Several factors are driving the market for solar cooling solutions. On one hand, the exploding demand for cooling and consequently, the demand for electricity to drive the conventional cooling machines is becoming not only very expensive but also endangering the stability of electricity grids. In several European countries, the peak electricity demand has already shifted from winter to summer and the demand for more comfort and cooling is rapidly increasing even in more moderate climates. Furthermore, the trend of larger solar thermal systems and higher solar fractions lead to more available solar heat output in summer than what is actually needed. These factors make solar thermal cooling a more and more attractive option. The number of installed systems has increased tremendously during the past few years and several new thermally driven cooling machines, especially, designed for smaller capacities and lower driving temperatures have entered the market. As this is a very new market, there are hardly any standards regarding the durability of solar thermal cooling systems so far. To ensure a good quality and to avoid any backlashes from low – or non-performing systems, quality requirements need to be developed soon, as solar cooling technology is developing very fast and high expectations are put in it. Solar cooling systems require high investments and therefore, any kind of guarantees on life expectancy, performance or durability are welcome to convince or reassure customers and investors.
Common Technical Problems
Some of the frequently detected technical problems which have been emphasized are:
• Solar collectors overheating due to insufficient thermal energy demand or difficulties to match solar energy availability and energy demand with the result of serious damages in solar collector field: leakages, degradation of the thermal insulation and absorbers, etc…
• Control and regulation sensors located in not suitable positions causing control failures and low system performance.
• Heat storage higher than necessary causing delay to start up the solar assisted cooling system and reduction of operating hours in solar cooling mode.
• Lack of suitable technical maintenance. Risk points are basically the solar collector field and the heat rejection system.
• Difficulties to combine the control of the conventional cooling system with that of the solar cooling system.
• System performance deviations against predicted performance due to failures in operation of the control system and the heat rejection system, resulting in an insufficient cooling capacity.
Seasonal Operation of Solar Assisted Cooling Systems
Chiller manufacturers state that seasonal operation of solar assisted cooling systems does not produce technical problems if during each commissioning, suitable regulation of all internal parameters is made according to the instructions provided by the manufacturer, although there is evidence that this could have influence on the switching valves. On the other hand it is also unlikely that the medium and large chiller can be operated as a heat pump for heating due to its low coefficient of performance (COP).
In terms of cost-effectiveness of solar cooling systems, seasonal operation leads to less operation hours per year and makes it more difficult to get the investment recovery. Anyway regarding to cost-effectiveness, seasonal operation is only an additional issue because the main problem is the actual high investment costs of the systems.
In case of long-term shutdown of absorption chillers, some manufacturers recommend to pressure the vapour space with nitrogen above atmospheric pressure so that oxygen cannot enter the equipment through any possible leakage caused by corrosion. Usually, chiller parameters are controlled during each annual commissioning at the beginning of operation, but they are not taken into account during idle operation.
Maintenance Requirements
Depending on the system, solar cooling systems can require exhaustive technical maintenance. Because different elements (solar thermal circuit, heat rejection circuit, chiller circuit and distribution circuit) need to be combined, the complexity of the hydraulic connections is higher for a solar cooling system than for a solar thermal systems to produce heat. Maintenance must be periodic and it cannot be neglected because the consequences are complex to solve afterwards.
The maintenance effort for chillers is usually low. Absorption machines, for example, are robust and inherently stable, its solution concentration adjusts to the imposed temperatures according to its thermodynamic properties to provide trouble free operation for the user and no active controls are required with the exception of controls to deal with crystallization issues. However their maintenance, in most of the cases, requires technicians trained by the own chiller manufacturer. As a result, it is usually expensive and the access to this maintenance uses to be difficult to get in some regional markets where chiller manufacturers have few products or there is no presence at all. After-sales management should be guaranteed.
Corrosion is usually the ultimate failure of absorption chillers. Because the life of the chiller is limited by corrosion, for long life, great attention must be paid to avoid introduction of air into the equipment and to ensure the corrosion inhibition regime is strictly followed. Maintenance of absorption chillers usually includes purging of non-condensable gases, addition of octyl alcohol (if it is used), addition of corrosion inhibitor and addition of pH buffer. The frequency of these tasks depends on variables such as the size of the chiller or the purging system and is specified by the chiller manufacturer.
Heat rejection systems such as cooling towers require exhaustive maintenance and in the case of wet cooling towers, periodic controls, regular water treatment and periodic water analysis depending on the national regulations are required. On the other hand, heat rejection systems such as geothermal probes, basically, require the checking of hydraulic pressure in the pipes. The solar collector field imposes an annual basic maintenance load. Air vents should be installed and periodically checked.
Critical Elements Regarding System Durability
Elements which need special attention regarding system durability are the heat rejection system at the top, the chiller and controls. Many technical problems with chillers are associated with improper maintenance on the heat rejection system. Insufficient heat rejection air flow due to dirt accumulation in the filters is frequently reported. Hence, the heat transfer capacity of the heat rejection system decreases and the return temperature of the chiller rises resulting in a decrease of the performance of the chiller.
In such situations, a safety control should be active to protect absorption chillers against crystallization. It is absolutely necessary to avoid or minimize the accidental introduction of oxygen during maintenance procedure. If vacuum must be broken for any reason, the vapour space should be filled with nitrogen or other inert gas to avoid introduction of oxygen. Regarding absorption chillers using silica gel, the return temperature provided by the heat rejection shall be above the temperature of the cooling circuit. Otherwise condensation may occur in the absorber. Critical components are the vacuum chamber and switching valves which have to switch every 5-10 minutes.
For desiccant systems, the air filters have to be cleaned or changed regularly, in order not to have accumulation of dust in the passages of the desiccant wheel resulting in performance degradation. Potential air leakage between the supply and exhaust air stream in the desiccant wheel has to be checked and prevented.
Additional to the maintenance procedures, it is strongly recommended to carry out a periodic assessment of performance against the values declared by the manufacturer in order to detect potential technical problems.
Overheating during periods without cooling demand (weekends, holidays, etc.) produces a great degradation of the solar collector field. Special caution has to be paid to release trapped air inside the circuits and collectors.
Since solar cooling systems are more complex than e.g. solar thermal systems for heating support, more effort has to be made for regular maintenance and system checks. Some aspects that should be tested or checked to improve the durability of solar cooling systems are:
Aspects to Promote Solar Cooling Installations
Even though the level of technological solutions of solar assisted cooling systems can be considered as quite mature, their penetration is mainly limited to demonstration and research installations.
At present the costs of solar assisted cooling systems are high as compared to other cooling technologies. The low degree of standardization and the lack of know-how in system design and of trained installers, technicians and designers are additional obstacles. Training of the installers, planners, etc… should be part of a quality assurance system.
For these reasons, economic support and incentives are still required to improve the market penetration of the technology. More demonstration projects for practical use with high quality should be installed. Their operating performance and energy savings should be analysed. Such information should be easily accessible by interested people.
Reference: QaiST – Quality assurance in solar heating and cooling technology Task Report 5.3.2 Date: 21/05/2012
AUTHORS CREDIT & PHOTOGRAPH
Ritesh J Mistry
M.E Cryogenic
L. D. Engineering Ahmedabad.
Zamil Air Conditioners India Pvt. Ltd. Ahmedabad
