Architectural integration of solar thermal systems

The amount of solar energy reaching the Earth in one hour equals the total amount of primary energy used in the world during one year. Thanks to the different solar technologies presently available, most energy needs of buildings can today be covered by solar energy in its various forms. Among mature solar technologies, active solar thermal (ST) systems for domestic hot water production (DHW) and space heating deserve to play a key role. Their proven high efficiency, low cost and short payback time naturally make them a prime choice for energy efficient buildings. Then why is this technology still largely underexploited when compared to photovoltaics (PV)?