Solar-driven Refrigerator for off-grid Regions

Refrigeration systems are necessary for people living in hot climates. A majority of tropical and subtropical countries uses electrical power as a source of cooling. During the seasons of high ambient temperature there is a significant cooling load due to increased level of energy consumption. Cooling systems are therefore necessary in African countries in order to keep medications and food in safe conditions. Furthermore, there is a power shortage crisis due to the high demand for cooling. TRNSYS software allows to simulate a complete solar-powered absorption cooling system. A model used in an experiment includes PV modules making it advantageous over a conventional cooling system. PV modules of assumed area are sufficient to maintain the temperature inside cooling device below 6°C over the whole year.


Solar System s for domestic cooling meet the increasing demand for air conditioning
Energy consumed for do mestic cooling and airconditioning constitutes significant share in energy demand of the build ings. Since the buildings are responsible for approximately 40% of the global energy consumption [1], situation on the energy market is noticeably affected by requirement for cooling and air-conditioning [1]. Studies have shown a rapid increase in the air-conditioning market around the world and predict further development. Additionally, capability of passive cooling has been restrained by global warming [2]. Those factors contribute to increasing amount of energy consumed for the purpose of cooling and air-conditioning world wide. In order to meet progressively more restrictive environmental goals established among the international co mmunity, i.e. reduction of primary energy consumption and greenhouse gases emission, a wide range of systems for cooling and airconditioning based on renewable or waste energy has been proposed. One of the most suitable option among other renewable energy sources is solar energy, as top airconditioning demand overlaps with the highest solar irradiance [3].
Co mpared to cooling systems powered solely by the public power grid, solar-driven systems are seen as mo re environment-friendly alternative [4]. These are ab le to utilize solar energy and therefore to lo wer the primary energy consumption. It is estimated that deployment of solar cooling systems in southern European and Mediterranean regions could lead to 40-50% savings of primary energy [5]. Introduction of solar cooling systems also allo ws to avoid some drawbacks of the regular g rid-powered vapour compression-based machines. Classic VC machines significantly burden the grid over the periods of greatest cooling demand, whereas solar cooling systems diminish grid peak load [1,6]. That feature is considered as convenient particularly during summers when the largest demand for cooling occurs [7]. Such a coincidence between cooling demand and availability of solar energy is the reason why solar cooling is a raising technology, arguably with greater potential than solar domestic heating [8].
Several studies indicate that deployment of solar cooling could be particularly suitable in tropical or subtropical areas [5]. Solar-based cooling systems are identified as pro mising technology in some reg ions of Asia, especially in the countries geographically aligned with equator as they take advantage of abundant solar energy [9].

Development of mul tifunctional solar cooling systems
It is reported that solar cooling systems are almost always mu ltifunctional and are used not only for cooling but also for supplying hot water and do mestic heating off-season [5,10]. That feature makes solar cooling more pro fitable [1]. Nu mber of reported solar cooling system installed worldwide is around 1200 (as for 2014) and is expected to increase due to quick development of solar cooling technology [1]. Taking into account that usually solar cooling systems are also heating systems, fro m now on they will be referred to as solar heating and cooling systems (SHC).
Given that mu ltiple solar technologies are available, SHC systems differ in utilised method of harvesting solar energy which may be obtained with photovoltaic solar panels or with solar thermal collectors [7]. Further classification o f SHC systems is based on distinction if the cooling effect is provided with electrically-driven or thermally-driven process [1,7]. Electrically-driven cooling is usually achieved by utilising vapour compression cycle and/or heat pumps [7]. Thermally-driven cooling technologies include absorption and adsorption chillers, ejector cycles as well as solid/liquid desiccant cycles [1, 5, 7 11]. Thermally-driven cooling requires heat source and thus could be realised either with PV solar panels or solar thermal collectors. As the main interest of the art icle is absorption cooling technology, it will be discussed more specifically to the extent necessary later on. Amongst listed SHC layouts the most popular one includes thermally-d riven chiller and solar thermal collector combined with a thermal storage system [7].
The main obstacle restraining development of SHC is high capital cost of those installations. If reasonable economic profitability is to be achieved, high capital cost must be balanced with low operating cost [7]. The payback period depends on a wide range of factors such as utilised components, geographic conditions and government subsidies [10]. As mentioned earlier, profitability could be increased by designing multifunctional SHC systems. The payback period of all types of solar cooling systems falls within the range of 7.6-11.4 years [1], while in case of coupling solar cooling with heating (SHC), payback period is evaluated to be in the range of 3-15 years [10]. PV-based SHC systems face additional barrier restrain ing their development, e.g. high price of battery storage required for continuous operation of the system over the periods of scarce solar energy. On the other hand, capital cost of photovoltaic solar panels is decreasing and that trend contributes to more favourable financial metrics [1].
Cooling with absorption chillers is well-established technology that has been widely used in air-conditioning but up till now absorption cycles have been driven main ly with natural gas or industrial waste heat [1]. Studies indicate that currently large-scale SHC absorption chillers are not competitive co mpared to conventional cooling, leaving aside government subsidies [1,9]. That applies especially to the buildings with limited area exposed to solar radiat ion. Nevertheless, SHC absorption chillers are expected to become a pro mising solution in addressing environmental issues [9] but special attention must be paid to improving their economical performance [1]. A mong various SHC systems, absorption chillers technology is the most developed commercially [5] and therefore considered to compete with conventional air-conditioning in foreseeable future [1].

Solar absorption chillers
Most common SHC systems shall consist of absorption chiller co mbined with solar thermal co llectors and thermal storage system [7] Fo r reference, about 59% of SHC in Europe were based on absorption cooling technology in 2007 [5]. In research particular attention has been paid to the systems based on single-effect chillers coupled with low-temperature solar thermal collectors [1] and such attention corresponds to the most popular layout of solar absorption chillers worldwide [10]. Subject of using high-temperature solar thermal collectors and mult ieffect chillers in SHC gained less attention [1] and generally those systems are less widespread among the world. Fro m that overview the fo llowing picture of the regular SHC system emerges: such a system is based on thermally-driven single -effect absorption cooling cycle and consists of four basic co mponents, namely an absorption chiller co mbined with a (lo wtemperature) solar thermal co llector, a storage tank and an auxiliary heater. That typical system is usually mu ltifunctional as it serves also for do mestic heating and/or hot water production.
In co mparison to conventional vapour compression chillers, absorption chiller is a device featuring such advantages as: quiet and low-vibrat ion operation, little maintenance and environmentalfriendly functioning [1,11]. Additionally, device realises cycle in which pressure of mediu m in liquid state is to be increased and that could be done economically with a pu mp as opposed to a situation occurring in vapour co mpression cycle when a compressor must be applied to increase pressure of med iu m in gaseous state. Therefore absorption chiller requires little mechanical energy input [1]. In comparison to other SHC systems, absorption chillers achieve higher COP than adsorption chillers [7].

Solar absorption chillers operating on NH3-H 2 O mixture
Performance of absorption chiller (AC) depends on properties of operating mixture consisting of refrigerant and absorbent. Two fluids must comply with a wide range of requirements. Their liquid phases must be miscib le in all temperatures reached in cooling cycle [1]. In such a range of temperature, flu ids and their mixture should be chemically stable, non-toxic and non-explosive. [1,12]. Another favourable property is possibly large difference between the boiling points of mixture and pure refrigerant [12]. Otherwise applying dephlegmator may be needed [1]. High heat of vaporization and capacity to reach high concentration within the absorbent is also considered as advantageous refrigerant property since it allows to realise cooling process with low circulat ion rate [12]. While reaching high concentration of refrigerant in absorbent is beneficial, increasing ammonia in itial concentration in mixture could decrease mass transfer rate and even stop the absorption process for ammonia init ial concentration of about 60% [8]. Non-corrosive, inexpensive and safe for environ ment fluid pairs are preferred [12]. Refrigerant should be volatile in order to enable easy separation in generator [1], wh ile absorbent ought to be non-volatile so rectification of absorbent will not be necessary after desorption [13].
Most prevalently used refrigerant-absorbent pairs are H 2 O-LiBr and NH 3 -H 2 O [1]. As ammonia freezing point in considered range of pressure is equal to -77°C, ACs utilising NH 3 -H 2 O are ab le to p roduce cooling temperature below 0°C [12] but they need higher temperatures in generators than H 2 O-LiBr-based machines [1]. They also achieve lower COP [1] in a range of 0.5-0.6 [6], while corresponding values for H 2 O-LiBr are 0.7-0.8. Due to their capability to achieve temperatures below 0°C, NH 3

-H 2 O A Cs are p referred for cooling, while H 2 O-LiBr
ACs for air-conditioning [1]. H 2 O-LiBr displays some corrosive properties thus addition of anti-corrosives is required [6].
Despite slightly lower COP and higher driving temperature required in case of NH 3 -H 2 O A Cs, they are free of some construction constraints present for H 2 O-LiBr A Cs. The second fluid pair cannot operate in temperatures lower than 0°C due to water being refrigerant which freezes in such temperature but also temperature of 40°C cannot be exceeded because the possibility of LiBr crystallisation fro m mixtu re occurs as the saturation point is achieved [3]. Crystallisation could also occur when solution rich in LiBr salt has high concentration. Crystallisation may lead to blocking of the pipes and valves [1] therefore operational temperature range is restrained and that could limit utilisation of H 2 O-LiBr A Cs in extreme weather conditions [1]. When it co mes to physical size of the system, H 2 O-LiBr A Cs are generally larger than their NH 3 -H 2 O counterparts, because vapour volume of refrigerant is significantly larger [6]. Since the difference between the boiling temperature of ammon ia and water is not very h igh, NH 3 -H 2 O absorption chillers also need a dephlegmator to separate ammonia and water vapour inside the generator [1].

Multiple-effect cycle s for solar absorption chillers
Another critical parameter affecting perfo rmance of ACs is number of times heat has been recycled before comp leting the loop. Single -, double-and triple -effect (alternatively: single-, double-, triple -stage) ACs could be listed as commercially available [1]. Nu mber of effects corresponds to number of generators in which heat flow is harvested to realise the process of desorption. Higher COP could be achieved with increasing number of cycles but that operation requires higher temperatures for every further cycle [1].
It is reported that in reg ions characterised by low solar resources, solar mu lti-effect ch illers are relatively inefficient thus single-effect is preferred choice [1]. Singleeffect ACs' COP is limited to around 0.7 [13] and they require driv ing temperature of 80-100°C (LiBr) [13]. In order to raise COP of single -effect ACs, solution heat exchangers are applied for preheating solution entering the generator with heat received fro m water returning to absorber [1].
Double-effect ACs ut ilise additional generator and operate between different pressure levels. They achieve higher COPs (up to 1.4) but require higher driving temperature [1]. Starting with double-stage chillers, generators could be connected in different ways through series, parallel and reverse-series flo w cycles [1] displaying non-obvious dependence between layout structure and performance of a ch iller [1]. Currently all co mmercially available double-effect A Cs operate on H 2 O-LiBr cycle since NH 3 -H 2 O requires higher pressure.
Studies have shown that variable effect (1.n-effect) absorption cycles are under development. Such systems allow vapour to undergo either single-or double-effect cycle and rat io of both vapour volumes subjected to those cycles is regulated by generators temperatures [10]. As no specific co mponents differ 1.n-effect cycle fro m single-effect cycle, [10] it could easily extend a range of ACs utilisation.
Three-effect configuration rarely occurs because driving temperature must be higher than in case of single-and double-effect ACs. There are some hybrid configurations in which single-effect cycle powered with solar thermal energy is used accompanied by double-stage gas-fired cycle. Those different methods could work independently or simultaneously [10].

Energy sources
The analyses were undertaken for Ouagadougou which is the capital of Burkina Faso. Bu rkina Faso is an African country classified as one of the Third World Countries. Its electrificat ion rate is estimated to be around 20%, meaning that more than 12.  become inexpensive and environment-friendly source of cooling for storage facilit ies. This paper describes an offgrid cold storage system wh ich is able to keep med ications cool for a long time. This installation may be cut off fro m external electrical network and powered with local s ources of renewable energy.

Fig. 2. Schematic diagram of a solar-powered absorption refrigerator system
The main elements of cooling system are shown in Fig. 2 as following: PV modules, absorption refrigerator, two heat exchangers e.g. evaporator and condenser, battery storage, electrical heating element and the control system. The incident solar radiat ion is converted to electric current by three PV modules of co mbined power equal to 480 W. After regulating its electrical parameters the current is supplied to connected 12V battery with capacity of 200 Ah and to electrical heating element. Control system switches power supply to heating element if the temperature of working mixture (NH 3 -H 2 O) is lower than 80°C or to the battery if this temperature is achieved. Ammon ia absorption refrigerator produces cooling effect in order to maintain the low temperature in analysed 90d m 3 tank by boiling working mixtu re and forcing its circu lation. Condensation, the subsequent step in the cooling process, is realised by rejection o f latent heat to the ambient. The refrigerant enters the evaporator as a low-quality saturated mixture, and it completely evaporates by absorbing heat from the refrigerated space. All the flows in refrigerator are based on convection, caused by changes of density. Therefore in presented system electrical heating element is the only device consuming considerable amount of energy. COP of the analysed installation was calcu lated as a 0.55 wh ich value is much lo wer than COP of systems based on vapour compression which is at the level of 3.5. There is a risk of ammon ia leakage which is toxic to people but it is minimized by tight seal of the system.

System simulation
Some crit ical parameters of the system were summarized in Table 1.  Figure 3 shows solar panels' energy output in following time steps, depending on irradiat ion of the area. Simu lation shows that the average annual energy production ensures work of solar refrigerator system cut off from external electrical network. The results were obtained through TRNSYS simu lation wh ich was run with 6 minutes time step. The analysed system has successfully maintained storage temperature below the point of 6°C, even while the amb ient temperature was over 40°C during the peak of summer season (Fig. 4).