Residential air conditioning global energy challenge
Due to the world population growth and the economic development, especially in developing countries, the energy consumption is increasing at alarming rates. Figure 1 shows the world primary energy demand categorized by fuel and the corresponding CO2 emissions for three different scenarios established by the International Energy Agency: a Current Policies scenario, in which the policies applied until now remain unchanged; a Stated Policies scenario, in which the proposed policies by the different countries are implemented; and a Sustainable Development scenario, which is an ideal scenario in which the sustainable energy goals are met. According to this figure, the increase in the global energy demand between 2000 and 2040 is expected to be around 91% in the Current Policies scenario (from 10037 to 19177 Mtoe). Moreover, from the total energy consumed in 2040, 78% will be coming from fossil fuels (whose combustion generates pollution and causes global warming). This makes practically no change if compared with the share of 80% that it represents today (IEA, 2019).
The building sector is the most energy demanding sector with 35% of the final energy consumption (IEA, 2019). In this sector, the need for space cooling represents nowadays 20% of the total electricity used, and this demand is growing faster than any other end use in buildings, reaching a share of 30% by 2050 (IEA, 2018a). Figure 2 shows that the final energy consumption for space cooling in the building sector has more than tripled in 26 years (from 1990 to 2016), and it is expected to triple again by 2050, with around 70% of this increase coming from the residential sector, mostly in emerging economies (IEA, 2018a).
Nearly all the energy consumed for space cooling (Figure 2) is in the form of electricity to drive vapor compression chillers. Only around 1% is in the form of natural gas to drive thermally driven absorption chillers in commercial buildings (IEA, 2018a). This dependence on electricity has generated a series of problems and challenges for the environment and the electricity distribution grid in hot-climate countries (Casals, 2006). Indeed, in some regions of the world, this cooling demand reaches over 70% of the peak residential electrical demand during hot days (IEA, 2018a).
The Mexican context
Mexico is an important player in the increasing demand for cooling. Its energy consumption in buildings for cooling has increased by more than 5-fold times from 1990 to 2016 (from 7 to 37 TWh), representing nowadays 9.8% of the total energy consumed in buildings, which is the second highest share in a country, just after the United States (IEA, 2018a). Moreover, this energy consumption is expected to increase by 4-fold times again by 2050 (IEA, 2018a).
Mexico is also the second country in the world with the highest share of electricity used for space cooling with a 14% already today. Even if the policies and targets announced by the government until now are implemented, this share is expected to increase to more than 23% by 2050, which entails that cooling will account for more than 25% of the increase in total CO2 emissions (the highest share of any country) (IEA, 2018a). Indeed, the need for cooling of Mexico, measured in cooling degree days (CDDs), is expected to increase of 36.8% from 2016 to 2050, which is the largest increase of any country in the world (IEA, 2018a).
Energetically and economically, Mexico is dependent on fossil fuels. In 2013, the oil sector generated 13% of the country’s export earnings and 33% of the government’s income (Alpizar-Castro and Rodríguez-Monroy, 2016). The instability in oil prices and its constant drop in recent years has generated economic problems and concern in the country. Until now, the increasing demand for energy is mainly covered by fossil fuels. In 2014, 93% of the total energy consumption came from oil (45%), natural gas (40%), and coal (8%) (Pérez-Denicia et al., 2017).
In Mexico, the recently approved energy reform has shown a lack of expertise to face the energetic challenges of the coming years. If no change in the Mexican energy policy is implemented, the dramatically increasing numbers regarding the energy demand for cooling and its dependence on fossil fuels forecast an economic and energy crisis in the coming decades.
Mexico has to operate a change in its energy policy. Among other options, efforts must be put on the implementation of systems that make use of renewable energy, such as solar energy, that are until now unexploited (Alpizar-Castro and Rodríguez-Monroy, 2016). Actions of this type would not only be useful to equilibrate the country energy flows and monetary incomes, but are also in line with the different international efforts to protect the ozone layer and reduce the impact of climate change such as the Montreal Protocol, the Kyoto Protocol, and the Paris Agreement.
Solar absorption chillers
The dependence on fossil fuels, the high cost of electricity, and global warming are some of the main obstacles to cover the increasing energy demands in the building sector. The use of clean and sustainable energy sources is the most promising alternative to generate clean energy without leaving CO2 footprints (Bellos et al., 2016). In this context, solar energy is one of the most important energy resources that can be used for air conditioning since it is abundant, widespread, endless, and clean (Bellos et al., 2016; Jafari and Poshtiri, 2017). In this context, Mexico possesses the advantage of being part of the sunbelt countries (countries between the 20th and 40th degrees of latitude in the northern and southern hemisphere), which are hot countries with high irradiation levels that can be used to drive solar thermal chillers.
Absorption chillers are thermally driven systems that replace the mechanical compression of standard refrigeration systems by a thermochemical compression. Different categories of thermodynamic cycle arrangement exist for absorption chillers. However, the single-stage configuration is the most advantageous for solar thermal cooling applications in the residential building sector in terms of cost, low driving temperatures, size, and simple configuration (Hassan and Mohamad, 2012). Even though there exist other thermally driven chiller technologies such as adsorption, desiccant, or ejector systems, the main advantage of absorption cooling systems is their higher coefficient of performance (COP) (Best and Rivera, 2015; Hassan and Mohamad, 2012). Moreover, its coupling with solar thermal energy is a very promising alternative, as the irradiation level is higher when cooling is most needed (Dube et al., 2017; Kim and Ferreira, 2008a). In addition, the solar thermal panels can also be used for space heating or domestic hot water during winter (IEA, 2018a).
The suitability of absorption chillers relies on several economic, political, industrial, and geographical conditions (Herold et al., 2016). In spite of their different advantages such as the use of renewable energies, low operating/maintenance costs, good thermal performance, and reduced noise; they are to date a niche technology. From the total energy consumed for space cooling (Figure 2), 99% comes in the form of electricity. The remaining 1% is natural gas mainly used to drive H2O-LiBr absorption chillers in the Far East (China, Japan, and South Korea), where regulatory incentives exist for gas-fired cooling (Herold et al., 2016). Regarding solar cooling, the niche market has experienced a slow but steady increase passing from 40 installed units in 2004 to 1,350 by the end of 2015 (Figure 3), of which 80% were in Europe (Mugnier and Jakob, 2015; Weiss et al., 2017).
The main obstacles for absorption chillers to be deployed on a much larger scale are their high initial investment cost and their lower compactness if compared with conventional vapor compression systems (Altamirano et al., 2019a). This is especially true for small-capacity application (Figure 4), such as in the residential cooling sector. Indeed, as observed in Figure 4, the conventional vapor compression systems require a different compressor technology at capacities higher than 250 kW to deal with the larger required refrigerant vapor flow, which increases their specific costs and make them even slightly more expensive than absorption chillers. The market of absorption chillers, however, is expected to grow as the fabrication costs are reduced. These costs have already been decreased by more than half since 2007, mainly thanks to the standardization of the equipment, making these systems to gain acceptance and seem like true alternative for cooling residential, light commercial, and industrial applications (Zhu and Gu, 2010). However, to date, they are still not cost-competitive with electrical chillers (IEA, 2018a).
Owing to their potential to cover a great part of the rising energy demand for space cooling, R&D in solar thermal cooling systems is part of the technology milestones to achieve energy efficient buildings in the IEA Energy Technology Perspectives report (IEA, 2012). Moreover, these systems are included in the IEA SHC Strategic Plan Key Technologies (IEA, 2018b). Solar cooling has already been the subject of four IEA SHC Taks (No. 25, No., 38, No. 48, and No. 53)., and a new Task (Task 65) has been created in 2020 to adapt, verify, and promote solar cooling as an affordable and reliable solution to the increasing cooling demand across the sunbelt countries such as Mexico (IEA, 2020).
In order to get a bigger market share in the residential cooling sector, from the technological point of view, absorption chillers still need to reduce their production costs (through the simplification of the manufacturing processes), increase their compactness, and increase their efficiency (Altamirano et al., 2020b, 2019a; IEA, 2018a; Mugnier et al., 2017). On the other hand, from the political point of view, the stimulation of the market is required through governmental regulations (Herold et al., 2016). Regarding the R&D concern, different solution options have already been studied like the search of the perfect working pair (refrigerant/absorbent combination), new heat and mass transfer exchanger technologies, the cost reduction through the employment of new manufacturing methods, the appropriate heat sink source, and innovative machine architectures (Altamirano et al., 2020b, 2019a).
Review of theoretical, experimental, and commercial cycles
The present section aims to provide an up-to-date global overview of the small-capacity single-stage continuous absorption systems operating on binary working fluids for cooling from the theoretical, experimental, and commercial points of view. Special focus is placed on comparing the cycle performance between the different families of refrigerants and working pairs as the choice of working fluid directly affects the performance of an absorption cycle (Hassan and Mohamad, 2012; Perez-Blanco, 1984). The main objective is to provide useful information about the cycles performance and the challenges faced by researchers in the development of such systems. It is not in the scope of this work to compare the brands or configurations. The main objective is to provide helpful information for a good understanding of the development of these systems over the past 40 years and of the main advantages and disadvantages of using different working pairs.
The performance of absorption systems depends on different factors and can be measured according to different criteria (Pons et al., 2012). These factors depend on internal parameters such as the working pair, the exchanger technology, or the monitoring procedure; and external parameters, such as the specific climate conditions or the type of source (liquid, solid or gas), which directly impact the operating temperatures and thermal transfers in the system. Compared with conventional compression systems, which possess two temperature sources (the heat rejection temperature and the cooling temperature), absorption cooling systems have three temperature sources. In addition to the heat rejection and the cooling temperature, there is an additional high-temperature driving source. All three temperature sources are of great importance for the system’s performance. In the case of air-cooled systems, the ambient temperature is fundamental (since it cannot be modified). While the performance of conventional compression AC is generally measured in terms of their electric coefficient of performance (COPelect, defined as the cold production divided by the electricity consumption (Eq. (I.1))), the performance of absorption cooling systems is usually measured by a thermal coefficient of performance (COP) defined by Eq. (I.2). Some authors in the literature also use a combination of the two definitions, adding the electricity consumption (mainly consisting of the pumping power) to the energy consumption in the definition of the COP. Even though the pumping electricity consumption is considered negligible compared to the generator heat transfer rate, some studies have shown that it might be nonnegligible. Therefore, the COPelect has earned a relevant importance in some studies (Zamora, 2012; Zamora et al., 2014).
COPelect = Q̇e (I.1)
COP = Q̇e (I.2)
Apart from the COP, which can be optimized by means of the first law (conservation of energy), the second law (quality of energy and material) can be used to maximize the exergetic efficiency and minimize the entropy generation within the system (Alefeld and Radermacher, 1993). The exergetic efficiency represents the capacity of the system to avoid the destruction of available energy and is defined as the ratio between the recovered exergy at the evaporator and the provided exergy at the desorber (Kilic and Kaynakli, 2007). However, for the sake of the present review, special focus is given to the COP as the main parameter for comparing the different systems. Another important parameter is the Carnot COP (function of the system’s operating temperatures), which represents the maximum possible coefficient of performance of the absorption cooling system. This is defined by Eq. (I.3).
COP = ( tgi − tai ) ( teo ) (I.3)
The COPcarnot implies a reversible process with isothermal heat transfers at the three temperature levels, which ideally requires infinite energy sources. Therefore, for an ideal system, the temperatures from the external heat transfer fluids should be taken into account at the inlet of the generator ( ), at the outlet of the evaporator ( ), and at the inlet of the absorber and the condenser ( and ), or in the case of air-cooled systems, the ambient temperature ( ). In most experimental studies, the same source is used in parallel for the intermediate temperature, and in these cases, some authors use the term “intermediate temperature” or “heat sink temperature” ( = = = ). The nominal external temperatures data of absorption chillers is provided by the manufacturers in their catalogs. However, theoretical studies generally use the temperatures of the internal working pair or refrigerant at the exit of the components. On the other hand, prototypes in the literature provide information depending on the research objectives and the specific configuration and instrumentation implemented in the machine (internal or external temperatures located in different sections), making it difficult to standardize the data. Therefore, this information was carefully extracted with the best possible accuracy if the values were not directly given and when the provided information allowed to, adding some degree of inaccuracy, to present the data contained in the tables and figures of the present review. Hence, these data may be interpreted with care and the reader must refer to the sources if more detailed information is required.
Ammonia-based working fluids
The general advantages of NH3 over H2O as the refrigerant are its low freezing point (-77°C) (Wang et al., 2011) and the higher working pressures, which allow for negative cooling temperatures and eliminate the need for special and expensive equipment for vacuum conditions (Wu et al., 2014). The present section covers the theoretical, experimental, and commercial systems operating on NH3-based systems.
Conventional ammonia-based working pair: NH3-H2O
NH3-H2O is, together with H2O-LiBr, the most widely used working fluid in absorption systems (Herold et al., 2016; Taha Al-Zubaydi, 2011). It is commonly used for applications where negative refrigeration temperatures are required (Wu et al., 2014). Compared with H2O-LiBr, it allows for a large range of operation with no risk of crystallization. However, a vapor purification process is needed at the output of the generator to remove the remaining water that evaporates together with the refrigerant (Medrano et al., 2001; Srikhirin et al., 2001; Wu et al., 2014). This process is composed of a distillation column with partial or total condensation, a rectifier (externally cooled device to condense water vapor), or a combination of both (Bogart, 1981). The term “rectifier” is commonly used to refer to the vapor purification process in general. Indeed, a poor vapor purification would lead to a low system performance due to the presence of water in the evaporator (Bogart, 1982), reducing the COP by over 30% and requiring higher driving temperatures (Dardouch et al., 2018).
NH3-H2O: Theoretical studies
Some theoretical studies have focused on identifying the parameters that most impact the performance of absorption systems. Very high negatively impact the COP, in which case an increase in the improves it (Kaushik and Bhardwaj, 1982). On the other hand, at low , a decrease in the and improves the COP (Best et al., 1987; Bulgan, 1995), with having the highest impact, and thus < is more preferable (Kaushik and Bhardwaj, 1982).
Other studies have focused on analyzing the optimum operating conditions of these systems. In the case of negative refrigeration, Alvares and Trepp (1987) concluded that for a refrigeration temperature of -10°C, a single-stage system with a rectifier, SHX, and RHX could reach a maximum COP of 0.6021 at =120°C, Pe=2.85 bar, and a =25°C. Clerx and Trezek (1987) performed a computer-aided thermodynamic analysis and concluded that the evaporator pressure must be between 0.21 and 0.41 MPa to maintain negative evaporator temperatures, while the generator pressure must be between 1.38 MPa (to force a of over 37.8°C and use air-cooled condensers) and 2.04 MPa (for a lower maintenance and safety requirements), the between 82.2°C (to maintain an acceptable COP over 0.2) and 204°C (due to the maximum pressure determined before and also to avoid excessive water evaporation), and the preheater, SHX, and rectifier must have at least an effectiveness of 70% to maintain a COP of no less than 0.5 for ice production.
In the case of positive cooling, Shiran et al. (1982) developed a program that evaluates the cooling and heating demands of buildings depending on the location and design the absorption cycle along with the solar collectors and auxiliaries. Other studies centered on observing the behavior of the system in local geographic conditions (Kouremenos et al., 1987) or in a specific working range (Butz and Stephan, 1989; Hammad and Habali, 2000). Special configurations have also been theoretically analyzed. Uppal et al. (1986) proposed a system with no moving parts for vaccine storage in remote locations, and Ahachad et al. (1992) proposed to substitute the rectifier column by a bubble exchanger fed with the strong solution coming out of the absorber (see Figure I.2a); the proposed system showed an improvement in the COP ranging from 15% to 35%.
Finally, optimization studies are also available in the literature. Bulgan (1995) observed that at fixed and , for each ammonia concentration level in the refrigerant and varying the , there is a local COPmax. A theoretical maximum COP of 0.888 could be achieved. Le Lostec et al. (2010) presented an optimization study for a system under steady-state conditions. Three optimum COP values were identified: a COP of 0.56 with a minimum heat exchange surface, a COP of 0.62 with a minimum overall irreversibility, and a COP of 0.69 with a maximum exergetic efficiency. Finally, the destruction of exergy takes place mostly in the absorber and the desorber.
NH3-H2O: Experimental prototypes development and experimentation
A summary of the small-capacity prototypes of NH3-H2O absorption cooling systems developed in laboratories and their operating conditions is presented in Table I.1. The prototypes described in the literature have often been built for the use of renewable energy sources. A prototype for the use of geothermal energy was constructed at the Cerro Prieto Geothermal Field (Mexico) to cool a small storage unit at negative temperatures. The system provided evaporating temperatures of – 10°C at generator temperatures of 125°C (Ayala Delgado, 1992). The use of solar energy for these systems is also of great interest. De Francisco et al. (2002) simulated and constructed a 2-kW air-cooled (by natural convection) solar system (with SHX and rectifier). The experimental COPs were lower than 0.05, compared with 0.53 in the simulations. There were expected improvements in the solar collectors, new fans for forced convection, an ice storage tank, a new pump system, and a new automatic control process. Sözen et al. (2002) developed and tested a solar prototype in Ankara (Turkey) driven by a parabolic collector. The system reached cooling temperatures as low as 3°C, and the evaporator and absorber possessed the highest exergy losses. Mendes et al. (2007) designed and constructed a solar system with SHX and RHX in which the rectification column was substituted by a refining spray of rich solution directly integrated in the desorber (see Figure I.2b). The system showed an increase of 1% in ammonia content in the refrigerant after the refining process (before refining, the NH3 content was in the range of 95.5–97.6%) at nominal conditions of 4 kW of cooling with a COP of 0.54. Said et al. (2015) designed and simulated a solar system (based on a design presented by the University of Stuttgart (Brendel et al., 2010; Zetzsche et al., 2010)) that recovered waste heat from the rectifier and contained a refrigerant storage unit. A total increase of 18% in the COP was observed compared with a conventional generator/rectifier configuration and the system (installed in Saudi Arabia) delivered cooling capacities between 4 and 10 kW with COP values between 0.4 and 0.7 (Said et al., 2016). Another innovative proposal was made by Zotter and Rieberer (2015), who adapted a commercial PinkChiller PC19 with a thermally driven solution pump (instead of an electrically driven pump). Thermally driven pumps can be a suitable option owing to the lower production cost, fewer leakage problems, and no electricity consumption; however, the experimental system’s COP was reduced by at least 0.1 (15%) compared with the system using an electrical pump. Boudéhenn et al. (2012) developed and constructed a compact and low-cost 5-kW system at INES (French National Institute for Solar Energy). The system, which uses compact plate heat exchangers (PHEs) available on the market, showed good potential. Nonetheless, further optimization opportunities exist for the SHX and the absorber.
Table of contents :
Residential air conditioning global energy challenge
The Mexican context
Solar absorption chillers
Chapter I. State of the art on single-stage absorption chillers
1.2. Review of theoretical, experimental, and commercial cycles
1.2.1. Comparative criteria
1.2.2. Ammonia-based working fluids
22.214.171.124. Conventional ammonia-based working pair: NH3-H2O
126.96.36.199. NH3-LiNO3 and NaSCN
188.8.131.52. NH3–Ionic Liquids: Theoretical studies
184.108.40.206. Comparisons of NH3-based absorption systems
1.2.3. Water-based working fluids
220.127.116.11. Conventional water-based working pair: H2O-LiBr
18.104.22.168. H2O-LiI and LiCl
22.214.171.124. H2O-Ionic Liquids: Theoretical studies and experimental prototypes
126.96.36.199. Comparisons of the water-based working fluids
1.2.4. Working fluids with other refrigerant bases
188.8.131.52. Alcohol-based working fluids
184.108.40.206. HFCs and HCFCs-based working fluids
220.127.116.11. Other refrigerant bases
1.2.5. General comparisons between working pairs
18.104.22.168. Theoretical general comparisons in the literature
22.214.171.124. Discussion of the developed prototypes and commercial systems
1.3. Review of compact exchanger technologies
1.3.1. Heat and mass transfer comparative criteria
1.3.2. Non-adiabatic exchangers
126.96.36.199. Falling film
188.8.131.52. Two-phase flow exchangers
184.108.40.206. Membrane-based exchanger
1.3.3. Adiabatic exchangers
220.127.116.11. Falling film
18.104.22.168. Two-phase flow
22.214.171.124. Membrane-based exchangers
1.3.4. General discussion
126.96.36.199. Technologies used in experimental prototypes
188.8.131.52. Comparison of the technologies studied
Chapter II. Thermal and mass effectivenesses as a tool for the characterization of absorption chillers and models comparison
2.2. Characterization of the sorption exchangers
2.2.1. Thermal and mass effectivenesses definition
2.2.2. Exchanger’s geometries and experimental conditions
2.3. Characterization methods applied to a NH3-LiNO3 single stage prototype
2.3.1. System description
2.3.2. Characterization of the system
184.108.40.206. Characteristic equation method
220.127.116.11. Adapted characteristic equation method and Carnot function model
18.104.22.168. Effectiveness model
2.3.3. Results and validation of the models
22.214.171.124. Characteristic equation method
126.96.36.199. Adapted characteristic equation method and Carnot function model
188.8.131.52. Effectiveness model
2.3.4. Discussion of the internal operating conditions through the effectiveness model
184.108.40.206. Solution concentration conditions
220.127.116.11. Equilibrium factor
18.104.22.168. Solution flow conditions
22.214.171.124. Equilibrium deviation temperatures
126.96.36.199. Thermal and mass effectivenesses of the sorption exchangers
Chapter III. Solar absorption air conditioner in the Mexican context
3.2. Selection of high-performance working fluid for a solar-geothermal absorption cooling system and techno-economic study in the northern Mexican conditio
3.2.1. Climatic conditions and ground temperature profiles
3.2.2. Thermodynamic comparison of the working fluids
188.8.131.52. Description of the system
184.108.40.206. Assumptions for the thermodynamic analysis
220.127.116.11. Thermodynamic analysis
18.104.22.168. Results and discussion
3.2.3. Integrated solar-geothermal system and economic feasibility study
22.214.171.124. Solar-geothermal absorption cooling system and dimensioning
126.96.36.199. Economic assessment
3.3. Solar absorption air conditioner with an innovative bi-adiabatic configuration: dynamic model, nominal conditions and typical day operation
3.3.1. System description
3.3.2. Dynamic model
3.3.2. Results and discussion
188.8.131.52. System’s optimum operating conditions
184.108.40.206. Typical day operation
Chapter IV. New-generation adiabatic falling film mass exchanger for absorption chillers
4.2. Experimental set-up
4.2.1. Experimental facility
4.2.2. Adiabatic sorption exchanger
4.3. Data reduction and operating conditions
4.3.1. Temperature equilibrium deviations
4.3.2. Desorbed mass and mass effectiveness
4.3.3. Design of experiments and uncertainties
4.4. Results and discussion
4.4.1. Mass effectiveness regression
4.4.2. Model interpretation
4.5. Comparison with other technologies
4.5.1. Membrane desorbers effectiveness
4.5.2. Performance comparison
4.5.3. Pressure drop comparison
Conclusions and future work