## 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 results population resides in regions with annual sunlight radiation rates of between 3.5 and 7.0 kW\(\cdot\)h\(\cdot\)m\({}^{-2}\cdot\)day\({}^{-1}\) or 150 to 300 W\(\cdot\)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 \({}^{\circ}\)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.