Renewable Energy Production and Distribution Volume 2: Solutions and Opportunities

Renewable Energy Production and Distribution: Solutions and Opportunities, Volume Four, the latest release in the Advances in Renewable Energy Technologies series, looks at the production performance of renewable energy sources and emerging production processes. Containing all major renewable energy technologies in individual chapters, this reference includes some of the most dynamic developments, good practices and future concepts in solar energy systems, energy storage, geothermal energy, bioenergy and hydrogen production. By reviewing these advances, considering them in real world applications and analyzing key challenges, this book provides readers with an up-to-date resource on renewable energy grid integration and its importance.

This newest volume will be of interest to sustainability, energy and engineering graduates, researchers, professors as well as industry professionals involved in the renewable energy sector.

• Highlights best practices and future ideas for a range of renewable energy technologies, including solar energy, energy storage and geothermal energy
• Discusses the latest challenges in emerging energy production processes
• Presents real-world applications to bridge the gap between energy research and practice

• Language: English
• Publisher: Academic Press
• Release date: Apr 16, 2023
• ISBN: 9780443184406

Book preview

Section 1

Solar thermal energy

Outline

Chapter 1. Solar thermal systems: applications, techno-economic assessment, and challenges

Chapter 2. Solar-operated vapor absorption cooling system

Chapter 1: Solar thermal systems

applications, techno-economic assessment, and challenges

Marwa Mortadi, and Abdellah El Fadar     Laboratory of Innovative Technologies, Department of Industrial and Electrical Engineering, Abdelmalek Essaadi University, National School of Applied Sciences, Tangier, Morocco

Abstract

Solar energy, if properly exploited, could offer an undeniable potential to fulfill the world energy demand and alleviate energy security concerns and environmental issues. This chapter introduces the solar thermal systems. It starts by presenting different solar thermal collectors technologies as well as the main applications such as power generation, heating, cooling, drying, and desalination. The advantages, limitations, and latest advancements associated with each technology are also discussed. Then, it provides an exhaustive analysis of the energy and economic indicators for assessing the solar systems' performance. Through this chapter, it was concluded that solar thermal systems could contribute strongly to sustainable energy production and utilization. Nonetheless, their widespread use is still limited by low performance, high capital cost, and intermittence shortcomings. In this context, the challenges for promoting these systems are discussed, and a series of recommendations are outlined to provide the readers with a clear idea about the future research trend in this field.

Keywords

Economic indicators; Performance assessment; Power generation; Research trend; Solar active system; Solar desiccant; Solar passive system; Solar sorption cooling; Solar thermal applications

List of symbols

A    area (m²)

A f    geometrical factor (−)

C    cost ($)

C p    specific heat capacity (J kg −¹ K −¹)

C r    concentration ratio (−)

d    discount rate (−)

E    energy (Wh)

F R    heat removal factor (−)

I    solar irradiance (W m −²)

i    inflation rate (−)

I∗    solar irradiation (Wh m − ²)

L    latent heat (J kg −¹)

m    mass (kg)

mc    moisture content (%)

ṁ    mass flow rate (kg s −¹)

n    lifespan (years)

S    savings ($)

t    time (s)

T    temperature (°C)

Greek symbols

τ    transmissivity (−)

α    absorptivity (−)

γ    intercept factor (−)

η    efficiency (−)

θ    incidence angle (°)

ρ    reflectance (−)

η c    heat to electricity conversion coefficient (−)

η p    primary energy efficiency (−)

Subscripts

abs    absorbed

amb    ambient

aux    auxiliary

con    consumed

d    drying

dis    distilled

ele    electric

in    inlet

inv    investment

out    outlet

rel    released

sol    solar

st    steam

sto    stored

th    thermal

u    useful

w    water

Abbreviations

AEP    annual electricity production (Wh)

ATB    adsorption thermal batteries

CB    cost benefit ($)

CF    capacity factor (−)

COP    coefficient of performance (−)

CPC    compound parabolic collector

CPVT    concentrating photovoltaic thermal

DPP    discounted payback period (year)

DR    drying rate (kg h −¹)

ETC    evacuated tube collector

FPC    flat plate collector

GOR    gain output ratio (−)

HTF    heat transfer fluid

HVAC    heating, ventilation and air-conditioning

LCOP    levelized cost of production

LFR    linear Fresnel reflector

NCF    net cash flow ($)

NCP    net capacity of plant (W)

NPV    net present value ($)

OMC    operation and maintenance cost ($)

PCM    phase change material

PDR    parabolic dish reflector

PTC    parabolic through collector

PV    photovoltaic

PVT    photovoltaic thermal

SABC    solar absorption cooling

SADC    solar adsorption cooling

SCOP    solar coefficient of performance (−)

SCP    specific cooling power (W kg −¹)

SF    solar fraction (−)

SHP    specific heating power (W kg −¹)

SIR    savings-to-investment ratio (−)

SME    specific moisture extraction rate (kg Wh −¹)

SPF    seasonal performance factor (−)

SPP    simple payback period (year)

SPT    solar power tower

UO    useful output

VCC    vapor compression cooling

1. Introduction

Since the beginning of mankind, the pursuit of energy resources was a vital concern of human beings for their survival, starting with wood to provide fire for heating and cooking needs. Afterward, the search for natural energy resources was extended to more efficient ones such as coal, oil, and natural gas labeled as fossil fuels. The industrial revolution and world development played a major role in the intensive exploitation of these energy resources, which could induce their rapid depletion in addition to their harmful environmental impacts such as the global warming caused by greenhouse gas emissions. Indeed, according to the International Energy Agency, in 2020, a high share (92%) of the world's CO2 emissions is attributed to the use of fossil fuels [1]. Moreover, the breakdown of the Covid-19 pandemic casts a shadow over the world's energy consumption which declined by 4% in 2020. Nevertheless, this consumption was projected to increase by 4.6% exceeding the registered values before the pandemic [2]. In fact, the world electricity demand increased by 6% in 2021 [3]. Furthermore, the invasion of Ukraine has been disturbing the world energy market, especially the oil supply, which increased the concerns about the world energy demand and supply balance. Therefore, the support and promotion of inexhaustible and clean energy resources are becoming of great importance. In particular, solar energy has a significant potential to meet a considerable share of the world energy demand given that 3.4 × 10²⁴ J of solar energy reaches the earth's surface annually [4].

Solar energy technologies are classified into two major categories, namely solar thermal and solar photovoltaic (PV) technologies. The first one exploits solar irradiation for thermal energy production by means of solar collectors and heat transfer thermal fluids to carry the absorbed solar energy to the end user. However, PV technology converts the absorbed solar energy into electricity by the use of semiconductor material, based upon the photovoltaic effect.

In solar thermal systems, solar collectors are vital components that collect solar energy and convert it into thermal energy for use in diverse applications. They are classified into two categories: nonconcentrating and concentrating solar collectors. The first category is a stationary technology where the collectors are mounted in a fixed position without tracking the sun, in contrast to the second one that needs a sun-tracking system to collect the direct solar irradiation and focus it onto a thermal receiver.

Historically, the first application of solar energy employed concentrating technology to set afire the Roman fleet by the Greek scientist Archimedes, where he used concave metallic mirrors to reflect the solar irradiation on the same fleet [5]. Since that time, many efforts were made to bring to light diverse solar thermal technologies such as solar furnaces which are composed of high concentrating collectors to achieve high temperatures that may exceed 1000°C and are usually employed for industrial applications such as materials processing. Additionally, solar thermal energy is used as well for electricity production through steam engines powered by the absorbed heat, supplied by the solar collector, to generate steam that drives a turbine for producing mechanical work. Furthermore, the building sector takes advantage of solar thermal energy for domestic hot water production in addition to space and swimming pool heating, as well as cooling production for air-conditioning by means of thermally driven cooling systems. Besides, the agricultural sector benefits from solar thermal energy for refrigeration purpose, through cooling systems, or drying for foods preservation along with plants cultivation via greenhouses by providing the appropriate climate. Moreover, the solar potential is harnessed in desalination processes to produce potable freshwater from seawater or brackish water. Furthermore, solar thermal energy can be used in different industrial sectors such as textile, pharmaceutical, wood, and cement industries as well as agri-foodstuff sectors (e.g., sterilization and pasteurization). Lastly, solar thermal energy can be exploited for carbon dioxide capture using generally a sorption system to separate carbon dioxide from the exhaust gases of an industrial process.

This chapter focuses on solar thermal systems, where an overview of the main applications of solar energy is provided, namely: solar thermal plants, solar heating and cooling systems, solar dryers, and solar desalination. Moreover, the key energetic and economic indicators for the evaluation of these systems are discussed along with the facing challenges for their progress. In this regard, the rest of the current chapter is organized as follows: Section 2 is devoted to the examination of the main solar thermal energy applications. Section 3 explains the energy and economic metrics used to evaluate the solar thermal systems. Finally, Section 4 discusses the challenges of these systems and provides a list of recommendations for their further development.

2. Solar thermal applications

In this section, the main solar thermal systems are described along with their research progress. Firstly, as key components, different technologies of solar thermal collectors are discussed. Then, the exploitation of solar thermal energy for power production is explored, followed by a review of the solar heating systems, including space and water heating. Afterward, the solar cooling systems are discussed to end with the description of solar dryers and desalination processes. The operating principle of these systems is described, and their advantages and shortcomings are highlighted.

2.1. Solar thermal collectors' technologies

2.1.1. Non-concentrating solar collectors

The non-concentrating or stationary solar collectors comprise mainly four categories: (1) flat plate collector (FPC) which includes a glass cover, absorber plate in thermal contact with a heat transfer fluid (HTF) flowing through a channel, usually copper tube, as depicted in Fig. 1.1A. Besides, a thermal insulation material is placed at the bottom and edges for reducing the thermal losses to the surroundings. The solar irradiations received by the glazing are transmitted to the absorber plate; then, the absorbed heat is transferred to the HTF through the flow channel. Water, antifreeze solution, or air can be used as HTF. Moreover, (2) evacuated tube collector (ETC) is made of multiple evacuated tubes where an absorber plate and a heat pipe are sealed inside, as seen in Fig. 1.1B. In a similar way to FPC, the solar irradiations, absorbed by the absorber fins, are transferred to the working fluid inside the heat pipe after passing through the glass tube. The working fluid is evaporated in the evaporator section of the heat pipe before being moved toward the condenser section where it is condensed, releasing its latent heat of condensation to an HTF circulating through the manifold (heat sink). Afterward, the condensed liquid returns to the evaporator section for re-evaporation [10]. Moreover, the presence of vacuum space is required to limit the convective heat losses, making ETC more efficient than FPC and able to work at higher temperatures [11]. The third stationary collector is (3) compound parabolic collector (CPC) in which a nonimaging parabolic-shaped concentrator is used to reflect the received solar energy onto a flat or tubular absorber placed at the bottom of the collector, as illustrated in Fig. 1.1C. Usually, the CPC is covered by a glass layer to create the greenhouse effect inside the CPC, but also to avoid the buildup of dust on the reflector, which could reduce its reflectivity. In contrast with FPC and ETC, CPC has a concentration ratio higher than 1. Finally, (4) photovoltaic thermal collector (PVT) is a hybrid type of stationary collectors. It consists of a PV panel mounted to a flow channel placed on the rear side to absorb the energy that could not be exploited for electricity production and wasted in form of heat, in addition to an insulation layer to reduce the thermal losses to the surroundings (Fig. 1.1D).

Figure 1.1  Stationary solar collectors (A) FPC [6], (B) ETC [7], (C) CPC [8], and (D) PVT [9]. Adapted with permission from Elsevier.

2.1.2. Concentrating solar collectors

Concentrating solar collector makes use of an imaging concentrator to focus the direct solar irradiation onto a small receiver area, aiming to reduce heat losses and produce higher temperatures compared to the non-concentrating collector. In this category, different solar collectors can be distinguished, namely: (1) parabolic through collector (PTC) which comprises a parabolic shaped mirror that receives and focuses the solar irradiation onto the focal line where a thermal receiver is placed, as depicted in Fig. 1.2A. The latter includes an absorber tube, where the HTF flows, covered by a glass tube to reduce convective thermal losses. PTC is considered as the most mature technology that could achieve high temperatures up to 400°C. Similarly to PTC, (2) linear Fresnel reflector (LFR) is a line focus system that encompasses an array of flat mirrors that concentrate the solar irradiation onto a stationary thermal receiver, placed at a specific height, as illustrated in Fig. 1.2B. Contrary to PTC, LFR has a simple design and lower concentration ratio. Moreover, (3) Parabolic dish reflector (PDR), known by its similar design to the satellite dish as seen in Fig. 1.2C, is composed of parabolic dish concentrator that focuses the solar rays onto the small receiver placed at the focal point. PDR can achieve higher temperatures, reaching 1500°C, due to its higher concentration ratio [5]. Furthermore, (4) solar power tower (SPT) is composed of reflectors' field, known as heliostat field, that concentrates the solar irradiation onto a focal point where the receiver is placed on the top of a tower which is positioned usually in the center of the heliostat field (Fig. 1.2D). Finally, hybrid concentrating photovoltaic thermal (CPVT) collector can be categorized as non-concentrating or concentrating solar collector, depending on the used technology. In fact, CPVT collector consists of using an imaging or non-imaging concentrator to focus the solar irradiation onto the receiver area where photovoltaic cells are mounted along with an HTF channel to absorb the waste heat [15]. Table 1.1 summarizes the main characteristics of non-concentrating and concentrating solar collectors.

Figure 1.2  Concentrating solar collectors (A) PTC [12], (B) LFR [13], (C) PDR [14], and (D) SPT [13]. Adapted with permission from Elsevier.

2.2. Solar thermal plants

Solar thermal plant is one of the most interesting applications of solar energy for power generation. The plant is composed mainly of a solar collector field and a power conversion system to convert thermal energy into electricity. It is noteworthy that three solar thermal power plants are distinguished based on the employed solar collector, namely: low, medium, and high-temperature technologies. The first one involves the use of FPC to drive a small heat engine; however, this type suffers from a very low efficiency and a high cost, which makes this technology outdated. By contrast, concentrating solar collectors are effectively used for medium and high-temperature technologies given their ability to produce higher temperatures. In medium-temperature solar power plants, the solar collector field, composed of PTC and LFR, focuses the direct irradiation onto a focal line; whereas in high-temperature systems, PDR and SPT are employed to focus the solar irradiation onto a focal point. Sun-tracking system is an important device to enhance the thermal efficiency of concentrating solar collectors by keeping the collectors' aperture in the optimal position that maximizes the received solar energy, which could be enhanced by 10%–100% in comparison with a nontracking solar system [18]. The sun trackers are divided into two groups: single-axis and dual-axis tracking systems. The first one rotates the solar collector using one axis that could be horizontal, vertical, or tilted axis, while the second one uses two axes, horizontal and vertical, and is the optimal mode despite its large land usage and capital cost, especially for large-scale solar power plants.

Table 1.1

Concerning the power conversion system, several thermodynamic cycles can be used to convert solar thermal energy into electric power. For instance, the Rankine cycle is the most popular one which is a vapor cycle composed of a generator (boiler), driven by solar energy, to evaporate a high-pressure liquid, a turbine to expand the working fluid and produce mechanical work, a condenser where the heat is rejected to a heat sink while the fluid becomes a saturated liquid, and a pump that pressurizes the liquid before being returned to the generator. Two distinctions are made in the Rankine cycle: steam and organic Rankine cycles. The former is the most common one that uses water as a working fluid, which makes the cycle more beneficial for medium and high temperatures sources, while the latter is based on organic fluids with a low boiling point, making the organic Rankine cycle more suitable for low and medium heat sources. On the other hand, the Brayton cycle is a gas turbine whose integration in solar thermal power plants is very recent. The basic cycle is composed of a compressor where the gas is compressed, a combustion chamber to heat the gas, a regenerator, frequently used, to preheat the gas by recovering the waste heat, an expansion turbine where the mechanical work is generated and a heat rejection device. Brayton cycle works with a high temperature, in the order of 700°C, and benefits from high cycle efficiency that could reach 50% [19]. Therefore, high-temperature collectors, especially SPT, can be harnessed. Moreover, the Stirling engine is commonly used with PDR for power generation. It is mounted on the collector's receiver to make use of the generated heat for mechanical work production through the movement of the engine's piston. This system is considered as standalone solar power generation one making it more suitable for use in rural areas. However, the power generation is of a low capacity, reported to be in the range of 0.01–0.4 MW [20]. It is worth noting that, to improve the cycle efficiency of the power conversion systems, the combined cycle could be used where the most common configuration is the Brayton cycle at the top stage and the Rankine cycle at the bottom stage. Moreover, the cooling tower is a necessary component to reject the waste heat of the working fluid and allow the good functioning of the power block. Three cooling technologies can be employed: wet, dry, and hybrid cooling towers. The wet category, the most frequently used in solar power plants, extracts the heat by means of evaporative cooling with high maintenance cost and water consumption; the dry type rejects the heat to the ambient air using heat exchangers and works with a low maintenance cost, while the hybrid cooling tower combines the two other technologies. It is important to note that a wet cooling tower is the most performing and cost-effective one, whereas hybrid and dry cooling towers are suitable in regions with water scarcity [21]. In this regard, Ref. [22] conducted a techno-economic study of PTC and SPT-based power plants with dry and wet cooling towers to evaluate the impact of the nominal capacity and thermal energy storage hours on the plants' performance. The results revealed that the dry-cooled PTC plant achieved the lowest net annual electricity production compared to the wet-cooled PTC and dry-cooled SPT plants. Besides, the SPT plant was found to be the most cost-effective one. Additionally, it was concluded that the installation of large-scale power plants can be economically beneficial.

Solar thermal power plants benefit from free solar energy for clean electricity production with low operational cost and greenhouse gases emissions. However, the major hurdle for developing these plants is the intermittence of solar energy leading to a mismatch of energy production with the energy demand. To overcome this issue, hybrid power plants are deployed, combining the solar energy source with a fossil one to enable power generation when solar energy is insufficient. Moreover, thermal energy storage systems are usually integrated into solar thermal power plants alone or with a backup system to overcome the intermittence problem. Indeed, the share of the implemented thermal energy storage systems was estimated in 2019 to be 65.9% of the total installed capacity in operational and under-development concentrating solar power plants [20]. One can distinguish three types of thermal energy storage technologies: sensible, latent, and thermo-chemical heat storage systems. In the first one, which is the most common, thermal energy is stored as sensible heat by raising the temperature of a liquid or solid material without phase change. The storage capacity of the system is highly dependent on the thermo-physical properties of the storage material such as the specific heat and density. As a liquid storage material, water is suitable for low temperature while thermal oils could be used in medium temperature due to their low thermal stability at high temperatures, in contrast to molten salt which is the most mature and used storage material that could be used at high-temperature applications (over 300°C). With the aim of enhancing the storage capacity of molten salt, Ref. [23] proposed a composite material based on chloride molten salt KNaCl2 and amorphous SiO2 nanoparticles. The results indicated that the addition of nanoparticles improved the specific heat capacity of the composed storage material by up to 7.87%. With regard to solid materials, such as ceramic and concrete, they can be used in a wide range of temperature variation, up to 1200°C, compared to the liquid ones that can achieve a hot temperature only up to 800°C. Sensible heat storage systems benefit from the high thermal conductivity of some storage materials, but the required size of the storage unit is large due to the limited energy density of these materials.

Regarding latent heat storage, phase change materials (PCMs) are used to store the thermal energy through the phase change process from a solid state to liquid one. Organic, inorganic, and eutectic materials are used as a storage medium for latent heat storage. Compared to sensible heat storage materials, PCMs have the advantage of higher energy density and they could operate in a small range of temperatures close to their phase change temperature. However, most PCMs suffer from low thermal conductivity leading to sluggish charge and discharge rates in addition to solid buildup in the heat transfer area. Therefore, composite latent heat materials could be used to enhance the thermal conductivity of PCM such as the composite of PCM–graphite, which is the most popular one.

Concerning thermo-chemical energy storage systems, the heat is stored as a chemical potential through a reversible endothermic chemical reaction where the solar energy drives the reaction in the charging cycle, while at the discharge process the reversed exothermic reaction occurs to recover the stored heat. Compounds such as ammonia, metal oxides, metal hydrides, hydroxides, and carbonates could be used as chemical storage materials. This type of heat storage is quite attractive due to its highest energy density, which can be 10 times greater than that of latent heat, in addition to the long duration of storage nearly at ambient temperature. However, thermo-chemical heat storage is still in the development phases to further investigate all the aspects of this system.

The integration of thermal energy storage systems can be carried out based on different concepts, which are categorized as active and passive systems. With regard to active systems, two distinctions can be made: (1) active direct system where the storage media is as well the HTF flowing into the solar collector field (Fig. 1.3A). Therefore, the use of heat exchangers is needless, yet the employed material needs to meet the required properties of both good HTF and storage medium. This type suffers from the high cost of the used materials in addition to the solidification risk of the storage fluid, especially molten salt, which increases the operation and maintenance costs. The second type is (2) an active indirect system where the thermal storage material is different from the HTF (Fig. 1.3B and C). Hence, a heat exchanger is required to transfer the absorbed solar energy to the storage medium. In an active storage system, two tanks can be used: hot and cold tanks where the hot and cold storage materials are stored, respectively. This configuration is considered as low-risk technique due to the separated storage tanks. Moreover, the solar field temperature could be raised which positively impacts the power block efficiency. Nevertheless, thermal losses to surroundings are increased as well. Besides, the two-tank system requires a large installation area and one tank space empty, which is considered as a drawback of this approach. Therefore, a single tank configuration is an alternative where the hot and cold fluids are both stored in the same tank and separated by stratification due to the temperature gradient and buoyancy effect. This kind is known also as thermocline referring to the space between the hot and cold fluids as depicted in Fig. 1.3C. The active single-tank system is characterized by lower cost in comparison with the two-tank one. However, thermal stratification is hard to maintain and requires appropriate devices to avoid the mixing of the two fluids. Thereby, filler materials are usually integrated to improve the thermocline effect. Lastly, concerning passive storage systems, the storage material is kept immobile and only the HTF flows into it during the charging or discharging processes. This configuration involves mainly a solid storage medium known as a packed bed of a storage material, which could be a sensible material, such as concrete, a PCM or even a thermo-chemical material (Fig. 1.3D). Ref. [25] studied the impact of thermal storage systems on the plant's techno-economic performance. Packed bed rock storage, direct and indirect two tanks storage systems were evaluated. It was found that the direct two tanks system enabled the highest electricity production, while the packed bed rock storage system with Therminol 55 as HTF achieved the lowest levelized cost of electricity.

Figure 1.3  (A) Active direct, (B) active indirect two tanks, (C) active indirect single tank, and (D) passive thermal storage systems [24]. Adapted with permission from Elsevier.

2.3. Solar heating systems

The solar heating system is a thermal process that enables the conversion of solar irradiation into useful heat energy exploited for space heating and domestic hot water production. In this section, the various approaches, passive and active, adopted for space and water heating purposes are discussed.

2.3.1. Solar space heating

First, passive techniques employed for space heating play a key role in reducing the building's heating demand by taking advantage of solar irradiation to supply thermal energy to the interior without using active components. In particular, the glazing surfaces are employed to transmit solar energy into indoors, known as direct heating. Therefore, the sizing of glazed areas is highly important given that a large area could induce space overheating, especially in hot climates with low heating demand. Additionally, the appropriate building's orientation is another approach to maximize the solar energy transferred inward; it is recommended to adopt a south/north orientation in the northern/southern hemisphere regions. Moreover, thermal insulation, and green roofs and walls are applicable for reducing heat losses from the inside to the external environment [26]. Besides, passive design techniques such as solar chimneys and Trombe walls are utilized to enhance the transfer of solar energy inward, known as indirect heating systems. Furthermore, storage materials are employed to improve the thermal mass of building's envelope such as PCMs [27]. Indeed, PCMs benefit from high storage capacity and can store the absorbed heat gains from solar irradiation during daytime in form of latent heat to release it into the inside during nighttime, as illustrated in Fig. 1.4. In this context, Ref. [29] proposed a dynamic Trombe wall integrating PCM, where the solar collector wall was equipped with rotating PCM panels to be oriented toward the exterior during the charging phase and to be facing the interior during the discharge period. The results indicated that the dynamic Trombe wall was 40.5% more efficient than the static configuration in terms of the period at which the indoor air temperature remained above 20°C during the discharge phase. In the same vein, Ref. [30] studied the impact of several thermo-physical parameters of PCM on indoor thermal comfort. The results showed that the integration of PCM into buildings can achieve an energy-saving rate of 20.76% and decrease the indoor discomfort degree-hours by 74.9%. Besides, it was found that the optimal phase transition temperature should be higher by 2°C than the lower temperature of thermal comfort with an inside position of the PCM panel for reducing the indoor temperature fluctuation. Moreover, adsorption thermal batteries (ATB) can be integrated to store solar thermal energy given their characteristics of a very high energy storage density, minimal thermal loss, and continuous storage cycle [31]. This storage system is classified as a thermo-chemical storage process relying on the binding of an adsorbate onto an adsorbent. The charging cycle is represented by the desorption process, driven by solar energy, during which heat is stored. When the building gets colder, the adsorption process occurs and releases its heat to warm up the space. ATB has the advantage of negligible thermal losses during the discharge phase in comparison with PCM. Nevertheless, its performance is highly dependent on the adsorbent material, because not all the available materials are suitable for such application, like zeolite and silica gel, according to their driving temperature and thermal storage capacity [31]. Besides, isolated heating is another type of passive space heating where the solar energy is absorbed and stored in a separate space, other than the interior, known as attached greenhouses. This type can prevent the overheating risk; however, it requires more space, and the shading effects should be properly controlled during winter and summer to maximize and minimize the heat gains, respectively. It is worth mentioning that the heat transfer in passive systems is based mainly on natural convection, conduction, and radiation; although, air vents and dampers can be used to promote the air flow and the natural convection.

Figure 1.4  Passive space heating system with phase change material [28]. Adapted with permission from Elsevier.

To sum up, passive space heating systems have the advantage of being a cheap solution to satisfy heating needs. However, these systems generally cannot cover the whole heat demand and are highly dependent on outdoor climate conditions; therefore, active heating systems are usually required. In this context, the main components of an active solar space heating system are: the solar collectors' field, a thermal storage tank where the absorbed heat is stored, an auxiliary heater in case of the insufficiency of solar energy to cover the heating demand, circulation pumps, and a terminal unit to supply the heat loads into the thermal zone, as seen in Fig. 1.5. The terminal unit can be heat exchangers such as radiators, heating floors, or heating walls where the working fluid circulates and releases its heat to the indoor space, or by means of heating, ventilation, and air-conditioning (HVAC) system in which the hot air is blown inside by a fan. Usually, space heating systems require a low temperature in the limit of 45°C; therefore, FPC and ETC are the most suitable collectors with air, water, or antifreeze solution as the working fluid. It is noteworthy that water is the most commonly used working fluid thanks to its high thermal conductance.

To enhance the performance of solar heating systems, solar-assisted heat pumps are employed. Three different configurations could be distinguished based on the coupling method of the solar system and the heat pump. First, (1) the parallel solar assisted heat pump configuration, which is considered as a hybrid solar heating system since another source is used to drive the heat pump such as ground or air; besides, the heat pump and the solar heating system complement each other to provide the heating loads, that is, when the solar system is unable to meet the heating loads, the heat pump works to supply it. Secondly, (2) the series configuration consists of using the solar system to heat up the working fluid in the heat pump's evaporator right after the solar collector or through the storage tank; this configuration is known as an indirect expansion solar-assisted heat pump. In this context, Ref. [33] conducted a comparative analysis of serial and parallel indirect expansion solar-assisted heat pumps in addition to a dual-source configuration where the heat pump's evaporator was driven by the hot water coming from the solar field along with air. The main findings revealed that the serial configuration achieved the highest seasonal performance factor of 5.5 followed by the parallel and dual-source types. However, the serial system required larger solar collectors' area leading to the highest payback period, whereas the lowest payback time was acquired by the parallel configuration. The third configuration is (3) the direct one in which the solar collector and the evaporator form one integrated unit, where the refrigerant is evaporated within the solar collector as depicted in Fig. 1.6. For this configuration, the use of an unglazed flat plate collector is recommended over the glazed one to avoid heat losses through the glass and benefit from both solar and air gains [34], since a part of solar irradiation will be reflected and absorbed by the glazing. Solar-assisted heat pumps allow an enhancement of heating production when the exploitation of solar energy through a direct space heating system is not enough to meet the heating loads. However, the performance of the system can decline if the solar radiation is far below the required level. Besides, these systems could not be considered as a clean one due to the use of non-friendly refrigerants such as R410A. Ref. [35] developed a direct expansion solar-assisted heat pump using a packed bed of composite PCM evaporator-collector. The aim of the proposed configuration was to prevent the decrease of the compressor's volumetric efficiency during the superheating periods, due to the high intensity of solar radiation, by storing the surplus of solar energy. The results revealed that the energy consumption of the compressor decreased by 0.5%–1.8% compared with a direct expansion flat plate evaporator-collector system. Additionally, the system's coefficient of performance (COP) was enhanced by 1%–9.5% with a payback period of 9.9 months.

Figure 1.5  Scheme of active space heating system [32]. Adapted with permission from Elsevier.

Figure 1.6  Scheme of direct expansion solar-assisted heat pump [34]. Adapted with permission from Elsevier.

On the other hand, an alternative solar-assisted heat pump which is more eco-friendly can be employed. It involves the use of sorption systems (absorption/adsorption) to supply the heating needs. These systems are powered by solar collectors to drive the desorption process, then the absorbate/adsorbate is condensed and heat, released at a medium temperature, can be employed for space heating. The liquid refrigerant is vaporized in the evaporator before being supplied to the absorbent/adsorbent where the absorption/adsorption process takes place. It can be noted that the evaporator extracts heat at a low temperature from the environment using an outdoor unit or a solar collector, and heat is released during the absorption/adsorption process at a medium temperature. The generation and heating temperatures of an absorption system are in the range of 75–170°C and 40–80°C, respectively, using H2O–LiBr or NH3–H2O as working solutions (refrigerant-absorbent) [36]. Regarding the adsorption heating system, the maximum achieved heating temperature is 55°C, while the generation temperature varies depending on the working pair, for example, for Silica gel-H2O, it is in the range of 60–100°C [37]. In contrast to the conventional solar-driven heat pumps, the solar-assisted sorption heat pumps have the advantage of using environmentally friendly refrigerants; however, they suffer from low performance and high capital cost in addition to the systems' bulkiness. To enhance the performance of solar absorption heat pump and extend its application in extremely cold climates, Ref. [38] studied a hybrid absorption-compression heat pump based on a generator-absorber heat exchanger. The results showed that the system was suitable for the building's floor heating with a primary energy ratio of 1.116 and a payback period of 7 years.

2.3.2. Solar water heating

Concerning solar water heating system, the solar energy is harnessed to heat up water for domestic use. Its main components include the solar collector, the storage tank, and pipes. One can distinguish between passive and active solar water heating systems. Compared to the passive system, the active one uses some electric components such as valves and pumps to provide a forced circulation of the working fluid. Otherwise, in the passive system, the circulation of HTF from the collector to the storage tank is ensured by natural convection where the tank needs to be in a higher level than the collector to promote the density difference principle; this system is known as the thermosiphon system. Usually, an auxiliary heater is integrated to further heat up the water when the solar energy is insufficient. Moreover, integrated collector storage is another variant of the passive water heating systems consisting of combining the thermal storage tank and the solar collector in one single unit. This system has the benefit of being simple and requiring less maintenance; however, it is easily affected by the climate conditions leading to heat losses, especially in cold