Solar heats up

Solar power prospects are looking good, but for solar heat as well as solar electricity. It is clear than solar photovoltaic power generation is booming around the world, with over 580 GW of PV solar installed by the end of 2019, and much more expected, but it’s worth noting that there is also a large amount of direct solar heating system capacity in use (470 GW thermal), and that too is also growing.  Most of this capacity is in the form of standard roof-top solar heat collectors, with China in the lead (330 GWth), but large community-scaled solar heating arrays have been developed in Europe and elsewhere, some of them linked to large inter-seasonal heat stores, allowing summer heat to be used for winter warming via local district heating networks.

Denmark leads in community solar

Denmark has been a leader in this field, with its flagship 13.5 MW Marstal project and many others. The heat stores typically involve large lined pits with floating insulating covers for heat retention.  In most cases, the solar input augments heat supplied by other means, including from biomass combustion, but new approaches are being adopted which enhance the solar and bioenergy input using large heat pumps. Although (fossil) gas fired heating still often has the edge, solar heating with heat stores can be competitive with other heating sources if district heating networks already exists, as they do in Denmark. And of course the carbon emissions associated with using fossil gas are then avoided.

Pit-type heat stores are not the only option. In some locations, use is made of Underground Thermal Energy Storage (UTES), with excess heat simply stored in the ground. There is a system using deep vertical boreholes, the Drake Landing inter-seasonal solar heat storage system, in Canada, where of course winters are very cold. UK company ICAX is developing similar ideas with commercial-scale solar-fed inter-seasonal heat stores, and there are many ‘shared ground loop’ heat storage projects fed by heat pumps using ambient (solar derived) energy.

So one way or another the idea of district solar heating and storage is spreading, with there being many direct solar heat network projects around the EU, including in Austria (e.g. in Gratz) and in Germany.   Storage is clearly an important area of development for solar as well as all the other renewables and heat is a key storage medium. One German project is looking at the use of molten salt heat stores, of the type used for storing heat from Concentrating Solar Power arrays and some new heat storage medium are also being developed. High tech heat storage at very high temperatures is also being looked at more generally as an alternative to battery electric storage. In one very advanced power producing system being developed at MIT, heat from solar or other sources is stored in molten silicon and when power output is required, the light from the white hot silicon is used to drive a high efficiency PV cell system. So that’s a very high tech hybrid thermo-photovoltaic power system. 

PV Thermal hybrids and solar cooling

Less complex, and more developed, are the various hybrid solar thermal/PV systems (‘PVT’). They avoid over-heating the PV cells, which can reduce their efficiency, by extracting usable heat. In some variants, a layer of semi-translucent PV sits on a solar thermal back plate, although other systems use focused solar in evacuated tubes. Essentially they are solar Combined Heat and Power systems, getting higher overall energy conversion efficiency and, by doubling up on PV and solar heat collection, better use of roof space.  Some of that heat produced could also be stored, making the PVT system more flexible.

Another possible area of expansion is direct solar cooling. As climate change impacts, this is likely to become urgent. Air conditioners can of course be run using solar PV power, but it is also possible to run absorption chillers or evaporation devices using solar heat and to develop community scale solar cooling networks.

While direct solar heating and cooling should continue to expand, hybrid systems, with solar heating and storage combined with the use of PV and wind electricity for heat production and storage, may be a way ahead for heating in some locations, helping with supply balancing, along, in some locations, with PVH and CPV/CSP hybrids.

Solar Hydrogen

Further ahead, we might also look to direct hydrogen production using focused solar heat for the thermal dissociation of water molecules: conversion efficiencies are low but improving.  A 100 kW Hydrosol project, using CSP heliostat mirrors, has been running in Spain since 2008, and some interesting ideas for novel approaches to high temperature dissociation have been explored elsewhere e.g. in the USA.  In one advanced concept, solar thermal hydrogen is stored to be used to generate power at night via a high efficiency combined gas/steam hydrogen-water cycle turbine. So instead of a heat store, as used with conventional CSP, use is made of a hydrogen store to enable 24/7 power generation. It is claimed that this so-called ‘Hytricty’ system could achieve an overall energy-from-sunlight capture efficiency of around 35%. There are many other direct solar heat based water-dissociation systems under development, some of them aiming to produce ‘solar fuels’ i.e. syngases and liquids, including aircraft fuel, using just solar heat, water and captured carbon dioxide.

Living on the sun

The use of solar heat actually has a long history, back into antiquity, but, more recently, roof top solar water heaters being quite widely used in California in the late 1800s, and from the 1970s onwards interest in it spreading globally. As the direct and indirect costs of using fossil energy for heating rise, it is now increasingly being seen as a viable zero carbon heat supply option in many locations, with minimal environmental impacts. Looking to the future, while PV solar electricity generation often gets much attention, as indicated above, solar heat, in its various applications, also has a big potential, with, in addition to direct solar heating and cooling, new applications also emerging, including for power balancing and new zero-carbon fuel production.