Black surfaces absorb solar heat. Tarmac roads tend to absorb the heat of the sun up to the point when they radiate heat as quickly as they are absorbing it: the surface temperature of tarmac can often reach 15°C higher than the ambient air temperature.
ICAX collects this naturally occurring energy using water circulating through an array of pipes embedded in the surface of the road and deposits it in ThermalBanks until needed for space heating in winter.