Solar PV vs solar thermal vs heat pumps: Which is best for water heating?
22Oct
by Sean Moolman
0
South Africans have been pummelled by large electricity price increases since electricity shortages and load shedding started in 2008. Electricity prices increased by more than 500% in real money terms over this period – in other words, 5 times more than inflation (see our blog article on this topic).
This rapid and sustained rise in electricity prices, combined with increasing concern about the health and environmental impacts of South Africa’s ‘dirty’ coal-fired electricity, has prompted a search for ways to reduce our dependence on the grid.
Water heating is the single biggest energy consumer in South African households at 40% of total household energy use(ref). That makes it an obvious target for reducing the electricity bill.
South Africa’s National Building Regulations also prescribes minimum energy efficiency standards (through Regulation XA), which requires all new residential properties to derive at least 50% of the annual household water heating requirement from an energy source other than grid electricity.
There are many options for heating water, including:
Solar PV (photovoltaic)
Solar thermal (also called ‘solar geysers’)
Heat pumps
Natural gas or LPG
Biomass
PVT (PV thermal – a combination of solar PV and solar thermal)
With so many options for water heating, the inevitable question is: which is best?
Like most things in life, the answer depends on your priorities and circumstances. Things like location & climate, size of household, budget, reliability of electricity supply and importance of environmental considerations all have an impact on the suitability of available water heating options.
However, there are clear advantages to some options over others. Let’s first look at cost.
When considering the cost of any system, it is important to also consider total lifetime cost. This includes the upfront cost of the system and installation (capital cost), as well as anyongoing costs, such as electricity or fuel costs, maintenance and repair costs. One must also consider the lifetime of the system.
There is a methodology to account for all the above factors called Levelized Cost of Energy (LCOE). It calculates the total cost per unit of energy (called a kWh or ‘kilowatt-hour’) for different energy options.
The graph below shows the overall cost or LCOE for several water heating options in South Africa as of 2021 (see tab “Levelized Cost of Energy” at ref):
It is clear from the above graph that solar PV and solar thermal water heating are by far the lowest cost options for heating water. The most expensive option is in fact electricity from Eskom or the municipality!
Heat pumps require maintenance and have ongoing electricity costs, while gas is expensive and fluctuates with changes in oil price.
Of course, there are several other considerations when deciding between different water heating options.
The table below compares some of these factors for the three most common alternative water heating options: solar PV, solar thermal and heat pumps.
Factor
Solar PV
Solar Thermal
Heat Pumps
Additional plumbing required?
No
Yes
Yes
Retrofit to standard electric geysers?
Yes
Some designs can be retrofitted
Requires modifications
Noise?
No noise
Some noise (pumped systems)
Yes
Works during power failures?
Yes
Yes for some (thermosiphon & pumped with solar PV)
No
Frost issues?
No
Yes (requires more expensive indirect system)
Reduced performance
Winter performance
Good
Not as good
Not as good
Overheating & water wastage?
No
Yes (stresses system & wastes water)
No
Aesthetics
Good
Not good for thermosiphon & requires stronger roof
Good (if hidden)
Maintenance required?
No
Yes – every few years
Yes – annual
Price
Comparable or less expensive than pumped solar thermal. Less expensive than heat pumps
Thermosiphon solar thermal systems are cheapest
Most expensive
Lifetime
30+ years
10 – 15 years
10 – 15 years
Roof space required
2 – 3 x more than solar thermal
2 – 4 m²
None (but requires space for system)
Lifetime cost of energy
Best
Good
Worst (continue paying for electricity)
Section 12B tax benefit for ‘build to rent’ property developers?
Yes
No
No
Based on cost and all the considerations in the above table, solar PV and solar thermal are clearly preferable to heat pumps. (There are some situations where heat pumps are the only practical option – for example multi-storey buildings with insufficient roof space for solar PV – although carports or other ground mounted solar PV can help overcome this issue).
If upfront cost is the overriding consideration, thermosiphon systems (the systems with the tank on the roof) are the lowest initial cost option. However, when system lifetime and maintenance cost are included in the calculations (in other words, when looking at lifetime cost), solar PV is the most cost effective.
When considering factors such as winter performance, noise, power failures, maintenance requirements, overheating and water wastage, solar PV is preferable to solar thermal.
Whichever option you prefer, it has never made more financial and environmental sense than now to switch to alternative ways of heating water!
