Cleantech, which stands for Clean Technology, is a broad gamut concept and it is more precisely defined by Clean Edge as under:

‘A diverse range of products, services, and processes that harness renewable materials and energy sources, dramatically reduce the use of natural resources, and cut or eliminate emissions and wastes.’

For all practical purposes, cleantech refers to the processes which can make any power-backed appliance or device more energy-efficient thereby reducing the carbon footprint on our globe. Talking of cloud computing specifically, cleantech will mean looking into ways and means to ensure that the switch to cloud will involve a greener approach and be restricted within the permissible limits of carbon emission. You can continue here :

http://blog.bootstraptoday.com/2011/09/22/how-can-cloud-aid-cleantech/