sorry khaoula i asked about your question? you said exergy efficiency decreases when using heat pump. but this comparison need tow things 1- heat pump and 2-?
With a heat pump you imput to the system mechanical energy, of high exergy content; the output is heat energy which is diffused in the surroundings, therefore with zero exergy value
Are you sure that the exery decrease if you using heat pump, I think it's related by the amount of the useful energy attained against the useful energy offer....