Solar PV & Thermal: One or both ?

Having both solar thermal & solar PV systems on our roof, over the years we've often been asked which of preferable: One, the other, or both. Here I'll attempt to address this question from a user viewpoint.

Technical

Both solar photovoltaic (PV) and solar thermal systems rely on collecting radiation emitted by the sun. It's really important at this stage to note that both forms of solar power rely on the intensity of light falling on their collectors, not heat, although ambient & operating temperatures can impact efficiency for both technologies.

Solar PV

Solar photovoltaic (PV) systems collect the sun's energy using cells constructed from semiconducting materials to create electricity. These materials don't require direct sunlight to work and they still generate some electricity on a cloudy day, although the amount generated depends on how bright conditions are. Electricity is created as direct current (DC) at voltages which vary with conditions and is normally converted by an inverter to match the mains alternating current (AC) voltage & frequency conditions which apply allowing the electricity to be used to run household appliances and lighting.

Solar Thermal

Domestic Hot Water from Solar - PV, Thermal or Both?

Solar thermal systems (also known as solar hot water systems) use solar panels which are normally installed on a roof to collect the sun's radiated energy through heating a surface which is then used to heat up water stored in a domestic hot water (DHW) cylinder or heat-bank. A central heating boiler or immersion heater is normally used as a back-up to heat the water when lighting conditions don't allow enough heat to be collected & stored, for example, during a period of cloudy skies or during the shorter daylight hours in winter which becomes more apparent as latitude increases..

Various manufacturer sources suggest that UK solar thermal installations which are correctly sized and matched to household demand provide around 90% of DHW in high Summer, and around 25% during the winter. The Energy Saving Trust (EST) field trial report of 2011 regarding solar thermal systems suggests that solar thermal should provide around 60% of annual domestic hot water requirements when installed and used correctly. Our own thermal collectors have a footprint similar to four PV panels and provide almost all of our hot water requirements from March to November and still contribute towards energy efficiency in the worst months by pre-heating mains water in the cylinder prior to the gas boiler raising the DHW to the required temperature.

Flat Plate Collectors

Flat plate panels normally comprise of a dark colour flat plate to absorb energy which is contained in an insulated box with a clear glass cover. Heat is transferred from the panel to a water storage cylinder within a domestic property by pumping a liquid, which normally contains a form of antifreeze, through tubes which are attached to the plate absorber. These types of panels tend to balance heat-loss inefficiencies through increasing physical dimensions and therefore collector surface area, however, in high latitude locations, cooler month heat-loss efficiency is far more relevant than in sunnier and warmer climes.

Evacuated Tube Collectors

Evacuated tube panels use a cylindrical glass double wall construction incorporating a partial vacuum, to insulate an internal collector surface against ambient temperatures, thereby reducing heat-loss as a Thermos flask would. This collector technology generally performs well in lower light or intermittent irradiation conditions caused by clouds and is therefore better suited to use in high latitude installations such as in the UK, extending the exclusive solar thermal heating season towards the winter solstice from both sides of the calendar. In the deepest, darkest months of winter evacuated tubes will contribute towards energy savings more effectively than flat panels, the small amount of heat collected being used to pre-heat the cold water fed into the DHW system thus reducing the load on other water heating energy sources.

Efficiency

Many highlight the unpredictability or efficiency of solar energy systems, but is that really an issue if they're performing their main function as a clean renewable energy source through reducing the need for fossil fuel generation. The planet spins at a remarkably consistent rate, therefore daylight hours are as predictable as anything is likely to get, the only issues are clouds, the possibly a blanket of snow, and of course, the odd eclipse. This applies equally to Solar PV and Solar Thermal.

When measuring efficiency in terms of potential kWh.t/year/sqm of roof coverage, solar thermal collectors are considerably more efficient than current technology photovoltaic panels. It is important to note that, because there is no ability to 'export' excess energy, solar thermal system maximum performance is limited as much by demand as weather conditions, so actual efficiency in terms of delivered kWh.t/year/sqm can be far lower. For solar thermal systems to perform well and be cost-efficient the entire system capacity must be assessed in order to balance collection, storage & heat-loss to anticipated demand, therefore over-sizing the system is not only capital inefficient, but can also lead to long-term maintenance and performance issues, whilst undersized systems will likely lead to disappointing water heating results.

If roof size is a limiting factor the higher efficiency of solar thermal will come in handy, it's also possible to extend the solar heating period by using vacuum tubes which, through having a far reduced heat-loss than standard flat-plate collectors perform better in colder periods or when clouds cause intermittent sunny conditions.

Investment & Returns

At the time of writing, if a major factor in the decision process is to maximise returns on investment, not only through energy saving, but through accessing support schemes designed to encourage uptake of clean energy technologies.

The UK Government operate two schemes which promote & encourage the uptake of solar technologies within the domestic environment, the Feed in Tariff (FiT) scheme for electricity microgeneration, and the Renewable Heat Incentive (RHI) for systems designed to provide heating. In terms of scheme returns from solar technologies, on a like-for-like basis, the opportunities available through accessing the FiT scheme afforded through the ability to export excess energy make it a much more attractive scheme than RHI. Over the full period that support is available, it's almost certain that FiT will return a higher proportion of the PV system purchase price than RHI would on solar thermal, however, there are factors other than ROI which need to be considered.

Storage

For most, as long as solar electricity generation covers household base-load that's good enough, and when the system's going to be belting out the electrons the more energy-efficiency aware amongst us attempt to take advantage by using high load appliances such dishwashers & washing machines. The simplicity of shifting demand to suit supply is pretty-well what many with Solar PV attempt to do daily to suit their pockets, not only that, attempting to match consumption to demand side generation also shifts demand away from the normal peak demand hours, thus helping smooth variations in the national daily demand curve.

The strive to match demand to self-generation with PV system results from the current absence of realistically priced electricity storage systems. Solar thermal systems address storage directly through accumulating heat in water cylinders, however, it is common practice for PV system owners to employ technology to divert excess generation to power immersion elements for the provision of DHW. It is anticipated that as battery production volumes increase to meet electric vehicle (EV) demand, global manufacturing economies of scale will drive considerable reductions in the cost of domestic electricity storage systems over the next 5 years, encouraging a substantial uptake in home batteries for electricity storage amongst the solar PV community, which will require a re-evaluation of storage priorities.

Conclusion: Solar PV & Thermal: One or both ?

As can be deduced from the above, the answer actually depends on a number of variables which apply differently to individual circumstances, but with the case for solar thermal the ability to store energy for later use has already been seriously challenged by excess generation diversion technologies will soon be overwhelmed by the advent of affordable electrical energy storage, which has an implicitly higher value to consumers because of it's flexibility.

Apart from cases where the higher energy density (kW/sqm) is essential due to space requirements, the argument for stand-alone solar thermal was lost quite a while ago, the position only being reinforced as solar PV costs continue to fall. Solar photovoltaic as a stand-alone solution provides the flexibility to power devices & appliances whilst still having the ability to perform the only function that thermal can provide, the heating of domestic water, but with the advent of justifiable battery costs, the argument only swings in favour of PV.

On an either/or basis it's pretty obvious that PV wins, so what about the case for both?. As mentioned previously, solar thermal does have an efficiency advantage in terms of energy collected per unit of area covered, therefore in situations where the intent is to maximise the useful of a limited roof area, it may be logical to consider a system which incorporates both technologies, utilising generated electricity to power the solar thermal pump and having the ability to use both sources to heat the DHW if necessary. Of course, introducing this form of complexity introduces far more cost than simply increasing the number of solar PV panels, it's also worth noting that if battery storage is added, the automated management system would need to be configured to prioritise that form of storage over DHW.

As PV prices continue to fall, the UK solar thermal industry needs to re-evaluate what seems to be a current position of relatively high margin complacency just to stand still. Photovoltaic and proportional diversion technologies have seriously eroded the base domestic hot water provision market position and the provision of affordable and justifiable battery storage systems will accelerate this process. Apart from a few cases where maximising performance over a limited area is important, unless the installed costs of UK solar thermal are seriously reduced over the next few years, it's highly likely that the market for the product will collapse, this being the case in many high-latitude countries.