Comparison between SWHS and PV

Are Solar Water Heating Systems (SWHS) or Photovoltaic rooftop systems (PV) more attractive for investors?

Why to compare?

Solar water heater and Photovoltaic systems use both the sunlight to generate heat or electricity. We do need heat and electricity and it is expected, that both technologies will become important pillars of the future energy system. However, if a building owner wants to invest in a solar system, he has to decide, if he installs a PV system, a SWHS, or a combination of both technologies. In the following the arguments for both are explained.

PV rooftop systems

Photovoltaic systems are composed of solar cells, which are converting sunlight directly in electricity, without any noise and emissions and which are placed in modules. The modules generate DC electricity, which is usually converted into AC electricity by an inverter. The AC electricity can be used within the building, stored in a battery or fed into the electricity grid.

PV systems exist in any size, from a small solar module with some Watt capacity which e.g. is used to charge a mobile phone, up to a large Megawatt size power plant which is feeding in electricity in the high voltage grid.

In the following PV rooftop systems are compared, since they are in competition with SWHS on the roof of buildings.

The Pros of PV rooftop systems are:

  • They are very flexible in size, since the electricity can be fed into the electricity grid
  • The modules are smaller than SWHS collectors and can be also installed as roof on the roof garden for shadowing
  • If the PV system is combined with a battery and a specific inverter, it can deliver electricity to the building during a black out
  • Electricity is very flexible, it can be used for electrical appliances, machines, as well as for heating or cooling purposes
  • PV systems need only little maintenance

SWHS

SWHS are generating hot water, which can be used as domestic hot water in residential homes or for industrial processes in industries. The solar collectors convert sunlight directly into heat, which is then transported by pipes to a storage tank or directly to the point of demand.

The size of the SWHS must be adapted to the heat demand, since the generated heat must be used in the building. Due to the mismatch between the sunshine (and therefore solar heat generation) and the heat demand, usually the solar system is delivering a part of the heat demanded (e.g. 80% of energy for domestic hot water heating). In the industry, often the heat demand is much higher than the heat which can be generated on the building roof, then the solar fraction is relatively low.

The Pros of SWHS are

  • SWHS are reducing fuel costs and stabilizing energy costs and
  • SWHS are reducing local emissions by reducing furnace oil or diesel consumption.

Comparison of SWHS and PV systems

SWHS PV
Type of energy generated Heat at temperatures of typically 60°C to 80°C Electricity (PV systems are usually connected to the grid)
Size of the solar system The size of the SWHS must be adapted to the heat demand in the building, which varies a lot in heat amount and temperature needed; in commercial and industrial buildings the heat demand is usually higher than the solar heat which can be generated by the SWHS on the roof. The size of the PV system is more flexible than for SWHS, since excess electricity can be fed into the grid, it can be oriented on roof area, electricity demand or financing conditions; in commercial and industrial buildings the electricity demand is usually higher than the electricity which can be generated by the PV system on the roof
Operation and performance Automated or manual operation, the performance depend a lot on Automatic operation
Performance The performance depends on the quality of components, the design, the operation, the maintenance and the heat demand (temperature level), typical system efficiency is 20% - 40% The performance depends on the quality of components, the design and the maintenance (eventually cleaning of modules), typical system efficiency is 10% - 15%
Monitoring The performance and energy yield is difficult to measure The performance and energy yield is easy to measure
Life time A lifetime of 15 years is expected if the SWHS is maintained, pumps may be replaced during lifetime A lifetime of more than 20 years is expected if the PV system is maintained, usually the inverter must be replaced after 10-15 years
Economy Depending on the design, the performance of the system and the price of the fuel, which is replaced by the SWHS; SWHS for process heating are usually profitable with a typical simple payback time of 2-3 years, if furnace oil or diesel replaced The economy of PV systems depends a lot on the electricity price and its expected development, the financing conditions (inclusive subsidies), and the business model (will the electricity mainly used in the building, will it replace electricity from the grid or electricity generated by the diesel generator); the PV simple payback time is about 6 years at an electricity price of 10 Rs/kWh