Solar Cooling (2017)Apunte Inglés
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Other Applications of Solar Thermal Power
Unlike the use of solar energy for heating purposes, which often take place during night time, a
system able to produce a cooling effect during daytime would be very appropriate, since the
matching of sun hours with the cooling load is almost exact. Moreover, the cooling load is
becoming more and more important, catching up to the heating load.
The idea is to use a thermal machine able to perform as follows: COOLING SYSTEM Useful cooling effect Low Temperature, Tc Thermal SOLAR COOLECTOR Driving heat High Temperature, Th Machine ENVIRONMENT Heat rejection Medium Temperature, Tm The maximum efficiency of such devices is given by: 𝐶𝑂𝑃𝑡𝑒𝑟𝑚,𝐶𝑎𝑟𝑛𝑜𝑡 = 𝑇𝑐 𝑇ℎ − 𝑇𝑚 · 𝑇ℎ 𝑇𝑚 − 𝑇𝑐 Basically, the idea is to produce cool effect with heat coming from the sun. The existing technologies that is able to perform in such way are the following: • • Absorption cooling.
o LiBr-H2O o Silica Gel-H2O Desiccant Evaporative Cooling.
Technological Options PV and Chiller One option to produce a cooling effect from the sun could be using already mature technology, such as PV system producing electricity that powers a mechanical vapor-compression chiller.
The PV system would have a moderate efficiency of around 15%, and the conventional chiller an approximated COPelec of 3.
CSP and Chiller The system would be similar to the previous, substituting the electricity generation of PV by a CSP plant. Since the CSP technology is not as mature as the PV one, the system would be much more complex and expensive, and the improvement of the electrical efficiency would be only a 5% higher. However, the use of CSP could open the path to the thermal storage.
Pau Carnero MUTEDS Si necesitas más apuntes puedes encontrarlos en Unybook.com buscando el usuario "pcarnero" CSP and Absorption Machine This option substitutes the compressor of the conventional chiller by an absorption machine driven by the heat produced of the solar collectors of a Concentrating Solar Plant, which does not run a conventional Rankine Cycle. The solar collectors’ efficiency is above the 50%, but the COPelec of the absorption machine cannot exceed 1. There are single-effect and dual-effect chillers, the latter are able to work at higher temperatures with better COP, but lower power.
In order to compare conventional chillers’ efficiency with absorption’s, the comparison must be made considering similar forms of energy, both thermal primary or both electrical final.
Desiccants This system, based on adsorption, is very complex. The process consists on producing cool dry air by making an air current pass through a desiccant, which captures the humidity present in the air process improved by a previous cooling effect. Then, the desiccant needs to be regenerated, which is made by means of heat appliance to it, while it’s showered onto an air current, which captures the humidity. There are systems with liquid desiccants, which have the problem of occasional carryover of the desiccant; and others with it in solid state. Both have the problem of high dependence on external weather conditions.
The efficiencies are similar to the previous technology.
Solid desiccant systems are becoming more used in the Air Handling Units in order to remove latent load from buildings. The most common materials are silica gel, lithium chlorate salt, etc.
Solar Collectors There are two classifications for the technologies of high temperature heat source, depending on whether they concentrate of not. With concentration there are the, already known, parabolic troughs and linear Fresnel; without concentration there are the evacuated tube and the flat plate ones. The most widely used are the flat plate ones, due to its simplicity.
The energy balance for the solar collectors is the following: 𝑄𝑢𝑠𝑒𝑓𝑢𝑙 = 𝑄𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 − 𝑄𝑙𝑜𝑠𝑡 = [𝐺𝛽 · 𝑆] − [𝑈𝐿 · 𝑆 · (𝑇𝑚,𝑎𝑏𝑠 − 𝑇𝑒𝑛𝑣 ) + 𝐺𝛽 · 𝑆 · (1 − 𝛼 · 𝜏)] Being the useful energy, the incident radiation minus the thermal loses due to convection and radiation to the ambient, and the optical losses due to the reflected radiation.
𝑄𝑢𝑠𝑒𝑓𝑢𝑙 = 𝑆 · [𝐺𝛽 · 𝛼 · 𝜏 − 𝑈𝐿 · (𝑇𝑚,𝑎𝑏𝑠 − 𝑇𝑒𝑛𝑣 )] Since the mean temperature of the absorber is very difficult to know, it’s estimated an ideal case where the absorber temperature would be equal to the inlet temperature of the heat transfer fluid (HTF), and a correction factor, FR.
𝑄𝑢𝑠𝑒𝑓𝑢𝑙 = 𝑆 · 𝐹𝑅 · [𝐺𝛽 · 𝛼 · 𝜏 − 𝑈𝐿 · (𝑇𝑖,𝑎𝑏𝑠 − 𝑇𝑒𝑛𝑣 )] The coefficient of heat losses depends itself from the operating temperatures, so it can be states that 𝐹𝑅 · 𝑈𝐿 = 𝑐1 + 𝑐2 · (𝑇𝑖,𝑎𝑏𝑠 − 𝑇𝑒𝑛𝑣 ) Then, the efficiency can be defined as: Pau Carnero MUTEDS Si necesitas más apuntes puedes encontrarlos en Unybook.com buscando el usuario "pcarnero" 2 𝜂= 𝑄𝑢𝑠𝑒𝑓𝑢𝑙 𝑐1 · (𝑇𝑖,𝑎𝑏𝑠 − 𝑇𝑒𝑛𝑣 ) 𝑐2 · (𝑇𝑖,𝑎𝑏𝑠 − 𝑇𝑒𝑛𝑣 ) = 𝐹𝑅 · 𝛼 · 𝜏 − + 𝐺𝛽 · 𝑆 𝐺𝛽 𝐺𝛽 Usually, for not very high applications, the quadratic term can be neglected. However, for evacuated tube collectors it must be taken into account.
It can be observed that the maximum efficiency is reached when the temperature inside the absorber is equal to the ambient, removing the thermal losses. This can be reached acting upon the mass flow rate and the inlet temperature, which would necessarily need to be lower than the ambient. In the practice, theses are not the optimal operating conditions. The installation must be able to withstand the stagnation temperature.
There’s an inverse relationship between the collector’s efficiency and the working range of temperatures, the order in this would be the following: desiccant systems>adsoption>1-effect absorption>2-effect absorption Heat Dissipation The most used systems for heat rejection are wet and dry cooling systems. It’s important to consider the effect of fans and pumps present in the heat dissipation system in the overall energy balance.
From an energy point of view, wet cooling systems are more interesting since they are able to work against the wet bulb temperature of the ambient, instead of the dry one, like the dry cooling systems. However, they are more expensive to install and operate.
Performance and Economical Feasibility The main issue with solar cooling is the high investment costs, currently is not possible to compete against the use of PV and a conventional chiller.
There are also market barriers like the low efficiencies of the technology, as well as poor experience and absence of mature and standard equipment.