The increasing amount of energy consumption by buildings has caused widespread global attention to the social, environmental, and economic implications associated with it. Research has shown that the building sector is responsible for 32% of the world’s total primary energy consumption [
1] and nearly 34% of direct greenhouse gas (GHG) emissions globally [
2]. In Europe, 39% of the total primary energy is consumed by buildings, among which 26% is for residential houses and 13% for commercial architectures [
3]. In China, the building industry accounts for 25–30% of the total national primary energy [
4], while in the USA buildings represent 40% of the total national energy consumption and 40% of CO
2 emissions [
5]. A similar situation happens in Australia, where the building industry consumes 40% of the national electric energy and contributes to 27% of the GHG emissions [
6]. Commercial buildings in particular consume approximately 61% total building energy consumption and contribute one third of total building GHG emissions in Australia. Additionally, the heating, ventilation, and air conditioning (HVAC) system installed in buildings is the largest energy consumption contributor, accounting for 68%, followed by 19% for lighting and 13% for others [
6].
Australia has a variety of climatic zones and is currently facing the challenge of dramatic peak electricity demand due to the high penetration rate of residential and commercial HVAC systems. Therefore, developing innovative HVAC technology towards sustainability is vitally crucial for Australia to decrease the nation’s electricity energy consumption and GHG emissions. Fortunately, the abundant solar energy resource in Australia makes solar cooling available [
7]. Because peak electricity demand due to wide use of air conditioning matches peak solar irradiance, it is feasible to assume that solar air conditioning technology would be highly desirable in Australia as a means to reduce peak demand, energy consumption and GHG emissions. In addition, solar air conditioning has been widely believed as an appealing alternative for traditional HVAC systems in the world because of its energy efficient, inexhaustible, and eco-friendly features [
8].
Therefore, this study aims at investigating the energy savings potential of different solar-assisted cooling systems for a typical office building in different Australian climates and assessing their economic feasibility. Specifically, this paper will compare the performance of solar desiccant-evaporative cooling (SDEC), combined solar desiccant-compression cooling (SDCC), and solar absorption cooling (SAC), with a referenced conventional vapor compression variable-air-volume (VAV) system, in terms of the technical, environmental, and economic aspects. This study will cover all Australian capital cities, including Adelaide, Brisbane, Canberra, Darwin, Hobart, Melbourne, Perth, and Sydney. The purpose of this investigation is to identify whether solar-assisted air conditioning systems are technically, environmentally and economically feasible for Australian commercial buildings.
1.1. Solar Energy in Australia
The solar energy resource in Australia is abundant. It is reported that the average solar radiation collected in Australia is about 58 million petajoules (PJ) per year, which is almost ten thousand times the nation’s annual energy consumption [
9].
Figure 1 shows the annual mean daily solar irradiation in Australia [
7]. It demonstrates that Western Australia, Northern Territory, and northern Queensland areas have excellent solar energy resources, with more than 22 MJ/m
2 per day. South Australia, southeast Queensland, and New South Wales have good solar energy potentials with about 19 MJ/m
2 per day, while Victoria, the Australian Capital Territory, and Tasmania have comparatively lower solar energy resources, with just below 16 MJ/m
2 per day.
There are three main methods to harness solar energy: active solar applications, passive solar strategies, and electricity generation through solar engines [
9]. Active solar technology uses solar collectors to convert sunlight into useful thermal heat actively [
10], which is normally used for domestic water heating, space heating and cooling. This technology is quite prevalent across Australia due to the merits of low running cost and government subsidies [
9]. Passive solar technology is more about improving the passive efficiency of buildings, such as optimizing the building design in terms of building envelope, building systems and building orientation [
10] in order to control the impact of solar radiation on the internal temperature of the building. In relation to electricity generation, solar thermal and solar photovoltaics (PV) are the technologies generally used for electricity production [
9].
Although Australia has rich available solar energy resources, the solar energy utilisation in Australia is still on a small scale. It was estimated that solar energy only accounted for 0.1% of Australia’s total primary energy depletion during 2007–2008 [
7] and 2.4% of all renewable energy use [
9]. However, solar energy has become increasingly popular in Australia recently for both electricity production and direct-use applications. According to [
6], there were 704,459 solar hot water systems installed around Australia in 2011, as well as many other low-temperature solar thermal applications such as solar ponds, solar air heating and solar air conditioning. The Australian PV Institute reported that since 2011, the solar PV installations in Australia have increased dramatically, reaching 1.7 million PV installations with a combined capacity of 6.2 gigawatts in 2017 [
11]. In addition, the Australian Energy Statistics 2016 reported that for 2014–2015, solar PV accounted for 21.5 PJ energy consumption compared with solar hot water of 14.8 PJ [
12]. It is believed that with the development of solar panels and thermal storage technologies, as well as government financial support, the cost of solar technology will reduce significantly and thus, solar energy utilisation in Australia will become more advantageous in the future.
1.2. Solar Air Conditioning Technology Review
Due to its environmentally friendly and energy efficient benefits, solar cooling has been widely recognised as a promising substitution for traditional air conditioning [
8]. Solar air conditioning is a technology which converts solar energy into useful cooling or air conditioning for buildings. According to [
13], solar cooling is divided into two broad groups: solar thermal cooling and solar electric cooling. Solar thermal cooling uses solar collectors to provide heat to drive a cooling process, which usually combines with thermally driven absorption or adsorption chillers. Solar electric cooling uses photovoltaics to generate electricity to drive classical motor driven vapour compression chillers. Nowadays, solar cooling applications have globally penetrated the world market in the USA, Europe, Japan, and China, with approximately 1000 solar cooling system installations [
14]. Baniyounes et al. [
6] indicates that solar absorption cooling systems are the most adopted solar thermal cooling technology in the global market, accounting for 70% of total installed solar thermal cooling systems. This is followed by solid solar desiccant cooling systems at 14%, solar adsorption cooling systems at 13%, liquid solar desiccant cooling systems at 2%, and others at 1%, which makes up the total market share percentage as is shown in
Figure 2 below.
In the last several decades, solar-assisted cooling technology has widely been evaluated worldwide, including solar electric cooling powered by PV [
15,
16,
17], solar absorption cooling [
18,
19,
20,
21,
22,
23], solar adsorption cooling [
24,
25], and solar desiccant cooling [
26,
27,
28,
29,
30,
31,
32,
33,
34,
35]. A theoretical modelling with experimental validation studied by Nie et al. [
36] demonstrated that the solid desiccant cooling assisted by heat pump was more efficient than the conventional cooling system due to high efficient dehumidification capacity. These research results have also indicated that based on different solar cooling technologies and different climates, the energy savings could be 25% to 90% compared with the traditional HVAC system. In addition, there are also a number of comparative studies on the performances within various solar cooling systems, which include the comparison of solar absorption cooling with solar electric cooling [
37,
38,
39,
40], solar desiccant cooling with solar absorption cooling [
41], and hybrid solar desiccant cooling with other solar cooling systems [
42,
43,
44,
45]. Gagliano et al. [
46] reported that the hybrid solar desiccant integrated vapour compression cooling system could achieve 40% primary energy savings compared to the solar absorption cooling, and 150% savings respect to the conventional vapour compression cooling system. Khan et al. [
47] found out that based on various collector areas, for Chennai city, the solar desiccant-assisted Dedicated Outdoor Air System (DOAS) integrated radiant cooling system could achieve 7.4% to 28.6% energy savings in comparison with the cooling coil-assisted DOAS radiant cooling system.
The comparison results between different solar cooling systems have shown that overall the PV-integrated solar cooling system has higher solar fraction and lower primary energy consumption than the solar thermal absorption cooling system. If considering the excess electricity generation by PV, the grid-connected solar PV cooling system outperforms the solar thermal absorption cooling system from both energy and economic respects.
In Australia, the solar air conditioning technology research and development is still in the early stage. Baniyounes et al. [
48] used the TRNSYS software to study the potential of solar absorption cooling for an office building under three subtropical climates in Australia. They indicated that by implementing 50 m
2 solar collectors and 1.8 m
3 hot water storage tank, the energy consumption of the solar absorption cooling system was only 20% of the conventional HVAC system. Alizadeh [
49] conducted a feasibility study of a solar liquid desiccant air-conditioner (LDAC) for a commercial building in Queensland, Australia. The author found that by using LDAC, the operating costs could be decreased significantly in comparison with the equivalent gas-fired conventional cooling system, and the payback period was only five years. Goldsworthy and White [
50] optimized a solar desiccant cooling system in Newcastle, Australia. They found that the system electric coefficient of performance (
COP) could be above 20 if the desiccant wheel regeneration temperature was 70 °C with the 0.67 process-to-regeneration air flow ratio and 0.3 indirect evaporative cooler secondary-to-primary air flow ratio. In their another study [
51], they found out that the frequency of high indoor temperature hours in Melbourne and Sydney could be reduced by improving the effectiveness of the indirect evaporative cooler, decreasing the regeneration temperature of the desiccant wheel, and increasing the solar collector areas. However, because of the high temperature and humidity ratio of the outdoor air, this effect was not dramatic in Darwin. Baniyounes et al. [
41] compared the performance of solar desiccant evaporative cooling with solar absorption cooling for a school building in Gladstone and Rockhampton based on a TRNSYS simulation. They indicated that increasing solar collector areas would result in improved system
COP and reduced energy consumption for both solar cooling systems. In addition, the solar desiccant evaporative cooling system had higher
COP and solar fraction (
SF) than the solar absorption cooling system. Kohlenbach and Dennis [
52] conducted a comparative study between a solar PV air conditioning system and a solar thermal absorption cooling system with a referenced conventional vapor compression cooling system from both economic and environmental aspects for a commercial building in Brisbane and Sydney. The financial parameters were assumed as 2.5% inflation rate, 8% discount rate, 20 years system lifetime, and 0.17
$/kWh electricity cost. They concluded that the solar absorption cooling system had a lower lifetime cost than the solar PV cooling system though they were both higher than the conventional cooling system. In addition, the solar thermal absorption cooling system was more economic until the electricity price exceeded 0.50
$/kWh, while the PV-based cooling system was more economic when the electricity price exceeded 0.55
$/kWh. In addition, the PV-based system resulted in the lowest GHG emissions due to the excess power generation over the lifetime.
From the above survey, it can be seen that the solar desiccant cooling technology is an appealing alternative to the conventional cooling system for the merits of low driving temperature, high
COP and relatively short payback period characteristics. Solar absorption cooling is another popular alternative, with a relatively low driving temperature and the potential for large energy conservation. However, the life cycle cost of the solar absorption cooling system is relatively high. In addition, the solar electric cooling technology has the largest energy savings potential but at the same time has high life cycle cost. Although there is some research about solar cooling in Australia, little studies have been evaluated on the comparison between different solar-assisted cooling systems under all Australian capital cities. Additionally, there is no comprehensive study on the feasibility of different solar-assisted cooling systems from the technical, environmental and economic aspects. Therefore, this paper will lead to the investigation and comparison of different solar-assisted cooling systems for all eight Australian capital cities. The results from this study are expected to contribute to the fulfilment of the Australian Government targets of 5% and 80% CO
2 emissions reduction on 2000 levels by 2020 and 2050 [
53].