NENK, 100-101

By their nature, buildings have large surface areas and consequently a large potential for thermally storing energy (Ortiz et al., 2010). Latent heat thermal energy storage such as that provided by PCM is a particularly attractive technique in buildings because it provides:

1. Temperature regulation 2. Energy conservation

3. Peak-load shifting to off-peak rate times

Within the literature reviewed, temperature regulation is the chief indicator of PCM performance followed by electricity consumption and then peak-load shifting.

The electricity consumption of a test space is recorded or predicted over regular time intervals to study the performance of PCMs in the space (Castell et al., 2010a). It may be recorded in addition to surface temperature of a building fabric (Khalifa and Abbas, 2009) or air temperature of the test space (Zhou et al., 2007).

Peak-load shifting also involves the collection of electricity consumption data, then using it to examine how PCMs can shift the peak consumption to off-peak times (Zhang et al., 2005; Atul Sharma, 2009; Diaconu and Cruceru, 2010). For off-peak electricity storage, PCM can be melted and solidified to store energy in the form of latent heat thermal energy and the energy is then available at a suitable time. So, if latent heat thermal energy storage systems are coupled with the active systems, it will help in reducing the peak load and thus electricity costs can be reduced by keeping the demand nearly constant, or shifting the peak to cheaper tariffs. This is only beneficial in countries that have varying electricity tariffs over the course of a day. Nigerian electricity tariffs are fixed for the different classes of users (NERC, 2013). Therefore, peak-load reduction is limited as an indicator of PCM performance in Nigeria.

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35

Like many other technologies, the success of PCM systems depends more in the way of use than in the PCM product itself. PCMs are thus evaluated in literature under two general classifications:

 Properties

 Applications

Both factors are heavily dependent upon effective melting and solidification of the PCM (Kuznik et al., 2008a).

Thirty-four studies have been reviewed in this chapter as shown in Table 2-1. Table 2-2 shows the variables have been studied and ranked according to how frequently the variables have been studied. The variables studied and their classifications are shown in Figure 2-2.

1 Ahmet, 2005 18 Kuznik and Virgone 2009b

2 Alawadhi 2008 19 Kuznik et al., 2011

3 Atul Sharma, 2009 20 Li et al., 2009

4 Cabeza et al., 2007 21 Mehling et al., 2008a 5 Castell et al., 2010a 22 Pasupathy et al. 2008a 6 Castello et al. 2008 23 Susman et al., 2010a 7 Castellón et al., 2010 24 Tay et al. 2012

8 Chen et al. 2008 25 Voelker et al., 2008

9 Darkwa and O’Callaghan 2006 26 Xiao et al. 2009 10 de Gracia et al., 2012 27 Xu et al., 2005 11 Diaconu and Cruceru 2010 28 Zalba et al., 2003

12 Farid et al., 2004 29 Zalba et al. 2004b

13 Gunther et al., 2007 30 Zhang, 2004

14 Isa et al., 2010 31 Zhang et al. 2005

15 Kelly, 2010 32 Zhang et al. 2006

16 Khalifa and Abbas, 2009 33 Zhou et al. 2007 17 Kuznik and Virgone, 2009a 34 Zhu et al., 2010 Table 2-1 Studies reviewed

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36

Properties Applications Applications

Variable Frequ ency

Studies Variable Frequency Studies Variable Frequ

ency

11. Analytic calculations 5 13, 31, 10, 27, 32, 22 20. Electricity consumption

Table 2-2 Authors of PCM studies and variable ranking

2-Literature review: Phase change materials as energy conservation mechanisms in buildings

37 Figure 2-2 Classification of variables studied in literature

PCM

2-Literature review: Phase change materials as energy conservation mechanisms in buildings

38 2.2.3 Climate

PCM performance has been shown to be dependent on transition temperature. The transition temperature itself is dependent on average room temperature and

subsequently on climate. In naturally ventilated buildings especially, effective melting and solidification is significantly affected by climate. Considering this, there is a conspicuous gap in literature covering the effect of climate specifically, on PCM performance across different climatic zones.

Location Climate Reference

Lleida, Spain Humid subtropical

Cabeza et al., 2007 and de Gracia et al., 2012 Germany Temperate Voelker et al., 2008

London Temperate Susman et al., 2010a Malaysia Hot and humid Isa et al., 2010 Beijing Humid

continental

Xiao et al. 2009, Xu et al., 2005, Zhang et al.

2005, Zhang et al. 2006, Zhou et al. 2007 Lyons, France Mediterranean Kuznik and Virgone, 2008b and 2011 Chennai- India Tropical wet and

dry

Pasupathy et al. 2008a Algeria Continental

temperate

Diaconu and Cruceru 2010 Table 2-3 Studies that examined the effect of climate

Within the studies reviewed, only a fourth is based on a climatic context. The climates are shown in Table 2-3. It is apparent that the eight climates covered are not

representative of the climates found in other parts of the world. None of the climates studied fall within a composite of hot and humid and hot and dry as experienced in a large swathe of Nigeria. None of the studies cover West Africa and the only hot and humid climate studied is a preliminary one (Isa et al., 2010) in Malaysia which is yet to publish measured or predicted results. On-going research is being conducted to evaluate the performance of copper foam enhanced PCM building fabric of a model of terrace houses in hot and humid Malaysia in a climatic laboratory chamber (Isa et al., 2010).

Diaconu and Cruceru (2010) study the influence of different parameters and system variables to establish a PCM system in Algeria, a continental temperate climate which – complemented with passive strategies (solar gains, natural ventilation) – reduces the

2-Literature review: Phase change materials as energy conservation mechanisms in buildings

39

thermal comfort related energy consumption in buildings. The system variables include transition temperature, air velocity and location in the building. In their investigation, PCM impregnated wallboards are combined in different configurations with insulation within the fabric of the building. They proposed a new type of PCM composite wall system for year-round thermal energy management; the novelty being that the two different PCMs have different values of the thermo-physical properties. Numerical test were carried out and results achieved are peak cooling/heating load reduction of 35.4%;

reduction of the total cooling load was 1%. However, it is expected that once the parameters of the wall system are adjusted, higher value of annual energy savings for cooling can be achieved; annual energy savings for heating is 12.8%. It should be noted that this study indicates the possibility of combining PCMs to achieve required thermo-physical properties.

The climate based thermo-physical properties of PCM in Nigeria such as transition temperature and thickness therefore remain unstudied as far as the literature studied are concerned.