a$.$ What are the potential application of hydrogen for doemsitic heating $?$ espeecially in U$.$K $.$ what are the advantages of it $,$ what about saftey $,$ cost $.$ and will there be need to change boiler? $.$ or is it principle same as normal boiler heater $.$

b$.$for sizing a ground heat exchangers what are the factors that need to be considered? from all point of view, infra, construction etc

c$.$what is a photovoltaic$/$thermal hybrid system? how is it different from traditional and what are its advantages $?$

how does it work? are there any extra parts in it$?$

d$.$ In COefficent of performance what is :

COP$($HP$)=$ COP$($R$)+1.$ what does it mean and why? coefficent of performance of heat pump is always better then refrigerator