I'm talking about energy, not costs. Money is arbitrary, joules are not, and you can't plug your computer to a stash of dollars and have it run. Generating energy comes at a cost to ecology, to workers and the supply is not unlimited.
RE capturing heat - heat is a low-grade form of energy and electricity is high-grade. You can't power a machine from heat at efficiency levels of electricity. And if you want an electric heater, buy a heat pump - these run at 500% (!) electric to thermal conversion efficiency (by using 1kW of electricity to move 5kW of heat around).
I'm talking about energy as well. In order to project how much energy bitcoin will use, you need to understand the incentives. Could you clarify what you meant by increasing energy use?
Isn't it true, that as time goes on, mining gets harder (= more energy intensive), so if you want to earn bitcoins you need to invest increasingly growing amount of energy? Isn't it also true that this process of mining needs to continue to ensure the integrity and trustworthiness of the blockchain?
No. Mining adjusts so that it always takes 10 minutes to find a block on average. This can happen on a single computer, or on a billion. If the price of bitcoin keeps on increasing, there's a larger inventive to mine, which will cause the difficulty to increase. The marginal profit of mining will always tend towards zero, and the total spent on trying to mine will tend towards subsidy plus fees. The subsidy halfs every so often.
Mining only appears always to increase as time goes on because Bitcoin is increasing in popularity, but the protocol itself doesn't mandate that at all. Like ikeboy said, difficulty is adjusted to keep the average time to find a new block at about 10 minutes. The network is perfectly content to let the rate stay still if no new computational power is added to the network.
RE capturing heat - heat is a low-grade form of energy and electricity is high-grade. You can't power a machine from heat at efficiency levels of electricity. And if you want an electric heater, buy a heat pump - these run at 500% (!) electric to thermal conversion efficiency (by using 1kW of electricity to move 5kW of heat around).