Consider how using the high heating value of a natural gas fuel could improve the efficiency of a home heating furnace compared to one that uses only the low heating value (i.e. the water vapor escapes from the heat exchanger). Would it be worth paying more for the furnace that condenses the water vapor and recovers the heat of transformation?

Electrical energy can be purchased more cheaply at night when the normal demand is reduced. Would it make sense to purchase electricity at a lower price and store it in a device that uses a phase change (solid to liquid) that could be reversed (liquid to solid) to release energy when electricity is more expensive? What transition temperature would be best for such a device? How big would it need to be in order to meet the demands for heating a 4 x 8 x 3 m room? What other data do you need?
References
Moran and Shapiro, Chapter 3
Reynolds and Perkins, Second Edition, Chapter 4
Black and Hartley, Chapter 2, Table B2.