Q4Non-Conventional Energy Sources

(a) Solar Cell vs Solar Cell Array; Photovoltaic Power Generating System

A solar cell (photovoltaic cell) is the basic, individual semiconductor device (typically made from crystalline or thin-film silicon, or other photovoltaic materials) that directly converts incident solar radiation into DC electrical energy through the photovoltaic effect, producing a relatively small voltage (approximately 0.5-0.7V for a typical silicon cell) and a current proportional to its area and the incident irradiance. A solar cell array (module/panel, and further combinations of modules into larger arrays) is formed by electrically connecting many individual solar cells in series (to build up a usable output voltage) and in parallel (to build up the required output current), packaged together with protective encapsulation, glass cover, and a supporting frame, to achieve a practical output voltage and power level suitable for a real application - so while a single cell might produce only about half a volt, a series-connected module of 36 or 60 cells produces a standard 12V or higher nominal output voltage suitable for practical charge-controller and inverter systems.

A basic photovoltaic power generating system consists of the PV array (generating DC power proportional to incident sunlight), a charge controller (regulating the charging of a battery bank, preventing overcharge or deep discharge damage), a battery bank (storing energy for use during periods of insufficient sunlight, essential in standalone/off-grid systems), and an inverter (converting the stored/generated DC power to AC power at the standard grid voltage and frequency, for use by conventional AC loads or for grid-tied export in a grid-connected system) - grid-tied systems, increasingly common for rooftop and utility-scale solar, often omit the battery bank entirely (or use it only for optional backup), instead exporting excess generated power directly to the grid and drawing from the grid when solar generation is insufficient, relying on the grid itself as the effective energy buffer rather than an on-site battery.

(b) Solar Pond

A solar pond is a body of saline water engineered to absorb and store solar thermal energy at its bottom layer by suppressing the natural convective heat loss that would otherwise occur in an ordinary pond, achieved by establishing a stable, increasing salinity (and hence density) gradient with depth - since warmer water is naturally less dense and would normally rise to the surface and lose heat to the atmosphere by convection, the deliberately imposed density gradient (denser, saltier water at the bottom) prevents this convective mixing even though the bottom layer becomes significantly hotter than the surface, allowing the pond to function as a long-term, large-volume, low-cost solar thermal energy collector and storage medium simultaneously.

A typical solar pond has three distinct zones: an upper convective zone (a thin, relatively fresh, well-mixed surface layer at near-ambient temperature), a non-convective gradient zone in the middle (where salinity, and hence density, increases steadily with depth, acting as the transparent insulating layer that both allows sunlight to pass through to the bottom while preventing convective heat loss back to the surface), and a lower convective zone (a dense, highly saline, well-mixed bottom layer where solar heat accumulates and is stored, reaching temperatures of 70-90 degrees Celsius even while the pond surface remains near ambient temperature).

• Applications: low-temperature process heat for industrial and agricultural drying, space heating, desalination, and, at larger scale, driving low-temperature organic Rankine cycle turbines for electricity generation.
• Benefits: solar ponds provide both collection and long-duration (seasonal) thermal storage in a single, low-cost structure without requiring separate collectors or storage tanks, and can supply heat even during periods of low or no sunlight by drawing on the stored thermal energy in the bottom layer.
• Drawbacks: solar ponds require large land area and a substantial volume of water and salt, ongoing maintenance to preserve the salinity gradient against gradual diffusion and mixing, water losses due to evaporation requiring periodic replenishment, and achieve comparatively low overall thermal-to-electrical conversion efficiency when used for power generation due to the relatively low bottom-layer temperature compared to concentrating solar thermal systems.

A closely related distinction to flat plate versus concentrating collectors is that between solar photovoltaic (PV) conversion and solar thermal collection: PV cells convert sunlight directly into electricity via the photovoltaic effect within a semiconductor junction, with no intermediate heat-engine stage, whereas flat plate and concentrating collectors instead collect solar energy as heat, which may be used directly (water/space heating) or further converted to electricity through a conventional thermodynamic heat-engine cycle - PV systems are consequently simpler (no moving fluid, turbine, or heat exchanger required) and scale down to very small sizes (a single small panel) far more gracefully than solar thermal power generation, which generally requires a certain minimum plant scale to achieve reasonable thermodynamic efficiency.

The efficiency of a practical silicon PV cell (typically in the range of 15-22% for commercial modules, with the highest-efficiency laboratory multi-junction cells exceeding 40% under concentrated sunlight) is fundamentally limited by the Shockley-Queisser limit, which arises because a given semiconductor material's bandgap can only efficiently convert photons within a certain energy range into usable electrical energy - photons with energy below the bandgap are not absorbed at all, while photons with energy above the bandgap have their excess energy lost as heat rather than contributing additional useful voltage, a fundamental physical constraint that multi-junction (tandem) cells partially overcome by stacking several different-bandgap semiconductor layers to capture a wider range of the solar spectrum.

In practical off-grid PV system design, the battery bank sizing (in terms of both capacity and depth-of-discharge rating) must be carefully matched to the expected load profile and the number of consecutive low-sunlight (autonomy) days the system must ride through, since undersizing the battery leads to load shedding during cloudy periods while oversizing unnecessarily increases system cost - lead-acid batteries have traditionally been used for their low cost, though lithium-ion batteries are increasingly favored in modern systems for their higher usable depth of discharge, longer cycle life, and better performance across a wider temperature range, despite their higher upfront cost per unit of storage capacity.

It is also worth noting the distinct wiring configuration flexibility that solar cell arrays provide compared to individual cells: because array modules can be connected together in further series and parallel combinations (strings of modules in series to reach a higher system voltage, and multiple such strings connected in parallel to reach the required total current/power), a photovoltaic power generating system can be readily scaled from a small few-hundred-watt residential rooftop installation up to a utility-scale multi-megawatt solar farm using the same basic module building block, simply by combining a correspondingly larger number of series-parallel module strings, illustrating the inherent modularity and scalability that has been a major factor in the rapid global cost reduction and deployment growth of photovoltaic technology over the past two decades.