Ok he said they are just called hydronic heaters - if you google a few different types come up. Not sure how big your nursery is though - we have a little tiny oil heater that we use in the kids room overnight and don't find it that expensive at all, when DD was a baby we used a blow heater with a safety shut off thing that worked well - again didn't use up too much power.

The oil ones are good as you can run them at a low temp and they just take that chill off the air - depends if you want something a bit more permanent though. Those thermofilm ones are pretty good too - my sister had one each in her kids rooms and they were great.