FIELD OF THE INVENTION
The invention relates generally to temperature controlled cabinets using diffusion absorption refrigeration cycle systems, in particular relating to cabinets for containing temperature-sensitive electrical and electronic equipment.
Many items of electrical and electronic equipment have increased susceptibility to failure, malfunction or generally accelerated degradation and shortened lifespan when exposed to large variations in temperature, humidity and other ambient conditions, The problem is particularly significant for items of equipment that must be left for extended periods of time in environments that are relatively unprotected from atmospheric conditions.
One example is items of control equipment, and in particular, the standby or backup battery power supplies thereof. Such control equipment may be found in power distribution, telecommunication, transport and security systems and may often be situated in isolated and exposed outdoor and indoor locations. Installing such equipment in an enclosure for protection from rain or other precipitation can often increase temperature variations, in that sunlight on the enclosure will tend to heat the contents of the enclosure to far higher temperatures than would otherwise be the case. Additionally, in some applications, heat emitting equipment situated close to the sensitive equipment may add to the thermal stress. Thus, there is a requirement to provide cooling or air conditioning to the most temperature sensitive items.
In particular, battery back-up power supplies for power distribution control systems and telecommunication systems in the field have been observed to have a service life substantially lower than expected largely due to degradation caused by temperature and/or humidity variation. Solutions in the prior art have provided temperature controlled enclosures for the sensitive equipment ranging from a simple ventilated enclosure through to complete air conditioning systems. These solutions and systems incorporate technologies such as thermoelectric devices, forced convection, heat pipes, phase change material and vapour compression cycles.
A problem to be addressed in such temperature controlled enclosures is to make them as thermally efficient as possible, whilst at the same time developing devices that have no moving components which removes the need for regular and expensive maintenance due to the failure of those components as a result of mechanical wear and tear. Components which can be removed include mechanical parts such as fans, pumps and compressors and consumables such as filters.
An alternative refrigeration cycle or cooling mechanism to those noted which can be adapted to be used with electronic and electrical equipment is the diffusion absorption cycle. This cycle completely avoids the use of mechanical energy and instead it relies on direct thermal energy as a power source. They also use environmentally benign fluids, are reliable, silent and relatively inexpensive to build and have no moving parts. However they have a relatively low refrigeration Coefficient of Performance ('COP'), which needs to be improved so that electronic and electrical equipment such as industrial reserve power batteries can efficiently be cooled.
Inside a conventional temperature controlled enclosure, an evaporator pipe of the diffusion absorption refrigeration cycle system extending into the enclosure has a tendency to cause a build-up of ice when the cooling system is in operation. This introduces safety issues for a cabinet containing electrical and electronic equipment. In addition, the ability of such a refrigerator system to extract heat from the cabinet is limited because the surface area of the evaporator pipe is typically small when compared to the contents of the cabinet. Simply adding a heat sink composed of a thermally conductive material to the evaporator pipe does not necessarily solve this problem, because the temperature distribution of the heat sink may not be uniform over a large surface area.
It is an object of the invention to improve the efficiency of cooling systems for temperature controlled cabinets.
- SUMMARY OF THE INVENTION
A further object of the invention is to reduce the variation in temperature throughout the interior of temperature controlled enclosures.
The invention provides a temperature-controllable equipment cabinet comprising a diffusion-absorption refrigeration cycle system, an evaporator pipe of the refrigeration system extending through a wall of the cabinet and passing through a sealed enclosure for containing a heat transfer liquid, the sealed enclosure extending across and forming part of an internal surface of the cabinet such that the refrigeration system in use extracts heat from within the cabinet to an external environment.
The sealed enclosure is preferably configured to provide a ‘thermo-siphon’ effect, i.e. where a change in density of a working liquid due to temperature variation is used to generate a pumping force through convention flow, thus improving the distribution of cooling throughout the enclosure and ensuring that there is no ice build up on the evaporator pipe coming into contact with any of the contents of the enclosure.
The enclosure may be attached to, or embedded in, the wall by various methods which include one or more of gluing, welding and mechanical fixing. The enclosure through which the evaporator pipe passes may form a part of the ceiling, floor or side wall of the enclosure. The wall may alternatively be a door of the cabinet. One or more sides of the enclosure may be made of different materials.
The wall of the cabinet preferably comprises a layer of thermal insulation through which the evaporator pipe extends.
The sealed enclosure is preferably substantially planar in construction, extending across the internal surface of the wall.
The evaporator pipe preferably passes in a horizontal direction through an upper portion of the sealed enclosure when the cabinet is oriented for use such that, in use, convective flow of the heat transfer liquid aids heat transfer from within the cabinet.
The invention enables improved temperature controlled enclosures for electrical and electronic components when used with a diffusion absorption refrigeration cycle system, leading to lower cost and greater energy efficiency and for a wider range of ambient temperatures.
The sealed enclosure forms a thermo-siphon configured through its location and shape to optimise and improve convection around the evaporator pipe, thus effectively providing additional surface area across which to transfer heat from within the cabinet.
Certain embodiments of the invention can be achieved through modification of an existing temperature controllable cabinet through the addition of a sealed enclosure for containing heat transfer fluid around the evaporator pipe of a diffusion-absorption refrigerator system.
The sealed enclosure may be made from one or a limited number of pieces of material, which improves the ease of manufacture of the enclosure and the ease of installation around the evaporator pipe.
Preferred embodiments of the invention require no moving parts, such as fans which would increase the maintenance costs of the equipment. Heat transfer from the evaporator pipe is instead effected without forced convection. Ice build-up around the evaporator pipe is also prevented, enabling electrical and electronic components within the cabinet to be safely cooled, and the temperature distribution within the cabinet made more uniform. Additional fans may, however, be used where increased heat transfer is required, though at the expense of additional maintenance cost and complexity.
The invention will now be described by way of example, and with reference to the appended drawing in FIG. 1 which shows a cross-sectional view of a diffusion-absorption refrigeration system.
With reference to FIG. 1 a diffusion absorption refrigeration system 5 is attached to a structural part of a cabinet, for example being attached to a door or a wall 9 of the cabinet. The wall 9 comprises a layer of insulation 10 to thermally isolate the internal volume 8 of the cabinet from the external environment 7.
A recess 14 is provided in the wall 9, for example within the layer of insulation 10, through which the evaporator pipe 4 passes. The recess allows the wall 9 to be kept relatively thin without unduly compromising the insulation of the internal volume 8 of the cabinet. The recess also allows the evaporator pipe 4 to be offset from other warmer parts of the system.
The evaporator pipe 4 of the refrigerator system 5 acts to draw heat from within the cabinet, and a heatsink 11 attached to the condenser of the system 5 conducts this heat to the external environment 7.
A liquid-filled enclosure, or thermo-siphon 3, is attached to the inside of the wall 9, forming a sealed vessel surrounding the evaporator pipe 4. The enclosure 3 comprises one or more filling points for introducing liquid 12 into the enclosure once it has been fixed in place around the evaporator pipe 4. The liquid filled enclosure 3 may be attached to the structural insulation 10, or to a material enclosing the insulation, by way of welding, gluing or other mechanical fixing methods, for example at fixing points 2 a, 2 b on the edge of the enclosure 3.
The enclosure 3 may have one or more sides or faces in common with the structural insulation 10 or a material enclosing the insulation, for example along an interface 13 between the internal volume of the enclosure 6 and the insulation 10. The external surface 15 of the enclosure 3 may be in direct contact with the contents of the temperature controlled enclosure, or may act as a cooling element across the internal wall 15 for cooling air within the cabinet.
The size of the thermo-siphon is preferably optimised to provide a balance between thermal efficiency in heat transfer, cost of manufacture, fit with the refrigeration cycle and weight of fluid. The embodiment shown illustrates a particular preferred embodiment, where the enclosure 3 is in a substantially planar form extending across the internal surface of the wall, so as to maximise the cooling effect within the cabinet and minimise the quantity of heat transfer liquid required.
Preferably, the evaporator pipe 4 is located towards an upper end of the enclosure 3, extending through the enclosure in a substantially horizontal direction. The upper location of the pipe 4 allows for the convection effect to be optimised, since cool liquid within the enclosure 3 in contact with the evaporator pipe 4 will sink away from the pipe 4. As the liquid 3 absorbs heat from the internal volume 8 of the cabinet, the liquid rises and is then cooled again by the evaporator pipe 4, creating a convection cycle between the evaporator pipe 4 and the bottom of the enclosure 3. Any volume of liquid above the evaporator pipe 4, however, is not able to contribute to the convection cycle, due to a thermocline being set up within the liquid 12 around the level of the evaporator pipe 4. The evaporator pipe 4 therefore preferably passes through an upper portion of the enclosure 3, and more preferably as near to the top of the enclosure as practical, so as to maximise the efficiency of the thermo-siphon effect.
Testing has indicated that a typical temperature difference ΔT between the contents of a temperature controlled enclosure, for example in the form of industrial batteries, and the external ambient environment of only around 15° C. can be achieved using standard 80 W diffusion absorption cycle refrigeration systems. In addition, the temperature of the contents of such a cabinet can vary by over 10° C. between the top and the bottom of the cabinet. Using the modifications to the cabinet described herein, this variation can be reduced to below 5° C. The invention also enables the refrigerator system to be used in elevated ambient temperatures (well above ‘domestic room’ temperatures) of up to 60° C., while maintaining the contents of the cabinet below 50° C. and with a reduced variation of temperature within the enclosure. An effective minimum ΔT of 15° C. can be maintained for the contents of the cabinet down to around room temperature ambient (around 20-25° C.).
Because the equipment cabinet is required to be thermally isolated from the external environment, a vent may be added to the cabinet to ensure that noxious or explosive gases (such as hydrogen) are dissipated to the external environment, thus avoiding any explosive build up of gas within the cabinet, which could be generated during operation of the equipment therein.
Other embodiments are intentionally within the scope of the invention, as defined by the appended claims.