When ‘Solar Energy’ is mentioned, people immediately think PV [Photo-voltaic], PV though is but tip of the iceberg…

Solar Energy from Irradiation

Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy including solar water heating, and solar architecture.

Wiki

As previously mentioned, Biomass is solar energy that is stored in plants through Photosynthesis and serves as the primary source of Hydrocarbons such as coal and petrol. Hydro energy is also derived from Solar Energy and the water cycle. Wind energy, along with most other renewable energy sources, also originate from the sun, which acts as a significant Nuclear Fusion Power Station emitting radiation that we are constantly exposed to.

The average annual solar radiation arriving at the top of the Earth’s atmosphere is roughly 1361 W/m2. The Sun’s rays are then attenuated as they pass through the atmosphere, leaving normal surface irradiance at approximately 1000 W/m2 at sea level on a clear day. This is the figure used when testing Solar Equipment to a fixed standard, as irradiation is different as shown in comparative image below. But lets first just explain some acronyms [abbreviations] used in solar:

• DNI, or Direct Normal Irradiance: This is the amount of solar radiation received per unit area by a surface that is always held perpendicular (or normal) to the rays that come in a straight line from the direction of the sun at its current position in the sky.
• DIF, or Diffuse Horizontal Irradiance: This is the terrestrial irradiance received by a horizontal surface which has been scattered or diffused by the atmosphere. It is the component of global horizontal irradiance which does not come from the beam of the sun (where “beam” is a 5° field of view concentric around the sun).
• GHI, or Global Horizontal Irradiance: This is the total amount of shortwave radiation received from above by a surface horizontal to the ground. This value is of particular interest to photovoltaic installations and includes both Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DHI).

The Southern hemisphere receives higher levels of irradiation, potentially due to the larger ratio of ocean to continent, or as a result of the “Ozone Hole” over Antarctica. Solar energy is typically categorized into Solar Thermal and Solar PV. However, it is important to note that solar energy production can be influenced by various factors such as weather, landscape, seasons, and orientation. For optimal efficiency, solar collectors should be positioned perpendicular to the incoming sunlight (DNI – Direct Normal Irradiation).

There are two main variations to consider: seasonal and daily. Seasonally, the sun’s movement shifts from North to South and back again, resulting in rays that hit the Earth at different angles in winter versus summer. On a daily basis, the sun rises in the East and sets in the West. Although tracking these movements can be complex, Isaac Newton developed an equation to calculate it, making it mathematically possible. This complexity is further heightened in Solar Power Towers, where thousands of mirrors must align to focus on a single point. Google has even become involved in this technology.

To implement solar thermal technology in your household, position the collector at an angle equal to your latitude, facing the opposite hemisphere. For instance, if you are in Cape Town at a latitude of 34° South, angle the collector 34° from horizontal facing North. If there is no space for a North or South-facing collector, consider positioning it in the Northwest (or Southwest in the Northern Hemisphere) to take advantage of the sun’s increased intensity in the afternoon.

For PV, the angle is reduced by 15° (flatter) from the location latitude in Summer. And in the Winter by adding 15° to the latitude (more upright). Seasonal angle adjustment can increase annual power output by up to 15%.

Solar Energy – Thermal

Solar thermal is diverse, but basically collecting the heat generated by Irradiation, a quick list is:

Ranges for temperatures as listed above are:

• Low – Temperature, this can basically considered ambient temperature up to 40C. Mostly used for space heating and will discuss this in Geothermal – as a good combination.
• Medium – Temperature, this the normal residential Solar Water Heaters, etc. up to 95C
• High – Temperature, this is basically above water boiling temperature, >95C. These systems are mostly for utility power generation.

Solar Water Heating

This opportunity is a major “low-hanging fruit” and should be considered as a top priority project if you have not implemented it yet. In South Africa, there was an initiative sponsored by Eskom to install solar water heaters, and similar programs may exist in your country as well. The economic benefits of this switch are clear – instead of using electricity to heat water, you can harness the power of the sun. Solar water heaters generally fall into two broad categories.

1. Active solar water heaters. Active solar water heaters use pumps [that the ‘active’ bit] to circulate water or some other fluid from the solar collectors, where it is heated by the sun, to the insulated storage tank, where the water remains until needed.
   An Active systems fall into two general groups based on freeze protection:
   * those using a fluid with a low freezing point (such as propylene glycol) in the collector loop
   * those using water in the loop, that is automatically drained when the sun is not shining.
2. Passive solar water heaters. Passive solar water heaters, which rely on gravity, are typically either integral collector/storage (ICS) systems or thermosyphon systems. The major advantage of these systems is that they don’t use controls, pumps, sensors, or other mechanical parts. This means that only minimal maintenance is required during their lifetime.
   They are less expensive than active solar systems and can only be used in warm sun-belt climates. Also, the roof structure must be able to support the load of the storage tanks used in passive systems.

NOTE: Because heat rises in Passive Systems the geyser [or hot water cylinder] must be installed above the SWH, hence for most retrofitting purposes must install an active system

Solar Energy collectors

In rising efficiencies, and cost:

Solar Pipe collectors

This is the simplest system, remember opening the garden hosepipe in summer… These collectors are the same principle, often used for swimming pools as can easily be integrated with the pool pump, and are basically just black pipe coiled on roof. Beware, water is heavy. For this image as shown the wood box can be painted black before installing pipe and the box can also be glazed with glass to retain heat.

Flat-plate collectors

A flat-plate collector is basically a panel-shaped box containing tubes mounted on a dark-coloured absorber with glazing to retain heat – and an upgraded form of solar pipe collectors. Suitable for both residential and non-residential use, it can also operate well in a humid climate, where haze creates more diffuse, rather than direct, sunlight. Another advantage is because ‘flat plate’ the volume of water, and hence wight, is less than the solar pipe collectors.

Evacuated-tube.

The evacuated tube collector (ETC) consists of a number of sealed glass tubes which have a thermally conductive copper rod or pipe inside a glass tube under vacuum (Evacuated) allowing for a much high thermal efficiency and working temperature compared to the flat plate solar collectors even during a freezing cold day. In evacuated-tube collector, a manifold heat exchanger holds the fluid to be heated, hollow copper tubes conduct solar heat to the manifold.

Each tube is surrounded by an outer glass tube, and a vacuum between the inner and outer tubes provides good insulation to reduce heat loss. The collector operates at high temperatures with high efficiency using direct and diffuse light. Like a vacuum flask for storing your hot water, this also helps this system to retain the heat, such as at night or in cold climates.

Parabolic collectors

A parabolic collector, aka ‘Solar Cookers‘, consists of a parabolic mirror that focuses the sun onto a spot in the centre of collector (called the focal point), almost like a magnifying glass. The cooker is moved according to sun position to keep concentrate the heat on bottom of pot. This ‘focussing’ of the suns rays is due to the parabolic shape of the reflecting dish.

Parabolic trough collectors are a modification of this – where long U-shaped parabolic troughs concentrate heat on a pipe mounted at the focal point – running the length of the trough. This highly efficient system typically tracks the sun and requires direct, not diffuse, sunlight.
The major use has been non-residential or institutional applications such as prisons and hospitals.

Solar Thermal system efficiencies

SWH do not operate as other RE devices as they do not make electricity. They can be used in powering a thermal generator with Parabolic devices. So difficult to judge this as they all will be in the 80% range! How they keep this energy though depends on insulation.

Glazing

To begin, apply black metal fireplace paint to the interior of the system, then cover it with a layer of glass. When exposed to high frequency sunlight, the black paint will absorb the light and heat up, emitting low frequency heat waves. However, these heat waves are reflected by the glass. Evacuated tubes enhance this glazing effect even further by creating a vacuum between the collector and outer glass. In colder regions with chilly nights, evacuated tubes can be used to trap heat as the vacuum prevents heat from escaping, similar to the concept of double glazing.

So basically, with the pipe on the roof scenario you will be looking at about 80% efficiency, but only for a limited time, as soon as the pipe heats up it will start radiating the collected energy out to ambient. So better to specify by temperature differences between the water in pipe and ambient:

• Unglazed, best for temperature differences of 0-10C
• Glazed, best for temperature differences of 10 – 50C above ambient
• Evacuated Tube, best for more than 50C temperature difference

Payback Time:

With collection possible between 9h00 and 15h00 on average of 6 hours per day and using augmented 1000 W/m2 Irradiation figure above. In South Africa a flat plate solar collector is sufficient and costs circa R5000 for 2.5m2 Kwikot system – these items should be stocked by your local geyser manufacturer.

• Cost of SWH: R5000
• Collection area: 2.5 m²
• Efficiency taken at 80%, Operational time of 6 hours/day
• Energy stored/year: 4380 kWh
• Electrical cost @ R3.30/kWh: R14 454
• Payback: 3½ months

It should be noted that geyser not included in price above, many new geysers have solar connections so easy to retrofit. For a system with geyser [hot water cylinder to rest of world?] the payback period is still within a year.

Solar Energy – PV

French physicist Edmond Becquerel first observed the ability of certain materials to generate an electrical charge when exposed to light in 1839. The early solar panels created were not efficient enough for basic electrical devices and were primarily used for light measurement. Progress was made sporadically until Bell Labs developed the Silicon chip, leading to significant advancements in solar technology. NASA became involved as solar power was essential for space missions where traditional energy sources were not viable. PV technology became the primary energy generation method in space due to its effectiveness in the absence of air.

PV ‘Flow Control’

A solar panel, also known as a solar electric panel, photo-voltaic (PV) module, or solar cell panel, is a collection of photo-voltaic cells mounted in a framework for installation. Multiple PV modules make up a PV panel, and a group of PV panels forms a PV array. These arrays supply solar electricity to electrical equipment. In terms of structure, PV cells, Light Emitting Diodes (LEDs), and Integrated Circuits (ICs found in laptops) are quite similar as they all operate on DC power.

Shade is detrimental to the proper functioning of PV systems. If even just one cell is shaded, it will produce no power and can create a short circuit that affects the entire panel’s output. The solution to this issue is the use of diodes, which act as one-way flow controls for electrons, ensuring that power can only flow out of the PV module.

Solar energy and Photo-Voltaic Panel Efficiencies:

Photo-Voltaic cells have different efficiencies, with from 15% to 50%:

PV Panels is slightly different in that already assembled batch of PV cells. PV cells have different efficiencies, but put together on a Panel the overall panel has a nameplate power rating. So a more efficient panel will just be smaller, and vice versa. Power rating normal done at standard irradiation of 1000W/m²

Photo-Voltaic Panel types

For the average household installation there are 3 types of PV panels:

1. Monocrystalline Solar Panels, aka single-crystalline cells: Manufactured from pure silicon. Like computer chips in the most stringent/cleanest of conditions. Produce a long rod. Solar cells then made by cutting the rod into wafers that will make the solar cells. Monocrystalline solar panels deliver the highest efficiency in standard test conditions, when compared to the other 2 types of solar cells. The current delivered monocrystalline solar panel efficiency stands at 22-27%. You can recognise a monocrystalline panel by the rounded edge and the dark colour.
2. Polycrystalline Solar Panels, aka multi crystalline cells: Solar panels made of polycrystalline solar panels are slightly less efficient than those made up of monocrystalline solar cells. This is due to the nature of production; Polycrystalline are often ‘offcuts’ from the monocrystalline manufacturing melted together again. The current delivered polycrystalline solar panel efficiency stands at 15-22%. You can recognise a polycrystalline solar panel by the square cut and blue speckled colour.
3. Thin Film Solar Panels: A thin-film solar cell is a second generation solar cell. Made by depositing one or more thin layers, or thin film (TF) of photovoltaic material on a substrate, such as glass, plastic or metal. Thin film solar panels are usually flexible and low in weight. Production of this kind of panels is less complex, but their output is 5% less than monocrystalline solar panel efficiency. Normally, thin film cells deliver between 15-22% solar panel efficiency.

Thin film solar panel technology has closed the efficiency gap with crystalline types of solar panels. Due to lower cost, thin film solar panels are often installed on large scale projects. Record-breaking solar power plants too.

Solar Energy PV Panel Payback

Looking at a 23% High Efficiency flexible 200W kit with controller, so nameplate power is 200W, but determining the actual power output quite complex. Discussed more in next sections – but historically we know that Power Output Factor for Cape Town is 18%. This is not panel efficiency, but anyway – so in a year the power generated is 0.18×0.2x24x365 = 315 kWh.

Cost of 200W PV flexible panel kit with associated controller: R5 198

EMS Courier service: R621

Vat@15%: R874

Total Panel Cost: R6 692

Yearly energy generation @ R3.30/kWh: R1040

Payback: 6.4 years

In my opinion, PV systems may not be sustainable due to their reliance on rare, poisonous, and expensive materials such as Gallium Arsenide, which is known to be carcinogenic. There is concern that we may be repeating the same pattern seen with lithium and lithium phosphate batteries, which are projected to be depleted within the next 10 years.

NEXT: Wind, the HYBRID Factor

PREVIOUS: Biomass Energy

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