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Solar energy is unlimited and free. The two main technologies that harness solar energy at the level of residential, commercial and industrial buildings are:

• Solar Heating and Cooling (SHC) or Solar Thermal Systems (STS), which collect solar energy to provide hot water for use and heating.
• Photovoltaics - PV, which directly convert solar energy into electricity.

Both of these technologies are used in Greece.

Solar Thermal systems are found in every home, in their simplest form, that of the solar water heater, while Central Solar Thermal Systems are the usual choice for hotel facilities where the requirements for hot water are even tougher and solar thermal systems offer the most economical way of its production.

Solar Thermal system

Photovoltaics are found both in domestic installations on the roofs of residential buildings, and in professional installations such as Photovoltaic parks.

Photovoltaic System

Both European and National legislation now provides particularly strict goals for the Energy Performance of buildings, Photovoltaic and Solar Thermal Systems will soon become necessary in almost all buildings.

In this article we will present the advantages and disadvantages of each system and mainly the energy impact of these two technologies.

Solar Thermal VS Photovoltaic

Both solar thermal and photovoltaic systems can make a significant impact in the direction of energy savings. Each of them in a different way. The following is a comparison between the two technologies with an indicative list of the advantages and disadvantages of each.

Solar Thermal - Advantages

• Two or three times higher efficiency than photovoltaics for water heating. For example, the efficiency of a solar collector is more than 60%, while a solar photovoltaic panel has an efficiency of about 20-25%.
• They require significantly less surface area. They are ideal where space is limited (roofs, roof tops of buildings, etc.).
• The technology of solar thermal systems is simple compared to photovoltaic systems in terms of production and storage.
• Through regular maintenance, the efficiency of the collectors will not decrease significantly as can be observed in photovoltaic systems over time.
• There are low-cost options available on the market (e.g. solar water heaters).
• Simple and cost-effective way of storage using a hot water tank.

Solar Thermal - Disadvantages

• Solar energy is only converted into thermal energy. They cannot be used to cover other electrical consumption of a building (e.g. lighting, electrical appliances).
• They include an additional hydraulic circuit.
• Requires hydraulic maintenance annually.
• Reduction in efficiency in the winter months, however the efficiency compared to photovoltaics remains at least twice as high.
• During the summer months there is oversupply of energy production (except for hotels where this is an advantage).

Photovoltaic - Advantages

• The electricity they produce could be used for any purpose (lighting, electrical appliances, cooling, heating, etc.).
• Any excess energy can be taken from the electricity network with a corresponding reduction in the bill.
• No additional hydraulic circuit and its maintenance is required.

Photovoltaic - Disadvantages

• Production of hot water with low efficiency compared to solar thermal (simple system).
• The efficiency of the collector decreases at a rate of 0.4-0.8% per year.
• Correspondingly, the storage performance of the batteries (if they exist) decreases and they have to be replaced after a certain period of time (about 5-10 years).

Summary

Solar and photovoltaic systems have advantages and disadvantages compared to each other in terms of using the unlimited solar energy.
Regarding water heating (either for hot water supply or for heating water - heating assistance), solar systems are more efficient than photovoltaic systems in terms of space saving, simplicity and economy.

For a residential house where the energy requirements for heating and domestic hot water are in the range of 60-70% the installation of a solar thermal system is the first step towards energy saving.

Photovoltaic systems can participate in the energy consumption of electrical appliances and lighting which is not possible with solar thermal systems.

For example, on a rooftop, solar thermal can produce more than 3 MWh of energy per year with a collector surface of less than 6.5 m², while a photovoltaic system would require more than 15 m² for an equivalent energy production, since the degree of efficiency of solar thermal in many cases exceeds 60%, while Photovoltaics hardly exceeds 20%.

Comparison of 3MWh Annual Energy Production with Solar Thermal (ST) or Photovoltaic (PV)

Solar Thermal & Photovoltaic Systems to the House of the Future

With energy legislation becoming stricter, climate change at its highest and the energy crisis overrun the entire planet, in the near future every home will be required, among other things, to have a Solar Thermal and Photovoltaic System.

Specifically, an installation in a new or energy-renovated house will usually include a hybrid system consisting of a Solar Thermal System, a Heat Pump and a Photovoltaic System, so that:

• The production of Domestic Hot Water be made exclusively by the Solar Thermal System. 
• The Solar Thermal System can also contribute to Space Heating up to 50%.
• The Heat Pump covers the other heating and cooling needs at a rate of 50% to 100% (on days when there is no sunshine).
• The Photovoltaic System, to produce the required energy for the operation of the Heat Pump, the lighting and the other devices of the building.

For example:

In order to achieve the above performance in a 100 m² residence, Energy Class A in Climate Zone B, is required: 

• One Solar Thermal System with a solar tank of 500lt capacity and 8 m² solar collectors surface.
• One Heat Pump 5KW (which consumes an average of 10KWh/day) and
• One Photovoltaic System 2.5KW with 12 m² panels surface, in order to produce the energy consumed by the Heat Pump.

A system as in the example above, it 's a fact that will have a very good payback (ROI – Return of Investment), that it will save the money of its initial purchase in less than 5 years, in relation to the cost of energy saved in October 2022 prices. subsidized programs such as "Save", the payback time can be reduced to 2.5 years. With the subsidized programs such as "Saving at Home", the payback time can be reduced to 2.5 years.