PCMs used in heat exchangers are a relatively new concept, with few commercial products available on the market. Contrary to PCM panels and boards, the phase change properties of PCMs employed in AC units are more dependent on the AC system design. Nonetheless, as shown by the following literatures, because these systems mainly operate under the free-cooling principle, the PCM phase change temperatures are close to the comfort conditions. This aids in the maximisation of the energy saving potential of PCMs.
Dolado et al. (2009) numerically studied an energy storage device consisting of a PCM plate arrangement placed in an air duct system under free cooling. The candidate PCMs were E21® (from EPS Ltd), RT27® (from Rubitherm GmbH) and C32® (from Climator AB). The authors conveyed the importance of the encapsulation geometry and chemical compatibility of different PCMs with their encapsulation. Aluminium encapsulation was used in their study. The PCM was used to collect ‘cold’ from the ambient air during the night to be released during the day (free-cooling), in a semi-active way to minimise overheating of the conditioned space and maintain comfortable indoor thermal
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conditions. The simulation results showed that the selected PCMs were not suitable for general rooms (comfort temperature of 20°C to 24°C) and ‘biotech labs’ (comfort temperature of 17°C to 27°C). They were however better suited to computer rooms requiring temperatures of 21°C to 40°C. This is because commercial PCMs tend to have large melting temperature ranges, and the associated latent capacity within the comfort temperature range is not always enough to maintain thermal comfort. This emphasises the fact that it is not the peak phase change temperature which is the crucial parameter, but rather, the melting range and enthalpy within the required comfort temperature limits.
Vakilaltojjar et al (2001) studied semi-analytically the heat transfer properties of an arrangement consisting of PCM plates placed in an air-conditioning system. The PCMs considered were calcium chloride hexahydrate and potassium fluoride tetrahydrate. The authors showed that for the same mass of PCM, increasing the plate thickness and air gap resulted in poorer heat transfer properties. Conversely, smaller air gaps and thinner PCM plates enhanced the heat transfer performance, at the expense of higher number of PCM containers, larger volume of the overall storage device and higher pressure drop.
Furthermore, it was found that the air velocity profile at the inlet did not affect the heat transfer characteristics considerably.
Medved and Arkar (2008) numerically investigated the free-cooling performance of PCM RT20® spheres packed in a cylinder to be incorporated into air conditioning system ducts, for six European cities. They found that a PCM with broader temperature range is more favourable for such applications due to the large variations in ambient air temperature. For the cases studied, the authors showed that the PCM peak melting temperature should not differ by more than ±2°C from the optimum indoor temperature in order to allow an effective use of the PCM. They showed that the optimum size of the PCM unit for free-cooling should be between 1 and 1.5 kg of PCM per m3/hr of fresh ventilation air, while the night ventilation rate should be approximately thrice the daytime flow rate.
Nagano et al (2006) experimentally studied the design of a floor supply AC system with granular PCM for the metropolitan area of Japan. The authors produced their own granules from a mixture of hexadecane and octadecane that were evenly distributed in the floor. The experiments were conducted in a test-cell mimicking an office building.
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The results showed that employing a PCM packed bed in the floor AC system improved the time constant of the room temperature by a factor 1.5-2.1, and that the use of night-ventilation at a rate of 16.3 m3/hr allowed 89% of the daytime cooling energy to be provided by the PCM.
Gowreesunker and Tassou (2013b) numerically investigated a CSM® panel arrangement retrofitted inside an airport’s displacement ventilation diffuser, using a TRNSYS-CFD quasi-dynamic simulation method. TRNSYS® was used to simulate the AC unit and control system, while the airflow and radiation within the airport terminal was simulated using CFD FLUENT®. The PCM system operated under the free-cooling principle during summer and mid-seasons, while the indoor air was re-circulated for winter recharge. The results showed that annual energy savings in the range of 22-34% are possible, and because of the operation schedule of the airport terminal, limiting the use of night ambient air for the PCM recharge is more beneficial.
Zalba et al (2004) experimentally studied an arrangement of PCM plates with RT-25®
and C-22® inside air conditioning ducts. They found that the heat transfer rate was faster with thinner plates, higher air flow rates, and greater temperature difference between the PCM and the incoming air. They further employed the Design of Experiment (DoE) statistical method to formulate the relationship between these three variables and the melting/solidification times. Using these relations to size the PCM system, an economic analysis was conducted to depict the performance of the system. This showed that using the PCM plate system would require an additional 9% investment, but will have a payback period of 3-4 years and will consume 9.4 times less energy than a conventional AC system.
Dolado et al (2011) studied a real scale PCM-air heat exchanger in a laboratory experimental setup. They used the Ruitherm CSM® plates with an organic PCM of mean phase change temperature of 26.5°C. They quantified the surface of the plates in terms of rugosity, and convection heat transfer analyses in the PCM were simplified using an effective thermal conductivity. The authors showed that for a constant PCM mass, although increasing the PCM thickness increases the air convective heat transfer coefficient due to an increase in the air velocity, the solidification/melting process is delayed. Conversely, decreasing the PCM thickness improves the absorption/release of heat because of the larger heat transfer unit area, for a given volume.
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A commercially available air-conditioning product with PCM is the TROXType FSL-B-PCM® (see Fig. 2.9), which is a semi active system that can be placed in suspended ceilings or under-window sill.
Fig. 2.9. TROX system (www.troxuk.co.uk, 2013)
The charging process occurs at night with cold air passing through the PCM, and the discharging process happens during the day, with hot outside air passing through the unit (free-cooling) before entering the space. The product uses either paraffin or salt hydrates with melting points between 20°C to 25°C. The manufacturer claims a cooling capacity of 280W when used for 5 hours, and an auxiliary heating capacity of 2 kW, done by electric heating (TROX UK Ltd, 2013).
TROXTechnik® produces other variations of AC units, comprising mainly of under-sill units and under floor units. During the UK-summer, the diurnal temperature fluctuation is around 10°C (De Saulles, 2009), which suggests that these systems may also provide semi-active heating at night, if a uniform temperature profile for both the day and night is required, without the need of electric heating. Nonetheless, auxiliary heating will be required for winter.
The Monodraught COOL-PHASE® unit is another commercially available AC unit, which incorporates PCM. The unit is placed on the ceiling and comprises of PCM plate heat exchangers. It operates by the free-cooling principle, using cold night air to recharge the PCM. For heating purposes, the unit recirculates indoor air to recharge the PCM accordingly. The manufacturer claims a reduction of 90% in the energy consumption relative to a conventional cooling system (MONODRAUGHT Ltd, 2013).
The introduction of these commercial PCM units shows the market’s interest in the concept of energy storage in reducing the energy consumption and CO2 emissions in buildings, with regards to the 2020 UK emission reduction targets.
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