https://orcid.org/0000-0001-6606-5411
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ABSTRACT
One of the most extensively used forms of energy is solar energy. It is a widely utilized, low-cost, eco-friendly, sustainable, emission-free renewable energy source, and it reduces power costs from coal, electricity, and other sources. Solar energy is utilized for several reasons, such as water heating in homes and industrial applications using solar collectors. Unlike other solar collector energy applications, flat plate solar collector techniques for water heating offer low maintenance and operational expenses. This article reviewed the innovations of flat plate solar collectors for improved thermal performance with reflectors in solar water heaters. The impact of combining flat plate solar collectors and reflectors for thermal performance improvements is discussed in this article. Integrating flat plate solar collectors using reflectors is an effective and cost-effective approach among the numerous geometries utilized for thermal performance improvements. In order to improve SWH performance, researchers have implemented a number of modifications so far. These modifications are associated with various types of reflector integration in SWH such as booster mirror reflector, plane reflector, side reflector, bottom reflector, top-edge reflector, inclination angle of reflector, gap between reflector and collector, trapezoidal reflector, parabolic reflector, polyline reflector etc. The major goal of this paper is to evaluate several studies wherein different reflectors have already been utilized to improve thermal performance and to recommend a design for the best performance. Furthermore, the research gap has been identified, along with suggested upgrades for future study, which should benefit researchers in extending the growth of this technology.
Nomenclature
| Ac | = | surface area of solar collector m2 |
| Cp | = | specific heat of HTF, (kJ/(kg·K)) |
| FR | = | heat removal factor; |
| G | = | global solar radiation intensity, (W/m2) |
| Gt | = | total solar radiation intensity (W/m2) |
| lg | = | gap length between collector and reflector, (m) |
| M | = | mass flow rate of HTF, kg/s |
| Q | = | rate of energy, (W) |
| Q_aux | = | auxiliary input to the tank W |
| Q_sol | = | solar input to the tank W |
| Qin | = | intensity of sun radiation acknowledged by the solar collector W |
| Qt | = | energy transferred from solar coil to storage tank W |
| Qu | = | the useful energy absorbed or gained by the solar collector W |
| Tavg | = | collector average temperature (°C) |
| Tcoll, in | = | temperature of entering water on collector (°C) |
| Tcoll, out | = | temperature of the water out at collector (°C) |
| TScoil, in | = | inlet temperature of solar coil (°C) |
| TScoil, out | = | outlet temperature of solar coil (°C) |
| UL | = | overall heat loss coefficient of collector, W/m2-K |
Greek Symbols
| Gdir_col | = | direct solar radiation on collector surface (W/m2) |
| Gin_r1 | = | incident solar radiation on the bottom reflector (W/m2) |
| Gin_r2 | = | incident solar radiation on the top reflector (W/m2) |
| Gin_sr1 | = | incident solar radiation on the left side reflector (W/m2) |
| Gref_r1 | = | reflected solar radiation from the bottom reflector which reached the collector surface (W/m2) |
| Gref_r2 | = | reflected solar radiation from the top reflector which reached the collector surface (W/m2) |
| Gref_sr1 | = | reflected solar radiation from the left side reflector which reached the collector surface (W/m2) |
| α | = | absorption coefficient of plate |
| α | = | solar altitude angle (o) |
| α1 | = | angle of the bottom reflector to the horizontal plane (o) |
| α2 | = | angle of the top reflector to the vertical plane (o) |
| β | = | collector’s tilt angle (o) |
| ε | = | emittance |
| ηcoll | = | collector efficiency |
| ηsys | = | system efficiency |
| θc | = | angle of collector from horizontal |
| θm | = | angle of the reflector from horizontal |
| τ | = | transmission efficiency of glazing |
| ϒ1 | = | angle of the left side reflector to the horizontal plane (o) |
| ϒ2 | = | angle of the right side reflector to the horizontal plane (o) |
| χ1 | = | incident angle from the bottom reflector (o) |
| χ2 | = | incident angle from the top reflector (o) |
| ϛ | = | off-South angle, deg. (0° to ±180°) |
Subscripts
| a | = | ambient |
| aux | = | auxiliary |
| coll | = | collector |
| in | = | input |
| out | = | output |
| s coil | = | solar coil |
| sol | = | solar |
| sys | = | system |
Abbreviations
| ETSC | = | evacuated tube solar collector |
| FPSC | = | flat plate solar collector |
| FPSWH | = | flat plate solar water heater |
| GHG | = | greenhouse gases |
| HTE | = | heat transfer enhancement |
| HTF | = | heat transfer fluid |
| ICS | = | integrated collector storage |
| MFR | = | mass flow rate |
| PDR | = | parabolic dish reflector |
| PTC | = | parabolic trough collector |
| RE | = | renewable energy |
| SC | = | solar collector |
| SWH | = | solar water heater |
| SWHS | = | solar water heating system |
| TE | = | thermal efficiency |
| TFI | = | thermal performance improvement |
Disclosure statement
No potential conflict of interest was reported by the author(s).