Growing population, depleting fossil fuels, economic expansions, and energy intensive life style demand are resulting in higher energy prices. We use energy as of heat and electricity, which can directly be obtained from sun using thermal collectors and solar cells. Solar thermal systems are gaining attention for water and space heating applications due to green aspects of solar energy. A solar thermal collector is a vital part of solar thermal energy system to absorb radiant energy from the sun. In this study, a solar water heating (SWH) system has been designed and simulated in the TRNSYS ® software using thermal and chemical properties of heat transfer fluids using REFPROP for dwellings located on ±31° latitudes (+31 Lahore in Pakistan and -31° Perth in Australia). We present an efficiency parametric optimization-based model for water and space heating. Simulation results for four types of solar thermal collectors are presented, and performance is analyzed on the basis of output temperature (), solar fraction (), and collector efficiency (). This study evaluates the comparative performance of evacuated tube collector (ETC), flat-plate collector (FPC), compound parabolic concentrator (CPC), and thermosiphon-driven systems. Our findings conclude the evacuated glass tube collector achieves the highest solar fraction, i.e., 50% of demand coverage during August in Pakistan and February in Australia, with an overall average of 43% annually.

1. Introduction

Energy demand is increasing globally due to population growth, fossil fuel depletion, and energy consumptive life styles [1]. US oil consumption was equal to local oil production in the 1950s, double in the 1980s, and equal again in the 2020s due to shale revolution in the 2000s. Energy shortages in winter, high cost of electricity in summer, depletion of conventional fossil fuels, and their environmental effects are leading the world to shift the focus towards renewable energy resources to empower the people beyond 2050 [2]. More than 72% CO2 emissions are related to energy and 21-37% to food production processes especially tilling, fertilizers, and cattle [3]. Heat and electricity account for 31% of CO2 emissions [4]. Rock dust in soil can remove 2 to 4 billion tons of CO2 annually whereas as solar energy can decarbonize energy sector especially 31% fossil fuels produced heat and electricity. Solar energy can easily be converted to useable forms, either solar thermal energy or solar electrical energy. Renewable energy technologies can easily be used for industrial and domestic applications [5]. Hydrogen systems have also shown promising results to be applied as an alternative fuel for domestic and industrial applications [6, 7]. Solar thermal systems are efficient to transform incident solar radiations into useful thermal energy in 40-120°C temperature range [8]. In the near future, application of intermittent renewable energy resources requires load management, power quality delivered, and increased focus on energy storage systems for backup [911].

Solar energy can be easily applied at domestic level to minimize gas and electric energy costs to meet water or space heating demands [12, 13]. A recent study shows 1 MWe peak load shaving may be attained by installing 1000 solar water heaters each with 100-liter volume [14]. Pakistan has large potential to utilize irradiant solar energy for meeting domestic hot water (DHW) demand, and Australia has high sunshine. Wasting coal and gas-fueled plant electricity in presence of abundant of solar energy is not sustainable way of life. Solar collector is a vital part of all the system designs which utilize solar energy, either photovoltaic (PV) or solar thermal energy [15]. Collectors act as a heat exchanging element and convert available solar insolation radiation into beneficial thermal energy. The collected thermal energy is conveyed to thermal storage tank by heat transfer fluid [16]. Heat transfer fluids may be synthetic yet natural refrigerants like water are more sustainable. Addition of table salt in water may increase heat transfer efficiency. The TRNSYS software is used for design of various types of solar collectors for domestic water heating. Flat-plate collector, evacuated glass tube collector, parabolic trough collector, linear Fresnel reflector, and compound parabolic collector are used for 45 to 400°C whereas point and line type solar collectors such as solar tower and parabolic dish are used for 1000 to 1500°C temperature [17, 18]. Flow control by optimum TRNSYS model may enhance effective efficiency up to 7% or more [19]. TRNSYS home models have demonstrated 37% to 68% solar fractions under diverse weather conditions [20]. Integration of PV-driven electromagnetic heating and phase change material storage increases lifecycle cost by 4 to 23% reducing CO2 emissions by 13 to 26.73 tons [21]. Storage of summer heat for winter heating and winter chill for summer cooling is emerging as yet a remote reality. Seasonal storage of summer heat holds key to elixir of decarbonozation at district heating level in big cities [22].

Thermal collectors are further classified into flat plate, evacuated tube, and concentrating type of collectors with the flat plate being least efficient, but economic, whereas concentrating collectors are employed only for applications requiring high temperatures. Generally, evacuated tube collectors are employed in most applications due to high efficiency and performance. Photovoltaic collectors branch out into 3 generations of solar cells each having their own pros and cons. Generally, in residential applications, only 1st-generation (mono and polycrystalline) silicon cells are being employed due to maturity and efficiency. Newer generation solar cells such as multijunction cells are efficient but currently used in selected areas as the price tag does not justify their use for residential applications. Furthermore, organic cells are promising technology but currently not highly efficient or reliable due to stability issues. Hybrid collectors and their implementations depend on various factors, and their role is ambiguous; some studies claim higher performance as compared to individual technologies, whereas others claim their findings in the favor of using a combination of individual collectors instead of hybrid single enclosure-based collectors. The performance of solar collectors depends on the selective surface, absorptivity (high), emissivity (low), coverings, spacing, and tilt angles ° or ° for domestic water or space heating and ° or ° for refrigeration/absorption due to dust accumulation on collectors. Solar water heaters usually employ FPC, ETC, or CPC with natural or forced circulation. Space heating, cooling, and refrigeration employ FPC, ETC, or CPC with forced circulation [23]. Mean energy efficiency of ETC varies from 80 to 90% compared to FPC from 50 to 60% in high sun countries [24]. ETC outperforms FTC in colder regions due to low conduction losses. ETC efficiency in cold regions is 30-45% but can achieve temperature as high as 170 to 200°C in hot regions. Application of solar energy for cooling and air-conditioning applications is also getting attention of the researchers [25, 26].

Water heating demand accounts for a significant amount of energy consumption in the world. The percentage of energy demand in the water heating process in a household is approximately 10–30%. As a result, selecting an adequate water heating system can have a significant impact on the increase in energy reserve. Using solar technologies as a result of increased energy demand has gotten significant attention because it is a safe, natural, and cost-free process to roll up hot water with solar energy [27, 28]. The performance analysis of solar thermal systems with various collectors, fluids, and conditions is gaining momentum. In this day and age, solar energy is a vital source of energy supply among all renewable energy resources. As a result, considerable progress is being made in harnessing solar energy through the use of technologies, such as solar collectors. Because of its excellent thermal efficiency and good performance in adverse weather condition especially in subzero temperature areas [29], evacuated tube collectors have received a lot of attention. Recent trends include use of nanofluids [3033], enhancement of heat pipe performance, use of latent heat storage [34], molten salts [35], heat loss reduction designs/insulations [36], geometric heat pipes [37, 38], flow control and design [19, 3941], and proposing holistic architectures such as CCHP [4244]. The existing literature emphasizes on individual detailed energy, exergy, and economic studies but lacks a comparative performance of various technologies. This work presents a much needed parametric performance comparison of various solar collectors. These collectors are studied for a typical dwelling. The main contributions of this paper include optimized parametric values for the highest collector efficiency and solar fraction in the specific ambient conditions. This work advocates the trial of multiple technologies to cater thermal needs of a specific area and provides evidence of how certain technologies might not be a good option in one condition but outperform in all other conditions.

2. Background Literature

Solar water heaters collect solar energy and export this irradiant energy to heat transporting medium to warm the water in a storage tank. These systems are efficient up to 70%, and this value is quite higher than the efficiency of PV-based systems, which is around 17% only. Passive and active are two basic categories of water heating system based on their working mechanism. Active systems circulate HTF using a pump, whereas passive systems circulate HTF via gravity forces. A survey presenting working, efficiency, and arrangements of evacuated tube-based collector is presented in [45]. Al-Joboory compared the enhancement performance of evacuated tube solar water heater systems in multiple configurations [46]. The study employed two identical solar water heaters with 120-liter capacity tanks and methanol as working fluid. One system incorporated thermosyphon and the other with heat pipes. The study revealed that heat pipe systems were better in performance by 22.5% (no load), 42.5% (intermittent loading), and 32.4% (continuous loading) conditions. District heating by employing design of a two-supply/one-return triple pipe structure was proposed by Prof. Xu and his team. The simulation analysis unveiled that distance between heating pipes played a key role in determining total heat loss, i.e., reducing spacing from 114.1 mm to 84.1 mm, and total heat loss reduced from 24.13 w/m to 20.16 w/m but also increased heat exchange between water and pipes [47]. Combination of nanofluids as HTF was tested for two-phase closed thermosyphon (TPCT) by Xu et al. [32] experimentally, and results showed hybrid nanofluid (Al2O3-TiO2-H2O) showed superior performance by increasing thermal efficiency and heat transfer coefficient. Nanofluids can enhance the performance of molten salts when used as heat storage fluids [33]. In heat transportation, storage, and conversion, the underground pipeline laying technique has gained attention recently because of suitability, quick construction, and low building cost. A study showed heat transfer analysis of large diameter L-type heat pipe network using the flow heat solid coupling method in the ANSYS workbench platform [48].

The sun is a gargantuan fusion reactor empowering wood, water, and wind cycles on earth. More than two-third solar energy is absorbed by ocean waters and less than one-third by land mass. Plants use photosynthesis process to run food supply chain system. The sun uses hydrogen fuel which is the most abundant element in universe. Star science inspired energy systems are under intensive research in USA (NIF), Europe (ITER), and China (EAST). Solar energy consists of light, heat, and radiations. The electromagnetic radiations range from X-rays to radio waves. Solar light spectrum ranges from ultraviolet to infrared. Solar energy is available as heat (IR) and light (visible) which we can harvest using solar thermal collectors and solar cells. Solar collectors use refrigerants/fluids to drive steam turbines, water heating systems, and empower refrigeration cycles for cooling. The energy inside solar spectrum is shown in Figure 1.

A solar thermal collector harvests heat by absorbing sunlight in gas or liquid fluids. Solar collectors are used for trigeneration, i.e., heating, cooling, and power generation applications [50]. Flat plate, evacuated tubes, and flat-plate evacuated solar collectors are preferred for water and space heating or cooling applications, whereas parabolic troughs, parabolic dishes, solar chimneys, and power towers are used for power generation [51]. Concentrating solar power (CSP) systems use lenses and mirrors to convert light into heat to drive heat engines or steam turbines. Solar thermal power plants are usually constructed in remote barren hot regions like Mojave Desert, USA. Solar photovoltaic panels and solar thermal collectors may be installed on rooftops for power generation, water heating, and space cooling in high-sun areas. Rooftop solar cells are common but solar thermal collectors are now becoming popular due to rising natural gas prices. Flat-plate and evacuated tube collectors are used to capture solar heat for water and space heating or cooling with absorption chillers. Solar hot water panels need no extra fluids but solar thermal collectors use refrigerants and heat exchangers to transfer heat to reservoir. All solar collectors may be used for water heating, air conditioning, and power plants at homes and industries.

2.1. Flat-Plate Collector

A flat-plate collector (FPC) is the most common form of solar thermal technology (80°C). It has insulated glazed absorbing plates that are planted in a casing with an air gap between glazing and plates to trap solar radiation. The cover is made up of sheets of glass. Absorber plates (flay, corrugated, and grooved) are mostly dull and dark (blackened) to absorb maximum radiations. The selective surfaces must be highly shortwave absorbent and transparent to long-wave thermal radiations [52]. Furthermore, the tubes are coated with high absorptive, low emittance layers to exploit maximum radiations. The tubes transfer the heat absorbed to a heat-carrying fluid inside the riser tubes. These riser tubes are connected to main header tubes at both top/bottom ends of the collector or another serpentine tube design. The serpentine tubes are often coupled with a pump as the natural flow is comparatively difficult owing to a complex tube shape. Flat-plate collectors are employed to heat water, refrigerants, air, etc. The collectors have significant life spans but are prone to damage due to extreme climatic conditions like hailing, floods, or thermal expansion that might damage glazing. Generally, copper tubes are the preferred medium of heat transfer owing to good conductivity properties and less prone to corrosion [53]. New polymer-based transparent insulating glazing (TIG) is introduced in literature [54] known as honeycomb collectors. The honeycomb-like structure helps trap air by ceasing rapid circulation, and polymer blocks infrared reradiation thus reducing convectional losses [55].

2.2. Evacuated Tube Collector

Evacuated tube collectors (ETC) employ heat pipes (copper) encapsulated within a vacuum-sealed tube. The ETC can be considered an upgraded form of FPC by creating a vacuum space around the receiver. The outer tube is transparent, and the inner tube is selectively coated for maximum absorption. Multiple tubes are connected to a common manifold to increase the heat collection area. Fin tubes are used to achieve high temperatures with selective surfaces. Also, evacuated tubes trap more radiations due to vacuum suppression. The design captures both direct and diffused radiations at lower incidence angles as compared to flat-plate collectors. The pipe uses a fluid that undergoes cycles of evaporation/condensation. Volatile liquid/gas is evaporated with radiations and converted into vapors which raised and condensed at sink points releasing its latent energy to another heat transfer fluid at the manifold. Condensed fluid returns due to gravity keeping the circuit alive. The heat transfer fluid through manifold is coupled to a thermal storage tank or directly used via heat exchangers. Temperature ranges from 100 to 130°C. ETCs are the most adopted thermal collectors worldwide with a major 77.8% followed by FPC at 17.9%, unglazed water collectors at 4.1%, and air collectors at 0.2% [56]. The technology is commercially available as shown in Figure 2.

Abas et al. [29] employed supercritical CO2 as mediating fluid for a solar water heating system and demonstrated a 10% increase in heat transfer efficiency. The thermal performance of ETCs is experimentally investigated by several researchers [57] under identical conditions. These experiments reveal higher efficiency achieved with evacuated collectors compared to other collectors. Nanofluids in heat pipes are getting popular in high population countries like China and India [58, 59]. Water-based CuO was selected as a carrier fluid, and study revealed that the thermal performance of thermosiphon increases by 30% with operating temperature. Li et al. [60] compared heat transfer performance characteristics of nanofluids (ZnO and MgO) in the solar collector and found ZnO as the most suitable option for solar energy utilization. Mahendran et al. [61] proposed a water-titanium oxide nanofluid to increase collector efficiency under clear skies. This study claimed 16.75% increase in efficiency during peak time 2 : 00 pm. A similar study with a slight increase in nanofluid volume concentration (1 to 3%) and modified flow rate claimed an efficiency increase by 42.5%.

2.3. Compound Parabolic Concentrators

Compound parabolic concentrators (CPCs) are nonimaging solar collectors. The CPC collectors use fin type absorbers in the form of flat, bifacial, wedge, or cylindrical configurations. Normally, CPC collectors use fin type absorbers. CPC may be designed with point and line focus and can integrate an inverted or inclined flat-plate absorber by reflecting light on it. Evacuated tube collectors also may be placed at absorber location to get more concentrated light. Line focus CPC collectors are usually preferred for thermal power plants. CPC area concentration ratio depends on acceptance half angle (). The CPC collector is not suitable for domestic water heating.

2.4. Evacuated Flat-Plate Collectors (EFPCs)

Evacuated flat-plate collectors (EFPCs) combine benefits of both FPC and ETC. An EFPC uses high-vacuum insulation inside and consists of glass and metal materials. EFPCs are most efficient nonconcentrating solar thermal collectors [62]. CERN made the first EFPC and simulation studies show it to be the best solar thermal collectors for air conditioning applications [63]. Solar collectors, due to their outdoor installation, face a wide range of environmental stresses. Apart from wind-blown objects, the collector materials undergo periodic thermal and humidity stresses. Oxidation of collector material reduces its heat collection efficiency. Silver paint increases reflectivity but soon suffers surface dullness. Mirrors faint over time due to humidity. Our experience shows steel collectors perform better than glass and silver-coated materials. Steel deposits lesser dust on the collector surface that is washed by rains and winds without affecting its overall performance. Evacuated glass tube collectors do not suffer weather wear and tear but easily break during hailing. High windblown objects also strike and break ETC glassware. Solar thermal heat collectors use steel and mirror heliostats. There is an urgent need to research more materials for solar collectors and storage containers. EFPCs are not commonly available in market.

2.5. Synthetic and Natural Refrigerants

During the refrigeration history spread over 160 years, nearly 50 materials had been under use as a heat transport medium. With passaging time, advent of new technologies, and increased environmental concerns, many of them were phased out and reasonable choices remain to be applied depending upon application. Because of the increased risks to the environment, including global warming, ozone depletion, and nuisance emissions, the global community reached at a consensus to stop using ozone damaging immediately and grant a strict time-based permission to replace existing cooling and heating systems with environmentally benign refrigerants. The most recent Paris Accord is a collective effort to limit average global temperature increase below 2°C. [64]. For the achievement of a carbon neutral goal at the earliest, research on replacement of old and development of new refrigerants has got significant attention. Requirement of refrigerants is increasing, and refrigeration technology has become mature and efficient than before. Resurrection of natural refrigerants has open new horizons for the researchers to comply with environmental protection protocols. Every year, 26th June is declared as world refrigeration day to acknowledge remarkable impact to the society made by cooling, air conditioning, and refrigeration technologies [65]. Conventional refrigerants, including chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), are in use in refrigeration industry for a longer period because of their excellent chemical and heat transfer capabilities. But these fluids are banned after the Montreal Protocol due to damaging effects to the ozone layer and increased global warming. Global protocols mandate nations to work in a unified manner for optimum solutions for existing refrigeration and heating systems [66]. Global Warming Potential (GWP) is a measurement index for the quantity of irradiant energy absorbed by the refrigerant. It is defined as the amount of infrared radiations absorbed by a gas comparative to CO2 spread over a period of 100 years [67].

In order to replace CFCs and HCFCs, the incumbent refrigerants should have lower flammability and toxicity, smaller atmospheric life span, zero ODP, and ultralow to zero GWP. For thermal and heat transportation systems where refrigerants are applied as mediating fluids, primary parameters for the selection of refrigerant must be GWP and ODP. In addition to CO2 and NH3 as purely natural refrigerants, eco-friendly refrigerants R600, R1233zd, R245fa, R410a, and R447a also exhibit great heat transportation outcomes without damaging the environment [68]. Low critical temperature value of 31°C of CO2 causes transcritical operation and high working pressure. NH3 has a reasonable pressure range, a high critical value of temperature, and higher enthalpy of vaporization. But it is limited due to toxic nature, and extra control mechanism is required for safe operation. [69]. Hydrocarbons occur naturally and possess various properties to be suitable refrigerants being energy efficient, critical point, soluble, and heat transportation. These have sound potential to be used as alternatives CFCs, CFCs, and HCFCs and having nearly zero ODP and relatively lower GWP [70].

3. Comparison of Solar Collector

A solar thermal collector captures heat by absorbing sunshine using synthetic or natural refrigerant. Role of refrigerant is to transfer heat by cooling collector surface. Common water heaters use water and glycol refrigerants. A simple collector may consist of copper spiral on aluminum plate and a working fluid often water. Plate and copper tubes are painted black to capture whole of solar spectrum. Commercial solar collectors may employ control mechanisms to maintain temperature. A solar collector may be concentrating or nonconcentrating type. Absorber area of nonconcentrating solar collector is the same as sunshine capturing area but concentrating solar collector has larger area than the absorber area. Solar air heaters need no refrigerants, and large-scale towers or parabolic collectors are often used in power generation industries. Common residential scale water heating systems employ flat plate [71], evacuated glass tube [72], and evacuated flat-tube collectors [73].

A flat-plate collector is made up of a plate coated with black absorbing medium and pipes to transfer heat at the bottom. A transparent cover is installed at the top and thermally insulated material at the bottom. Black surface absorbs solar energy, and thermal energy is supplied to tank through heat transfer fluid. Efficiency of flat-plate collector lowers in cold, cloudy, and windy environment and further decreases with decay of tubes and the insulation material by weather conditions with the passage of time [74]. Evacuated tube designs among all collector designs have high efficiency due to ability of heat transport in unpleasant weather conditions particularly. These have low cost, simple construction, and easy installation and can be applied for thermal energy requirement ranging from 70 to 120°C [75]. A single evacuated tube is made up of enormously sturdy borosilicate glass material. Outer tube allows radiation to pass through it with high transmission power and low reflectivity. Absorption of radiant heat from sun is increased by applying a layer of selective coating material on the inner tube which also decreases the reflection [29]. Compound parabolic concentrator is constructed like two meeting parabolic reflectors having application where temperature demand is over 100°C [76]. These can collect nondirect radiation using less amount of material in the manufacturing of reflector, and reflectance is increased two-third times [77]. CPC can collect radiation received with large angular spread and then concentrate it on to linear receivers of lesser transverse width. Thermosiphon SWHs have wide application for water heating. These systems work on natural water circulation principle also called thermosiphon effect. A density difference is produced by variations in temperature making the warm water to rise up and the cold water to flow down [78].

This research is intended to perform a comparative analysis of different types of collectors to analyze efficiency to select the best type collector for domestic hot water usage with higher energy conversion capability. Four types of typical selected collector models evacuated glass tube (type 71), flat plate (type 73), compound parabolic concentrator (type 74) and thermosiphon heating loop system (type 45a) are considered.

4. Thermal Energy Storage

Thermal energy storage be it heat or cold-based employs a heat transfer fluid (HTF) and a medium to store heat to or extract heat from. In some cases, the same HTF can also be employed as a medium. The basic goal of TES is to maintain the temperature of the medium temporarily to be used later. Any increase or decrease of temperature in cold or heat storage results in lower exergy. Exergy in layman terms is the net capability of doing work. If heat storage which is required to maintain heat for a specific purpose loses heat exergy reduces as the system must be supplied the heat that is lost, similarly cold storage in which temperature increases, further electric or other cooling techniques have to be employed to drop back to the desired temperature. The energy storage efficiency of a conventional two-tank system is higher but is not economical as one tank remains empty. Thermocline tanks have replaced two-tank systems with tradeoffs of such as lowered stratification due to water mixing in the same containment. Convective heat transfer between a moving fluid and solid medium is proportional to the heat transfer coefficient, area between fluid, and medium.

It is an efficient storage technology that stores heating/cooling in some medium from which it can be extracted later. The stored energy can be passed through a ranking cycle to produce electricity, can be employed in thermoelectric effect, or used directly as hydronic heating. It is usually integrated into distributed generation to promote renewable energy penetration but can be employed as a standalone system in local residential applications as well. This storage technology is capable of shifting peak energy demand and is believed to play a significant role in future demand response protocols [79]. It is also believed that TESS systems can aid in supply/demand mismatch [80]. TESS systems are the ideal candidates for domestic/residential hot water (DHW/RHW) applications, hot water coupled refrigeration systems, space heating, and cooling. The major challenge to be addressed is to prolong the storage duration in these technologies [79] with better insulations, increasing thermal efficiency, promoting passive heating/cooling, and modified architectures. Large-scale projects employ molten salt as storage medium but residential setups use water and directly use it for mentioned purposes. Large-scale projects employ concentrating solar panels (CSP) whereas residential applications employ evacuated glass tube-based solar thermal collectors (STCs) [81]. TESS systems are further classified into three types sensible energy storage (SES), latent heat storage (LHS), and thermochemical storage. The grid-connected capacity of thermal systems is dominated by molten salt thermal storage (MSTS) at 88.11%, followed by chilled water thermal storage (CWTS) at 5.1%, heat thermal storage (HTS) at 4.2%, and ice thermal storage (ITS) at 2.59% [82]. Comparison of solar thermal energy storage with rest of developed alternative technologies is shown in Figure 3.

4.1. Sensible Heat Storage

Store heat without phase change by heating/cooling a material/medium depending on its heat capacity and thermal diffusivity [83]. The material can be solid (rockbed), liquid (water), or gas (carbon dioxide), and mode of heat transfer can be radiative, convective, and conductive. Sensible heat storage systems (SHS) are believed to be the most appropriate option (70-90% efficient) for storing solar heat. The process is reversible with have high life cycle, no environmental concerns, and residential/commercial deployable technology. The most common residential application includes solar water heater (40 to 80°C) with insulated storage tanks [84] as passive heating as shown in the figure. The water is heated by solar infrared radiations, and through thermosiphon action, it starts circulating to storage or/and heat exchangers. Such setups are coupled with pipes to realize hydronic heating in built environments.

where accounts for heat stored is equivalent to product of mass of medium and heat capacity integrated over the difference between initial and final times. Other than the residential water thermal storage, there are more types of thermal storage underground thermal storage, molten salt storage, and aquifer thermal storage (ATS). The most notable sensible heat storage installation is near London, the UK, which consists of three 8 boilers and two 2 MWth CHP with storage with a capacity of 2500 m3 water providing hot water services to some 3256 residential buildings, 50 commercial buildings, and 3 schools [85]. Rockbed storage systems have low energy density compared to water, thus requires large areas (three times) to accommodate the same amount of heat. A pilot project is presented which was 300 m3 pebble beds which stored surplus heat from solar collectors during the day to provide hot water and heating services at night [84, 86].

4.2. Stratified Thermal Water Storage

Sensible storages make use of raising/dropping temperature to increase heat storage in a particular medium. The medium solid/liquid/gas should be selected on the basis of high specific heat (c) and high density such that maximum heat is accumulated per volume: water (R718) bearing specific heat (4.2 J/kg.C) and heat content per volume (4.2 MJ/m3K) satisfying both requirements and is potentially harmless. Water-based sensible storage ranges from 0°C to 100°C. Generally employed storage tanks are made up of stainless steel, concrete, plastic, watertight encapsulations preferably with lower heat loss and lower thermal conductivity. Baffle plates are often employed to reflect and trap a certain amount of heat loss. An ideal fluid must be capable of operating at high temperature and low pressure to transfer heat from source (solar collectors) to sink (stratified storage tank).

Hot water storages are designed with few important characteristics in mind being heat storage capacity, heat loss, heat exchange rate, heat exchange capacity, and temperature gradient stratification. Heat content accumulation can be estimated with the product of heat capacity and temperature swing (). Maximizing heat content accumulation reduces storage size. The basic requirement of thermal energy storage is to employ fluid or material with maximum (). A hot fluid at a temperature () with flow rate () can deliver heat energy with thermal power (). We know that sensible energy storage stores heat by increasing its internal energy by creating temperature difference.

where is the number of materials, is the mass of material (kg), is the specific heat of material (J/kg.K), and is the density of material (kg/m3). The volume required to store can be calculated using mass of the storage medium and density in (Kg.m3).

The heat exchange capacity and rate should be high both while charging and discharging to attain efficient results. Additionally, stratification in both charging/discharging periods improves performance, reduces, or eliminates reliance on auxiliary heating. Collector side temperature should tend to be lower while the extraction side temperature higher comparatively to maintain good stratification. Temperature and pressure influence certain physical properties of fluids. When we increase the temperature of water, its density and viscosity decrease and naturally hot water rises on top of the storage tank. Similarly, the thermal conductivity increases with increased temperature, and stratification is formed quickly. Moreover, it is observed that tall and thin tanks establish and maintain stratification better than bulky and short tanks.

Proper placement of thermal bridges strongly influences the performance of stratified tanks. A thermal bridge applied on the top node hot water side forces the bridge to maintain a higher temperature by causing internal convection from the tank to bridge with higher heat loss at the bridge. The placement of the thermal bridge at the bottom or cold side of the tank forms a cold stagnant layer above the thermal bridge minimizing heat loss. Moreover, rational piping can also improve or deteriorate the performance of the system. Piping installed on the top or sides of hot water stores results in high heat loss through internal circulations, etc. It is suggested to install pipes at the bottom of hot water stores and directed downwards to circumvent/prevent internal circulations. Additionally, heat traps are introduced outside tanks to minimize losses. Furthermore, to avoid thermosyphoning in the pipe loop, it is essential to implement controllable valves that block the piping in the absence of flow [87]. Coldwater inlets need special design attention as to minimize mixing during inflow and draw-offs, and baffle plates are often integrated to reduce mixing losses and improve the overall performance up to 5% [88, 89]. Convective losses from the main storage tank can be captured and accommodated using another sensible storage layer surrounding the main storage with fans and blowers to provide space heating.

The most common form of a storage tank is a steel-based watertight encapsulation for solar domestic hot water (S-DHW) [90]. Geometric rearrangements and modifications in parameters such as height, diameter, length, inlet/outlet positioning, layer thickness, and weight can improve performance up to 20% in certain cases. Research shows that extracting the closest desired temperature from the most appropriate node of stratified varying temperature ranges improves the performance. Smart storage tanks can satisfy variable draw-offs using rule-based algorithms. Research [88] shows that having multiple draw-off nodes can improve performance by 5%. Moreover, smart tanks are often equipped with auxiliary heating units [91] making use of solar, electric, and gas-based heating in different combinations improving reliability, desirable range of temperatures, and performance improvement up to 25%. A generic illustration of a smart tank is shown in Figure 4.

An ideal thermal storage tank must have no heat losses and be able to draw off the same heat as was stored. But realistically stratified tanks are employed which mix water upon draw-off affecting the temperature of the fluid. Floating controllable insulation baffles are integrated as single stratified tanks to maintain the stored heat and be able to mimic ideal behavior. A tank with height and vertical coordinate form an efficiency equation in which numerator denotes real discharge while denominators denote ideal discharge.

For ideal thermal storage (), the temperature should be maintained to value of which means while charging and discharging the fluid, it maintains its temperature by reducing mixing-based heat losses and other losses. Temperature degradation after draw-off is inevitable due to heat exchange rate between storage medium and fluid. One way if improving efficiency extrinsically is to support systems by using auxiliary heating to preheat the tank to . The other way is to mathematically evaluate and design a comparatively larger system that can accumulate additional heat prior to draw off. Mostly, storage tank is comparatively at a higher temperature than surrounding ambient temperature. This large temperature difference forces heat movement as heat losses. To avoid the losses, insulation has to be provided with materials that have low thermal conductivity (<1 W/m.K) and can sustain high temperatures without degradation of the material layer itself or negatively affecting the efficiency of the process. The prime objective is to minimize losses by regulating temperature. A range of organic (cotton, wool, pulp, cellulose, cane, polymers, fibers, etc.) and inorganic (glass, tock, vermiculite, ceramic, etc.) insulation materials exist. The latest trends include nanostructured aerogels, powdered graphite added to polystyrenes lowering conductivity by 20% [36, 92]. Several commonly employed insulation materials are tabulated in Table 1.

5. Mathematical Modeling

Mathematical equations of output parameters of collectors give outlet heat gain of the respective collector. Collector outlet temperature, storage tank outlet temperature, useful energy gains, and heat produced are monitored and used for the calculation of solar fraction () and collector efficiency (). The thermal performance of ETC array may be estimated by the following [93]:

Collector efficiency of a flat-plate collector array is given by the following equation [93]:

Effective reflectance of the compound parabolic concentrating reflector system is defined to be

Temperature of thermosiphon system is investigated by applying Bernoulli’s equation and is given by the following equation [93]:

The flow rate at the collector outlet for evacuated, flat plate, and concentric collectors is equal at inlet and outlet valves:

For the calculation of useful energy gain, the following equation is used:

The energy removal rate from the tank of thermosiphon heating system to supply the load may be given by the following:

The rate of energy transfer from the heat source of thermosiphon heating system to the storage tank may be calculated by the following:

The efficiency of a thermal solar collector is calculated using the Hottel Whillier equation: where is collected heat and is available solar insolation. The solar fraction may be approximated by the following [94]:

Here is the heat produced by collector, and is the supplementary energy required to meet the DHW demand. A simulation software gives average flow rates for a typical heating and cooling system. Solar water heating and space heating is direct use of solar heat and power in winter. A solar photovoltaic and thermal model was developed for combined water and space heating. Two buildings were selected for water and space heating as shown in Figure 5.

Electric and thermodynamic equations for this model may be written as follows [95]:

; therefore, ,

Solar thermal [96]

; therefore, ,

The mass balance equations () for each component of the plant are given by [97]

Solar collector

Heat exchanger #1

Heat exchanger #2

Fan-driven air heater

Water tank

Water pump

6. System Design and Modeling

A solar-assisted water heating system for a typical dwelling house is modeled and simulated in TRNSYS® for the weather of Lahore, Punjab, Pakistan (31.5204° N, 74.3587° E), which also applies to Perth area in Australia. The proposed SWH system comprises an evacuated glass tube solar collector, a fluid filled storage tank with an immersed heat exchanger, and a feed pump to keep heat exchanger fluid flowing in the loop, as shown in Figure 6.

In order to analyze the performance of various types of collectors, it set the initial parameters of the proposed SWH system as a collector area of 5 m2, optimized flow rate of 3 kg/hr.m2, and thermal storage tank of volume 0.3 m3. Water is used as mediating fluid with fluid specific heat 4.19 kJ/kg.K. The hot water inside the thermal storage tank keeps flowing via the load side of the tank. The hot water withdrawing from the water tank passes through an auxiliary heater which warms water at the demanded temperature value for human comfort [98]. During summer in Lahore, ambient temperature is quite high and favorable to harness solar energy for DHW purpose. Hot water usage profile for two to three dwellers of a single family residence was adopted from [99]. Simulations are performed for the metrological data of the whole year (1-8760 hours), and results are presented. Time step for the simulation study is chosen one hour, and the average of monthly values are picked up to represent the output graphs. Parameters selected for system analysis are solar irradiation, collector energy delivery rate, tank energy delivery to load, auxiliary heating rate, collector and tank output temperatures (), solar fraction (), and collector efficiency ().

7. Results and Discussions

For the analysis of the system designed with optimized parameters, output parameters for the calculation of efficiency and solar fraction are selected. To study the efficiency of the collector and the calculation of solar fraction, temperature at the outlet of the collector, and useful energy gain are measured on the hourly basis. In the thermal loop from storage tank to hot water delivery, temperature at the outlet of the storage tank and energy storage rate are observed. DHW energy requirement, auxiliary heating rate, and solar irradiance are the parameters which are used for the system performance in the mathematical model explained above. ETC model is employed from standard TESS library type 71 to analyze solar water heater performance under selected weather. Curves of collector and tank outlet temperatures, solar fraction, and collector efficiency for ETC are shown in Figure 7.

A maximum solar fraction value of 0.5 is achieved in August, resulting a collector efficiency of 54%. Average monthly temperatures at the collector outlet are noted from June to September (60-63°C) during peak of summer in Lahore. Tank outlet temperature which is delivered to load is 97°C during this time. It is the temperature of the fluid that flows from the upper half of the stratified storage tank to be delivered to the load. The maximum value of thermal efficiency was observed during month of September during which month wise average of outlet temperatures of the collector and tank attain the maximum values. Significantly lower efficiency is accomplished in the month of December with reduced solar fraction () at the value of 0.14 only. Maximum thermal energy delivery was recorded during May to September for water heating loop being the peak summer season having maximum sunshine available.

TRNSYS type 73 module performs a perfect flat-plate collector in simulation environment dynamically. Keeping other parameters as fixed, the type 73 is tested for a year round performance under the same weather to record and compare the results. Figure 8 shows the solar fraction and efficiency reaches a peak value of 0.4 in September, and a good collector efficiency of 88% is observed in May. The maximum value of monthly temperature at collector outlet is recorded 51°C during September. The peak monthly value of fluid temperature that flows towards the outlet node from the tank top was recorded 90°C during this time. System has shown maximum efficiency in the September month for hot water delivery to the residents with the opted parameters.

Various meteorological parameters, including temperature of the ambient, speed of wind, and available solar insolation, may significantly affect the efficiency of the system for year round hot water production.

Parabolic curve model concentrating solar collectors reflects the incident solar radiation on the focal line using surface-coated reflecting materials. Incident solar irradiant energy is concentrated towards the receiver, which raises the temperature of the mediating fluid flowing inside. For achieving maximum efficiency from CPC, it may be installed in a longitudinal plane containing the surface azimuth or in a transverse plane at an angle of 90° from the longitudinal plane. For the analysis of the contracting collector’s performance to provide water heating at the domestic level, type 74 module is simulated in TRNSYS simulations. Results of the simulation analysis of CPC collector are shown in Figure 9. The maximum type 74 collector efficiency and solar fractions are noted as 49% and 0.29 during September, respectively. Average monthly temperature at collector outlet is 70°C during September, and the top node temperature of the tank outlet was recorded 91°C. CPC collectors are also used along photovoltaic systems as a hybrid collector, desalination, photo degradation of wastewater, and hydrogen production systems.

Thermosiphon-based system offers more energy efficiency as no need of eternal energy to run the pumps for circulating heat transfer fluid in the loop. Component module type 45a is modeled and simulated in TRNSYS for the analysis of the thermosiphon solar water heating system, and results are presented in Figure 10. Maximum value of collector efficiency and solar fraction are recorded 0.75 and 0.2, respectively, in May, thus exhibiting maximum efficiency of the collector. Maximum monthly temperature at collector outlet is 38°C during June as the peak of summer in Lahore. Tank outlet temperature of the fluid to be provided to load is 52°C during this time.

A comparative graph of solar fraction for all the collector under study is presented in Figure 11. It is observed that the highest value of solar fraction during whole year is attained in case of evacuated glass tube solar collector. This is also validated in literature that evacuated tube-based collectors exhibit greater efficiency on average as compared to flat plate and unglazed collectors [100]. With standard solar radiation available, an EGTC performs 28% more efficiently compared to a flat-plate water heating system [101]. The FPC and CPC stand at second and third number, respectively. Thermosiphon system has shown relatively lower value of solar fraction. Maximum value of is 0.5 in case of ETC, so 50% of the DWH demand is supplied by the solar collector, and an auxiliary heating unit fulfilled the rest of the demand. These results are in accordance with the collector performance, as reported in literature of previous work.

Solar collector efficiency varies from 45% to 49% with a water-based system, 50% to 53% with nanofluids like Al2O3 and 49% to 52% with salty water [102]. Addition of salt in water lowers the freezing point and slightly raises the boiling point. Water heats quickly, yet it has some corrosive actions. Flat-plate collectors show higher efficiency at low temperature and low at higher temperature, whereas conventional evacuated glass tube collectors have moderate efficiencies at all temperatures. [103]. Performance comparison of flat plate and evacuated glass tube is shown in Figure 12.

Evacuated glass tubes were 10 to 15% more expensive than flat plate, yet their prices are falling fast. Flat-tube collectors shed ice easily compared to evacuated glass tubes, yet later perform better in rainy winds because of their lower resistance. Earlier studies suggest evacuated glass tube collectors because of their higher efficiency and wider temperature range [104]. Flat-plate collectors heat from 40 to 70°C whereas evacuated glass tubes heat even better.

8. Conclusions

A comparison to evaluate better performance collector from evacuated glass tube, flat plate, compound parabolic concentrating, and thermosiphon is performed on the simulation grounds. A water heating test rig has been designed, modeled, and simulated in the TRNSYS software under weather of Lahore, which are alike Perth, Australia because of similar latitudes. TRNSYS simulation was conducted in Australia, and experiments were performed in Lahore, Pakistan. System performance is studied for parameter solar irradiation, heating requirement, tank energy delivery, and auxiliary heating demand. From these parameters, a mathematical-based model evaluates solar fraction and collector efficiency to fulfill the DHW demand. This TRNSYS simulation study showed a solar fraction of ETC varied from 0.4 to 0.50 peaking to 0.52 in August. The performance of ETC is better than FPC in high sunshine regions, whereas FPC does not function in freezing cold regions. The efficiency at higher temperatures is better for ETC than for FPC. ETC has lower convection and conduction heat losses compared to FPC. ETC temperature range is 60 to 120 degrees compared 60 to 80 degrees by FPC. ETC continues working down to -18°C where FPC freezes and breaks. It is much easier to replace the glass tube compared to expensive repair of the FPC requiring additional heat exchanger. ETC supplies hot water for an average 350 days per year compared to 300 days by FPC. However, the average life of ETC is 11 to 12 years compared to 25 years for FPC. ETC does not require grouting, which is essential for FPC. Results showed that highest solar fraction and collector efficiency values are attained in case of evacuated tube collector compared to flat-tube collector. This system can provide 50% water heating demand for a residential building with two to three occupants. It shows comprising results during the winter season by providing a considerable amount of hot water during the cold season. It is concluded from results that ETC better suits to ±31° latitude metrological conditions worldwide.

Data Availability

Data are available on request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.


The author acknowledges Swinburne University of Technology for the Postgraduate Research Scholarship (SUPRA).