A Rectangular Spiral Inward–Outward Alternating-Flow Polymer Thermal Collector for a Solar Water Heating System—A Preliminary Investigation in the Climate of Seri Iskandar, Malaysia

Abstract

A flat-plate unglazed solar water heater (SWH) with a polymer thermal absorber was developed and experimented with. Polymer thermal absorbers could be a viable alternative to metal thermal absorbers for SWH systems. The performance of this polymer SWH system was measured based on inlet and outlet water temperature, water flow rate, ambient air temperature and solar irradiance. The polymer thermal absorbers were hollow Polyvinyl Chloride (PVC) tubes with a 20 mm external diameter and 3 mm thickness and were painted black to enhance radiation absorption. The pipes are arranged in a rectangular spiral inward–outward alternating-flow (RSioaf) pattern. The collector pipes were placed in a 1 m × 1 m enclosure with bottom insulation and a reflective surface for maximized radiation absorption. Water circulated through a closed loop with an uninsulated 16 L storage tank, driven by a pump and controlled by two valves to maintain a mass flow rate of 0.0031 to 0.0034 kg·s⁻¹. The test was conducted under a partially clouded sky from 9 a.m. to 5 p.m., with solar irradiance between 105 and 1003 W·m⁻² and an ambient air temperature of 27–36 °C. This SWH system produced outlet hot water at 65 °C by midday and maintained the storage temperature at 63 °C until the end of the test period. Photothermal energy conversion was recorded, showing a maximum value of 23%. Results indicate that a flat-plate solar water heater with a polymer thermal absorber in an RSioaf design can be an effective alternative to an SWH with a metal thermal absorber. Its performance can be improved with glazing and optimized tube sizing.

1. Introduction

The outer layer of the earth’s atmosphere receives 174 petawatts (PW) of solar radiation [1]. Around 30% of this radiation is bounced back to space, and the remaining 122 PW is absorbed by clouds, oceans and land masses. The amount of solar energy reaching land surfaces exceeds the world’s energy demand. However, due to the intermittent availability of sunshine as well as the low energy conversion efficiencies of solar energy conversion systems, a large quantity of this free energy remains untapped. Most of the earth’s population resides in regions with annual sunlight radiation rates of between 3.5 and 7.0 kW·h·m⁻²·day⁻¹ or 150 to 300 W·m⁻². With this tremendous amount of solar energy reaching the surface of the earth, there should be more efforts to benefit from the most abundant free energy available. Solar energy can be harnessed by various technologies, mainly categorized into photovoltaic and solar thermal. The use of solar energy would significantly reduce dependence on non-renewable energy resources.

Most of the solar irradiation reaching the earth’s ground has a wavelength within the range of 300–2500 nm, which covers UV light (<380 nm), visible light (380–780 nm) and near infrared (NIR) light (>780 nm). The thermal band of solar irradiance can be converted to thermal energy in solid, liquid and gaseous heat transfer fluids such as water, air and nanofluids [2]. Among all of these heat transfer media, liquid allows the easiest transport of energy. One of the simplest techniques is direct water heating that is widely used in residential and commercial sectors. A solar water heater (SWH) system typically comprises solar collectors, a storage tank, heat exchangers and pumps. An SWH system can achieve thermal conversion efficiency of over 80% [3], as compared to around 15–20% for a photovoltaic system [4]. This is because solar energy conversion through heating is more efficient than that through excitation of energy within photovoltaic cells, and the fluid (liquid) allows easy transport. The main factor that can significantly improve the photothermal efficiency of SWH is the convective heat transfer coefficient, which is influenced by many factors, including substance properties, flow behavior, and the temperature gradient between materials in contact [5].

Water heating normally comprises 25% of total household energy usage [6]. In 2010, around 70 million houses worldwide benefited from SWH systems. SWHs are not only environmentally friendly but also require minimal maintenance and operation costs compared to other solar energy applications. The normal SWH investment payback period is between 2 and 4 years, depending on the size and load of the system installed [5]. There are four major classifications of SWHs according to their thermal absorbing mechanisms:

1. Flat-plate collectors [7];

2. Hybrid photovoltaic/thermal collectors producing both electricity and heat energy [8];

3. Evacuated tube collectors;

4. Compound parabolic collectors [9].

Flat-plate solar collectors in particular are generally designed for working temperatures between 40 and 60 °C, making them ideal for domestic hot water applications [10]. Evacuated tube SWHs have the best efficiency and the fastest heating rate but incur the highest initial cost. The global market for solar water heaters was valued at 4.7 billion dollars in 2019 and is anticipated to increase at a rate of 6.1% from 2020 to 2027 to reach USD 6.7 billion [11].

In Malaysia, SWHs are becoming more popular in residential areas due to their ability to reduce household electricity costs. As most SWH systems are equipped with pumps, there are still some electricity costs to power the pump that drives the water circulation. However, the electricity cost to operate an SWH system is much lower than the cost to operate an electric water heater. The range of prices for an SWH system in Malaysia is around MYR 4000 to MYR 10,000 (USD 950 to USD 2400), depending on the capacity of the tank in the system. The price range is considerably higher than that of electric water heaters, which are normally priced between MYR 400 and MYR 900 per unit. Most of the SWHs available to Malaysia’s market are passive flat-plate-type devices equipped with metal heat absorbers. Tubes made of metals such as copper or aluminum are used as heat absorbers due to their high thermal conductivity. The unfortunate downside of metal heat absorbers is that they are more susceptible to internal tube scale formation, which eventually leads to early failure [12]. Over an extended operation duration, as the solubility range of the heat transfer fluid in metal tubes exceeds certain limits, ions leave the solution and adhere to the metal pipe walls. Over time, these ions accumulate to form what appear to be scales all over the inside of the pipe. Scale build-up in the long term can constrict water flow as it decreases the effective pipe diameter. Additionally, scale build-up in the metal pipe of a heat absorber will affect the thermal conductance of the pipe, thereby decreasing the heat absorber’s efficiency.

One way to overcome the high initial cost and maintenance challenge is to use an alternative material for thermal absorber tubes. Among others, polymer tubes, which are widely available and reasonably priced, are a very practical option. Polymers tubes are cheaper, non-corrodible and easy to fabricate into thermal collectors. By optimizing the geometry, layout, fluid flow rate and other related factors, comparable performance can be achieved. Many polymer and plastic materials have been tested for SWH systems. Sopian et al. used thermoplastic natural rubber (TPNR) in a thermosyphon-type SWH, which produced 65 °C hot water with 72% photothermal conversion efficiency [12]. The used of ethylene propylene diene monomer (EPDM), a petroleum-based product, has also been explored [13]. Other polymer materials exploited include polyethylene (PE) and polypropylene (PP), which were used in a thermosyphon SWH system with over 90% maximum photothermal conversion efficiency, producing 60 °C hot water when operating under 6.11 kW·h·m⁻²·day⁻¹ solar irradiance [14]. Polyethylene tubes can replace aluminum tubes with up to 95% thermal efficiency while being four times cheaper to manufacture [15].

Another factor affecting the performance of an SWH is its tube geometry and layout, which should optimize heat transfer fluid residence time in the thermal collector. In addition, the heat transfer rate is increased by elevating turbulence. Typical SWH thermal collector tubes range between 14 mm and 18 mm internal diameter. Collector tube improvements can be achieved by a number of techniques such as twisted tape inserts, perforated twisted tape inserts, wire coil inserts and wire mesh in the flow passage [9]. For example, inserting steel wire coil in the collector tube increased the heat transfer rate by 200% for mass flow rates in the range of 0.011–0.047 kg·s⁻¹, producing a 14–31% increase in average thermal efficiency [16,17]. Inserting helical twisted tape with different twist ratios inside the copper tubes resulted in increased turbulence and thus reduced the collector area requirement by 8–24% for a given efficiency [18]. In terms of tube layout, the most common designs are straight riser tubes, spirals and serpentines.

In this work, a novel tube layout designated “RSioaf” (rectangular spiral inward–outward alternating flow) was developed and tested. The tube layout combined the benefit of using polymer material with increasing water residence time in the thermal collector. This was achieved by optimizing the water flow rate (L/s) and the trajectory of water within the alternating cold and hot flow in the rectangular spiral layout. PVC was selected as the tube material due to its cost and availability. PVC possesses a good maximum temperature range of 75–100 °C and excellent hydrolysis stability. PVC’s poor UV radiation resistance can pose a challenge in the long run [19], but this can be partially overcome with a coating material that prolongs PVC’s life and increases solar absorption.

2. Materials and Methods

2.1. RSioaf Polymer Thermal Collector

The polymer thermal collector designed in this study is a system that consists of several components. The most crucial component of the SWH system is the rectangular spiral inward–outward alternating-flow (RSioaf) polymer thermal absorber. The polymer thermal absorbers are made from hollow Polyvinyl Chloride (PVC) tubes with 20 mm external diameter and 3 mm thickness. The rectangular spiral is made by connecting 62 straight tubes at various lengths with elbows. The tubes are cut to various lengths and inter-connected with elbows using a special plastic glue—VT-300 PVC Solvent Cement. To form the RSioaf patterns, the pipe is connected initially from both the inlet and outlet pipe, working inward to the middle of the pattern. The distance between adjacent tubes centers is 4 cm. The resulting total length is 24 m. It can be deduced that if more tubes are filled within the collector, higher thermal absorption can be achieved, but this is not the case for this design. The distance between adjacent tubes was chosen for fabrication practicality and balance between material quantity/cost and effective area for thermal absorption. High resistance to water flux is expected in a tube of such length; thus, pushing water through with a pump is required. To further aid heat transfer, the tubes are coated with black paint—“Red Fox” Acrylic Spray Paint No. 39 (210)—to ensure an effective heat radiation absorption. A 2-D drawing of the RSioaf design, including the direction of water flow in the tubes, is shown in Figure 1. The solar collector enclosure that houses the RSioaf polymer thermal absorber is a 1 m² square box made from plywood. The bottom of the box is insulated with polystyrene foam and covered with a zinc sheet reflector to maximize thermal absorbance by reflecting the incoming solar radiance to the lower part of the tubes as shown in Figure 2.

Table 1. Parameters specification and comparison with a typical copper tube collector.

Parameter	Polymer Tube	Metal Tube 1
Absorber tube material	Polyvinyl chloride	Copper
Absorber tube design	Rectangular spiral inward–outward alternating flow	Serpentine
Dimension of absorber plate	1.0 m2	0.6 m2
Pipe inner diameter	14 mm	10 mm
Pipe thickness	3 mm	1.5 mm
Glazing material	None	Tempered glass
Insulation	PVC foam	Glass wool and cork sheet
Collector tilt angle	4.5°	27°
Absorber thermal conductivity	0.19 W·m−1·K−1	401 W·m−1·K−1
Ambient temperature	27–36 °C	30 °C
Solar irradiance	105–1004 W·m−2	900 W·m−2

¹ Data from previous work on similar subject [20].

lists some of the main parameters of the tested polymer SWH system tested with reference to a copper tube serpentine SWH system that was previously researched [20].

2.2. Experimental Setup and Procedures

The schematic of the system experiment is shown in Figure 3. The physical setting is shown in Figure 4. The flat-plate collector was mounted on the stand, inclined at 4.5° from horizontal, facing south, corresponding to the location of the test (Seri Iskandar, Malaysia 4.3590° N, 100.9849° E), thus focusing on direct solar irradiance at noontime. The system utilized a closed-loop water circulation with an uninsulated 16 L storage tank, powered by a 0.5 hp centrifugal pump. Two valves, at the inlet and outlet of the water tank, were used to maintain a mass flow rate in the range of 0.00318–0.00345 kg·s⁻¹ which is consequently measured via flowmeters connected to the inlet and outlet of the flat-plate collector. Initially, the RSioaf tubes were filled with water and connected to the pump and storage tank. As the pump was powered, water flowed from the collector into the pump suction and was discharged towards the storage tank.

Solar irradiance data were measured at every 1 min interval using a photodiode pyranometer. The inlet and outlet water temperatures as well as the ambient temperature were simultaneously monitored by k-type thermocouples. The volume flow rate of the water was measured by turbine flowmeters (OMEGA model FLT-40/C/05/W/S/N, Signapore) at the thermal collector inlet and exit. The entire data acquisition process was overseen using a Hioki MEMORY HiLOGGER LR8431 (Ueda, Japan) data logger and subsequently processed on a desktop computer. The experimental period, which took place under partially cloudy skies, spanned from 9 a.m. to 5 p.m. in April 2023 at Universiti Teknologi PETRONAS Solar Research Park. The data were averaged for each hour, reported and analyzed accordingly. Throughout the experiment, the tropical climate exhibited solar irradiance levels ranging between 105 and 1004 W·m⁻², with ambient air temperatures fluctuating between 27–36 °C, as illustrated in Figure 5. The ambient temperature remained relatively high even during periods of lower irradiance in the afternoon, attributable to significant cloud shading during those intervals.

2.3. Energy Model

The energy absorbed by the solar collector is given in Equation (1). This equation calculates the energy Q_a absorbed by the thermal collector, it is measured in units of energy (joules or kilojoules). The effect of reflector is considered proportional to the gap between tubes; therefore, the total area of absorption was estimated as the area of thermal collector box. The magnitude of Q_a is determined by the product of the water mass flow rate m, the specific heat capacity of water c_p and the temperature difference between the outlet temperature T_o and the inlet temperature T_i. The mass flow rate, measured in kilograms per second (kg·s⁻¹), determines the rate at which heat is transferred from the collector to the water. Higher flow rates can increase the efficiency of heat transfer but may require more energy to pump the water. Whereas the specific heat capacity, measured in joules per kilogram-degree Celsius (J·kg⁻¹·°C⁻¹), determines how much heat energy is required to raise the temperature of the water in the collector. The temperature difference, T_o − T_i, measured in degrees Celsius (°C), indicates the amount of heat absorbed by the water as it flows through the collector. A larger temperature difference implies more heat absorption. This formula represents the basic principle of energy transfer in the collector, where the water absorbs heat as it passes through the collector, ultimately contributing to the overall thermal efficiency of the system.

The photothermal conversion efficiency, ɳ_th, is derived from Equation (2). It is calculated based on the heat absorbed by the collector (Q_a) and the solar energy received by the collector is calculated by multiplying the solar irradiance (Is) with thermal collector surface area (Ac). Efficiency is dependent on several factors; F_R is the heat removal factor, representing the fraction of absorbed heat that is effectively transferred to the working fluid. It is influenced by factors such as the collector design and operating conditions. τ is the emissivity of the glazing material, which affects the amount of heat retained by the collector (1 in the case of an unglazed system). A lower emissivity means that the material retains more heat, increasing the efficiency of the collector. α is the absorptivity of the thermal collector tube, indicating the fraction of incident solar radiation absorbed by the collector. A higher absorptivity means that the collector absorbs more solar radiation, increasing its efficiency. U_L is the overall heat loss coefficient, representing the efficiency of heat transfer from the absorber surface to the working fluid. It depends on the collector’s design and fluid properties. Ultimately, the equation shows how the efficiency of the system is influenced by factors related to the collector’s design, operating conditions and environmental factors.

$$Q_a=\dot{m}{c}_p(T_o-T_i)$$

(1)

$${\eta }_{th}=F_R\tau \alpha -F_R{U}_L\left({\displaystyle \frac{T_i-T_a}{I_s}}\right)={\displaystyle \frac{Q_a}{I_s{A}_c}}$$

(2)

3. Results and Discussion

The hourly recorded water temperatures at the collector inlet and outlet, as illustrated in Figure 6, provide insight into the system’s thermal behavior. Starting from 26.4 °C at the inlet and 28.1 °C at the outlet, the temperatures steadily rose, reaching peak values of 61.1 °C and 65 °C, respectively, at 1:00 p.m. Subsequently, both temperatures remained relatively constant at around 65 °C for the duration of the test, indicating a thermal equilibrium where heat gain and loss were balanced. The narrowing temperature difference between the inlet and outlet temperatures is a result of the closed-loop water circulation nature of the system, demonstrating its ability to maintain stability in operation.

Figure 7 depicts the mass flow rate of water at the thermal collector inlet throughout the test period. Ranging from 3.18 g·s⁻¹ to 3.45 g·s⁻¹, the flow rate is inversely related to water temperature. As temperatures increased, the mass flow rate decreased due to water expansion within the system. This phenomenon reduces both the amount and rate of heat absorption, contributing to a drop in efficiency at higher water circulation temperatures. Careful adjustment of the flow rate can lead to higher photothermal conversion efficiency. The control strategy is not developed for this experiment.

The photothermal conversion efficiency, shown in Figure 8, was calculated based on a closed-loop water circulation system, allowing for gradual heating of the water over the course of the day. Efficiency peaked at 24% in the first hour, gradually declining as the day progressed. This decline can be attributed to the narrowing temperature difference between the inlet and outlet temperatures over time, coupled with increasing solar irradiance towards midday. After 1:00 p.m., the efficiency gradually declined and nearly reached zero by 2:00 p.m. This decrease was largely due to minimal temperature differences between the inlet and outlet, as shown in Figure 6. When the temperature difference approaches zero, the energy absorbed by the collector (Qa) also approaches zero, leading to an efficiency of zero according to Equation (2). Although using metal tube may reach a similar maximum temperature more quickly due to its absorptivity and thermal conductivity, achieving the same efficiency with a polymer tube is feasible with appropriate mass flow rate adjustments. Increasing the residence time of water in the collector tube could lead to a higher collector outlet temperature. Since the experiment was performed in a closed-loop water circulation, the maximum temperature was reached at a faster rate as compared to an opened-loop water flow with a fixed inlet temperature. Additionally, the absence of a transparent cover, called “glazing”, limits the system’s performance. With glazing, more heat could be retained in the collector, enhancing heat transfer from the enclosed air to the collector tubes.

The collector efficiency η_th plotted against (T_i − T_a)/I_s as shown in Figure 9. The slope of this curve, −F_RU_L, explained in Equation (2), represents the rate of heat loss from the collector. Since this collector is unglazed, steeper slopes can be seen especially at the small temperature difference between inlet water and ambient temperature. The maximum collector efficiency, F_Rτα, significantly depends on the optical properties of the collector. The moderately selective black paint applied on the thermal absorbers improved the overall thermal efficiency due to increased radiation absorption.

Overall, this polymer tube solar water heater has demonstrated comparable performance to devices using metal tubes. Even though the thermal conductivity of PVC is 0.05% of that of copper, it can still produce hot water at a temperature of between 60 °C and 70 °C, which is a typical temperature setting for household applications. Increasing water residence time in the thermal collector through spiraling and alternating cold and hot water flow may be the design advantage that enabled RSioaf to produce the observed results. The photothermal conversion efficiency can be further expanded and improved with open-loop water circulation, where inlet water is kept constant around ambient temperatures. This will lead to a more comprehensive characterization of this renewable energy system.

The material used in the work was polyvinyl chloride (PVC), which, in addition to being used as a thermal collector for water heating applications in a tropical climate, is also a widely used plastic material in construction and plumbing in general. Generally, up to two years of use in this condition, it shows no significant changes in its physical properties. Further use results in deterioration such as discoloration, decreased strength and reduced flexibility [21]. However, with black paint coating application as in this work, we expect that the deterioration can be delayed. Further, in the next phase of the project, a synthetic rubber—EPDM or polypropylene (PP)—will be used; a previous study suggests that they can last up to 50 years [22]. They will be mixed with metal oxide to yield better thermal properties and functional durability. Additionally, the 0.5 hp pump will be replaced with a 72 W solar powered pump, which has been verified to be able to provide the required water flow rate. Finally, further parametric and geometric refinement including optimization of tube size, inclusion of glazing and regulation of water flow will be executed to ensure the high system efficiency of this solar-powered water heating system.

4. Conclusions

A solar water heater with a polymer thermal absorber (PVC tube) has been briefly tested and verified. The tubes are arranged in a novel design called rectangular spiral inward–outward alternating flow (RSioaf), with the aim of increasing water residence time in the thermal collector and providing a base for well-distributed water heating. Despite low thermal conductivity compared to copper tubes, this solar water heating system produced hot water up to 64 °C, which is suitable for household applications. At a flow rate range of 3.10 g·s⁻¹ to 3.45 g·s⁻¹, thermal conversion efficiency up to 23.94% was achieved. This work indicates that using PVC, a polymer thermal absorber, for solar water heaters is a practical solution for a cost-competitive and technically simpler option. The performance of this system at the preliminary stage has shown promising results. With further refinement, by optimizing the flow rate, installing glazing and optimizing the tube material selection and sizing, the system is expected to serve in its intended application of household hot water generation.