Solar thermal vs solar PV panels

The difference between solar thermal and solar photovoltaic (PV) panels is a matter of technology and application. Solar thermal and solar PV both depend on the sun to produce energy, but that’s where their paths diverge. In a nutshell, a solar thermal system harvests sunlight to generate heat. A solar photovoltaic system uses sunlight to generate electricity. Both use solar panels, but it’s easy to distinguish between thermal energy and solar energy panels by sight.

What is solar thermal?

Solar thermal panels capture the sun’s energy in order to provide hot water. There are two different types of solar panels used for this.

Flat-plate collectors

How does solar thermal energy work? That depends on the panel. This type looks similar to PV panels, in that they’re flat, dark plates mounted on a roof. These plates are often referred to as “solar thermal collectors” and are essentially heat exchangers. They comprise of:

• Black surface – the absorber plate, which is typically a sheet of copper or aluminium for good heat conductivity. The plate is black to efficiently absorb solar radiation.
• Support structure – an insulated metal or wooden box that protects the components and holds them securely in place.
• Glazing sheet – a transparent cover made of either glass or plastic to protect the absorber plate while creating an insulating gap of air.
  • The air gap traps heat, preventing it from escaping.
  • The absorber plate transfers heat to the fluid within the collector, but it also loses heat to its surroundings.
  • The bottom and sides are insulated with high temperature rigid foam or aluminium foil insulation to minimise heat loss.
• Copper pipes or tubes – also called risers. These contain the heat transfer fluid, typically water, and are bonded, soldered or brased to the absorber plate for optimal surface contact and heat transfer.
  • As the sun heats the absorber plate, the temperature increases.
  • The heat is conducted through the risers and absorbed by the water flowing inside the copper pipes.
  • The hot water then flows to a hot water tank, where it’s stored to be used by the occupants.

Evacuated tube collectors

These systems are more efficient than flat-plate panels, particularly in cold weather. On the other hand, they risk overheating in warm temperatures, losing efficiency.

Components include:

• Transparent glass tubes – multiple rows of parallel tubes with each consisting of a thinner glass inner tube.
  • Inner tubes are called twin-glass tubes or thermos-flask tubes and are covered with a special coating that absorbs solar energy but limits heat loss.
  • Inner tubes are made of borosilicate or soda lime glass due to its strength and resistance to high temperatures, and has a high transmittance for solar irradiation.
  • While flat-panel collectors heat water directly within tubes, evacuated tube collectors evacuate air from the space between tubes, creating a vacuum.
  • Vacuum acts as an insulator, which significantly reduces heat loss to the atmosphere. This is why it’s more efficient than flat plate collector’s internal insulation.
• Absorber plates – Instead of being one, large sheet as found in flat-plate collectors, these absorber plates, or fins, are smaller and contained within the transparent glass tubes.
• Heat pipes – stainless steel or copper pipes run through the inner tubes. Attached to the heat pipes are copper or aluminium absorber plates, or fins.
  • The absorber plate’s special coating transfers heat to the fluid – typically water – circulating through the pipe.
  • The heat pipe transfers the heat through convection of its internal heat transfer fluid to a “hot bulb pipe.”
  • The hot bulb heats a copper manifold, which is located in a header tank, just above the panel.

In addition to the panels, or collectors, solar thermal systems also use a pump – which can be powered by solar PV systems – to move the fluid around the cycle, and a control system to prevent liquid cooling the tank on cold days.

Solar thermal pros and cons

Photovoltaic panels

Solar thermal efficiency vs PV systems isn’t much of a contest. PV solar panels aren’t nearly as efficient as thermal panels, turning about 20% of captured sunlight into electricity. Compare that to solar thermal energy systems, which harvest 70% of energy captured. But when they serve different purposes, any comparison is only a point of interest.

Below are the components that comprise photovoltaic solar panels:

• Solar photovoltaic cells – PV cells are made of a layer or two of a semiconducting material, typically silicon. When sunrays hit the cell, it generates an electric field. The more intense the light is, the greater the flow of electricity.
  • The cells are wired together to form a solar power panel, also called a module
  • The panels send the generated direct current (DC) to an inverter – a separate piece of equipment – which turns it into alternating current (AC)
• Junction box and connectors – small enclosure located behind the panel to attach the cables necessary to interconnect the panels.
  • Protects interconnections from moisture and debris.
  • Also houses the bypass diodes, which prevent back current due to cells being shaded or dirty.
• Tempered glass – designed to resist mechanical loads and extreme temperature changes while protecting the PV cells from the weather and debris.
  • An anti-reflective coating on the underside improves light transmission.
• Aluminium frame – protects the edge of the laminate section that holds the cells. The frame enables the solar panel to be mounted securely into position.
  • Lightweight and stiff, it’s designed to stand up to stress and loading from high winds and the forces of nature.
• EVA film – transparent ethylene vinyl acetate (EVA) encapsulates the cells.
  • Incredibly durable and tolerates extreme temperatures and humidity.
  • Prevents moisture and dirt ingress.
  • Positioned on both sides of the cells, providing shock absorption and protecting cells and wires from vibrations and sudden impact.
• Back sheet – a barrier against moisture while providing mechanical protection and electrical insulation.
  • Typically made of various polypropylene (PP), polyethylene terephthalate (PET) or Polyvinyl fluoride (PVF) for protection, thermal stability and long-term UV resistance.
  • Instead of a back sheet, some panels use a rear glass panel instead for more durability and longer life.

What about solar PV heating systems?

Solar PV panels can power appliances and just about anything else in the home, as long as that appliance or device depends on electricity. This means that domestic solar panels can heat a home only if an electric heating system is in place. This extends to some furnaces, hot water tanks and gas or oil boilers, which might have electrical components.

What about photovoltaic thermal energy?

Yes, you can heat water with solar PV panels by using an immersion optimiser. This technology detects when a surplus of solar PV generation is sent to the grid and diverts that energy into heating the water tank. This enables the occupant to optimise their energy usage and store hot water for use later on during the day.

What else should you know?

You can learn more about PV solar panels and systems in these helpful articles:

Basics of solar PV

Different types of solar PV equipment

Solar PV inverters

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