Chapter: Energy from the Sun
Welcome to your study notes on Energy from the Sun! In this chapter, we will explore how we capture energy from the most abundant renewable resource available to our planet: the Sun. Whether you are aiming for top marks or just trying to get your head around the key concepts, these notes break down everything step-by-step.
By the end of this chapter, you will understand:
• How solar radiation reaches the Earth and what factors influence its intensity.
• How Solar Thermal Systems capture heat for domestic hot water.
• How Solar Photovoltaic (PV) Systems generate clean electricity.
• How Passive Solar Design uses architectural features to heat and cool buildings naturally.
1. Understanding Solar Radiation
What is Solar Energy?
The Sun is a massive nuclear fusion reactor that radiates energy into space in the form of electromagnetic waves. When this energy reaches Earth, we refer to it as solar radiation.
We receive solar radiation in two main forms:
• Direct Radiation: Solar radiation that travels in a straight line from the Sun directly to the Earth's surface without being scattered by clouds or atmospheric particles.
• Diffuse Radiation: Solar radiation that has been scattered by clouds, dust, and moisture in the atmosphere before reaching the ground. On an overcast day in Northern Ireland or the UK, most of our solar energy arrives as diffuse radiation!
Global Solar Radiation is the total sum of both direct and diffuse radiation hitting a surface:
\(\text{Global Radiation} = \text{Direct Radiation} + \text{Diffuse Radiation}\)
Solar Insolation and the Solar Constant
• Solar Constant: The average amount of solar energy received per unit area at the outer edge of Earth's atmosphere, roughly equal to \(1.36 \text{ kW/m}^2\) (or \(1360 \text{ W/m}^2\)).
• Solar Insolation: The total amount of solar energy that actually reaches a specific surface area on Earth over a given period, typically measured in \(\text{kWh/m}^2\text{/day}\) or \(\text{kWh/m}^2\text{/year}\).
Factors Affecting Solar Energy Received
Why do some locations receive more solar energy than others? Let us look at the primary factors:
1. Latitude: Near the Equator, the Sun is directly overhead, concentrating energy over a smaller surface area. Closer to the poles, the Sun strikes at an oblique angle, spreading the same amount of energy over a much larger area and passing through more atmosphere.
2. Season: Earth's axis is tilted at \(23.5^\circ\). In summer, our hemisphere is tilted toward the Sun (longer daylight hours and higher sun angles); in winter, it is tilted away.
3. Time of Day: The Sun's angle is highest at solar noon, giving maximum intensity.
4. Cloud Cover and Atmospheric Conditions: Heavy cloud cover reflects and scatters incoming radiation, reducing overall insolation.
5. Orientation (Azimuth) and Tilt Angle: In the Northern Hemisphere, collectors facing due South tilted between \(30^\circ\) and \(40^\circ\) capture the maximum annual solar energy.
Did you know? Even on a completely overcast, drizzly day, solar panels still generate electricity and hot water thanks to diffuse radiation!
Key Takeaway for Section 1: Total solar energy hitting a surface consists of direct and diffuse radiation. Its intensity depends on latitude, time of year, weather, and the collector's orientation and tilt.
2. Solar Thermal Systems (Solar Water Heating)
Solar Thermal technology uses the Sun's energy to produce heat (thermal energy), primarily used to heat domestic hot water (DHW) and occasionally to support space heating.
Types of Solar Thermal Collectors
There are two primary types of solar thermal collectors used on buildings:
A. Flat Plate Collectors (FPC)
A flat plate collector is a dark, insulated box covered with glass.
Main Components:
• Glazing (Glass Cover): Toughened, low-iron glass that lets sunlight in and creates a mini-greenhouse effect while protecting inner parts.
• Absorber Plate: A dark, highly selective coated metal sheet (usually copper or aluminium) designed to absorb maximum solar radiation and emit minimal thermal radiation.
• Riser Pipes (Tubes): Metal tubes attached to the absorber plate through which a heat-transfer fluid flows.
• Insulation: Located at the base and sides to prevent heat escaping through conduction.
• Casing/Enclosure: Weatherproof outer box.
B. Evacuated Tube Collectors (ETC)
Evacuated tube collectors consist of several parallel glass tubes. Each tube is made of two concentric glass layers with a vacuum between them.
Why the vacuum matters: Heat cannot travel through a vacuum by conduction or convection. This almost completely eliminates heat loss to the outside air!
How a "Heat Pipe" Evacuated Tube Works:
1. Sunlight strikes the dark absorber strip inside the inner glass tube.
2. Inside the copper heat pipe is a small amount of volatile liquid (such as an alcohol-water mixture).
3. The liquid heats up rapidly, vaporises (boils), and the hot vapour rises to a condenser bulb at the top of the tube.
4. The condenser transfers its heat to the main manifold where the system's heat-transfer fluid flows.
5. Once cooled, the vapour condenses back into liquid and trickles down to the bottom of the tube to repeat the cycle.
Comparing Flat Plate and Evacuated Tube Collectors
• Efficiency: Evacuated tubes are more efficient, especially in cold, windy, or overcast conditions, because the vacuum prevents convective heat loss.
• Cost: Flat plate collectors are generally less expensive to manufacture and install.
• Aesthetics & Durability: Flat plates look more like skylights/roof windows and are very robust. Evacuated tubes can be more fragile, though individual tubes can be replaced without draining the entire system.
The Complete Solar Thermal Circuit (Active System)
A typical domestic active solar thermal system contains:
• Solar Collector: Absorbs sunlight on the roof.
• Heat Transfer Fluid: Water mixed with glycol (antifreeze) to prevent freezing in winter and corrosion inside the pipework.
• Circulation Pump: Moves the fluid through the closed circuit.
• Differential Temperature Controller: A smart electronic unit that compares the temperature at the collector (\(T_1\)) with the temperature in the hot water cylinder (\(T_2\)). It turns the pump on only when the collector is hotter than the water cylinder (\(T_1 > T_2\)).
• Dual-Coil Hot Water Cylinder: The lower coil receives solar heat from the solar loop. The upper coil connects to a backup boiler or immersion heater for days with low sunshine.
• Expansion Vessel and Pressure Relief Valve: Accommodates fluid expansion as temperatures rise and protects against overpressure.
Key Takeaway for Section 2: Solar thermal converts solar radiation to heat. Evacuated tubes provide higher efficiency in cold climates due to vacuum insulation, while flat plates are robust and cost-effective. A differential controller ensures the pump runs only when useful heat can be gained.
3. Solar Photovoltaics (Solar PV)
While solar thermal makes heat, Solar Photovoltaic (PV) systems convert sunlight directly into electricity.
The Photovoltaic Effect (Step-by-Step)
Don't worry if this seems tricky at first—let's break down the physics simply:
1. Semiconductor Material: PV cells are made from semiconductor materials, most commonly silicon.
2. Doping and the p-n Junction: A silicon wafer is treated (doped) to create two layers:
• n-type layer (negative): Doped with phosphorus, giving it extra free electrons.
• p-type layer (positive): Doped with boron, giving it an abundance of "holes" (missing electrons).
• Where these two layers meet is the p-n junction, which creates an internal electric field.
3. Photon Absorption: When sunlight hits the cell, light particles called photons collide with silicon atoms, knocking electrons free.
4. Electron Flow: The electric field at the p-n junction forces these free electrons to flow in one direction toward metal contacts on the cell surface.
5. Electricity Generation: Connecting an external circuit allows these moving electrons to flow as Direct Current (DC) electricity.
Types of Solar PV Cells
1. Monocrystalline Silicon (Mono-Si)
• Structure: Made from a single, continuous, highly pure silicon crystal ingot sliced into wafers.
• Appearance: Uniform dark black colour with cut-off corners (octagonal appearance).
• Efficiency: Highest efficiency (typically \(18\%\text{ to }22\%\)).
• Pros & Cons: Best space efficiency, long lifespan, but highest manufacturing cost.
2. Polycrystalline Silicon (Poly-Si / Multicrystalline)
• Structure: Made by melting multiple silicon crystal fragments together and pouring them into a square mould.
• Appearance: Distinctive blue, speckled/shimmering look.
• Efficiency: Moderate efficiency (typically \(15\%\text{ to }17\%\)).
• Pros & Cons: Cheaper to produce with less silicon waste, but requires more roof space to produce the same power as monocrystalline.
3. Thin-Film Solar Cells (Amorphous Silicon / CdTe / CIGS)
• Structure: Made by depositing extremely thin layers of photovoltaic material onto glass, plastic, or metal substrates.
• Appearance: Smooth, uniform, often flexible.
• Efficiency: Lower efficiency (typically \(10\%\text{ to }13\%\)).
• Pros & Cons: Lightweight, flexible, performs well in low-light and high temperatures, but degrades faster and requires significant roof area.
Components of a Grid-Connected Solar PV System
• PV Array: Multiple PV panels (modules) wired together in series and parallel on the roof to collect sunlight and generate DC electricity.
• Inverter: The crucial brain of the system. It converts Direct Current (DC) produced by the panels into Alternating Current (AC) at \(230\text{ V}\), which is required by household appliances and the national grid.
• Generation Meter: Measures the total kilowatt-hours (\(\text{kWh}\)) of electricity produced by the system.
• Consumer Unit (Fuse Box): Distributes the AC electricity to appliances in the building.
• Import/Export Meter: Tracks how much electricity is bought from the grid versus how much surplus solar electricity is exported back to the grid.
• Battery Storage (Optional): Stores surplus daytime electricity for use during the evening and night.
Factors Affecting PV Output
• Orientation and Tilt: South-facing at \(30^\circ\text{ to }40^\circ\) tilt gives optimal output in the UK/Ireland.
• Shading: Even partial shading on one panel (from chimneys, trees) can drastically reduce output across an entire string of panels.
• Temperature (The Temperature Coefficient): Contrary to popular belief, PV panels do not like extreme heat! As the temperature of a PV cell rises above \(25^\circ\text{C}\), its electrical efficiency drops. Good ventilation behind panels helps keep them cool.
• Dirt and Soiling: Dust, bird droppings, and leaves block light, reducing system output.
Memory Aid for PV vs Thermal:
• PV = Power / Photons (Electricity)
• Thermal = Temperature (Hot water)
Key Takeaway for Section 3: PV panels convert photons into DC electricity using semiconductor p-n junctions. Monocrystalline is the most efficient, polycrystalline is mid-range, and thin-film is flexible/low-cost. An inverter converts the generated DC into usable AC electricity.
4. Passive Solar Building Design
Passive Solar Design refers to designing the structure, layout, and materials of a building to harness solar energy directly for heating, lighting, and cooling without using mechanical pumps or fans.
Core Principles of Passive Solar Heating
1. Building Orientation and Window Placement
• Buildings should have their main facade and largest window areas facing within \(30^\circ\) of true South.
• Zoning: Living rooms and main workspaces are placed on the sunny south side; utility rooms, bathrooms, and garages (which require less heating and light) are placed on the colder north side with minimal glazing.
2. Direct Solar Gain and Glazing
• High-performance double or triple glazing allows shortwave solar radiation to enter the building.
• The interior surfaces absorb this radiation and re-radiate it as longwave infrared (heat), which cannot easily escape through the glass (the greenhouse effect).
3. Thermal Mass
• What is it? Thermal mass is the ability of high-density materials to absorb, store, and slowly release heat.
• Materials: Concrete, dense brick, stone, ceramic tiles, and water tanks.
• How it works: During the day, dense floors and walls absorb excess solar heat, preventing the building from overheating. At night, as the indoor air cools, the stored heat is slowly radiated back into the rooms.
4. Indirect Gain: Trombe Walls and Sunspaces
• Trombe Wall: A thick, dark masonry wall placed on the south side of a building, with external glazing separated by a small air space. The wall absorbs heat during the day and transfers it slowly to the interior living space several hours later via conduction and convective air vents.
• Sunspace / Conservatory: An attached glazed room on the south side that collects heat during sunny periods, which can be ducted or vented into the main living areas.
5. Overhangs and Shading (Preventing Summer Overheating)
• A major risk in passive solar design is summer overheating.
• Because the Sun is high in the sky during summer and low in the sky during winter, correctly sized fixed roof overhangs or brise-soleil (louvres) block direct summer sun while allowing the low winter sun to penetrate deep inside the rooms.
Key Takeaway for Section 4: Passive solar design uses south-facing glazing, heavy thermal mass, calculated overhangs, and smart room zoning to regulate building temperatures naturally without relying on external energy inputs.
5. Quick Review & Common Exam Mistakes
Common Mistakes to Avoid:
• Confusing Solar PV with Solar Thermal: Never say "PV heats water directly" or "solar thermal generates grid electricity". Keep electricity (PV) and heat (thermal) strictly separated.
• Thinking PV works better when it is hot: High temperatures actually reduce PV efficiency. PV requires light intensity (photons), not ambient heat.
• Forgetting the Inverter: Always mention the inverter when describing a grid-connected PV system—appliances cannot run directly on the raw DC from panels.
• Ignoring Diffuse Light: Do not write that solar systems shut down when it is cloudy; they still generate output using diffuse solar radiation.
Quick Formula & Concept Checklist:
• Global Radiation: \(\text{Direct} + \text{Diffuse}\)
• Collector Direction (Northern Hemisphere): Due South
• Optimal Tilt (UK/NI): \(30^\circ\text{ to }40^\circ\)
• Inverter Conversion: \(\text{DC (Direct Current)} \longrightarrow \text{AC (Alternating Current at } 230\text{ V)}\)
• Thermal Storage: Materials with high thermal mass (concrete/brick/stone) smooth out indoor temperature swings.