological advancements which set it apart from its competitors. Specifically addressing all areas of the system which contribute to combined system performance, and not just focusing on collector performance alone.
The LaZer2 collector’s internal manifold has been developed to maximise efficiency when extracting the energy from the collectors with the minimum of losses. It incorporates among other things, a direct series manifold arrangement whereby each panel, comprising of vacuum collector tubes, designed such that the heat transfer fluid passes through the length of each collector tube twice and all 9 tubes in turn, before it joins the flow pipe work stretch. This enables an enhanced temperature change in each circulation of the system. This also increases turbulence and surface area the fluid comes into contact with, within the collector’s manifold and so enhances the efficiency of temperature exchange between the conducting profile, manifold U-tube and heat transfer fluid. This design is unique to the LaZer2 panel and enables the system to focus the energy towards the desired heat destination, and reduce the retention of usable energy in the panel material mass, making the LaZer2 panel more thermally responsive.
Multiples of panels are grouped together in a bank/array to achieve the system size desired for any given application. Examples of the arrangements that are possible can be seen in the following sections, under; Applications and Utility, and in Sizing of Systems.