## 1 Introduction
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 tubesare 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. [PERSON] et al. used thermoplastic natural rubber (TPNR) in a thermosyphon-type SWH, which produced 65 \({}^{\circ}\)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 \({}^{\circ}\)C hot water when operating under 6.11 kW\(\cdot\)h\(\cdot\)m\({}^{-2}\)\(\cdot\)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\(\cdot\)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 laser 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.