Today’s Earth Day Celebration: Fun Times But a Bit of Dubious Science
I just realized I was flimflammed at the Earth Day celebration in Santa Barbara from which my daughter Valerie and I just returned. Val has designs on a hot tub, and was drawn to a booth that offered a product that was there via its claim of super-efficiency.
“Normal tubs of this size will raise your electricity bill by $50 – $100 per month, but you can run this one for a fraction of that, say $10 – $15,” its representative said with a smile.
“No kidding,” I replied. “How?”
“We take the waste heat from the pump and we put it through a heat-exchanger.”
“Great concept!” I responded absently, taking his card, ambling on to the next booth.
On the drive home I started to think about this. You have a pump to circulate a few hundred gallons of water? How powerful does that need to be? One-quarter horsepower (about 200 watts) at most? Let’s say the pump is 80% efficient, leaving you with 20% heat, or 40 watts, and that you miraculously turn all of that into heating the water. 40 watts will warm 100 gallons of water 1 degree C in 10 hours. That’s a pretty tepid hot tub. I had been baited, bagged, buncoed, and bamboozled.
Fortunately, the vast majority of the exhibits were great – a terrific father-daughter experience.
Yes, the salesman lied; his explanation was total nonsense. However, it should be possible to make hot tubs more efficient by using a heat pump or solar heat to heat them.
The best way to make a hot tub more efficient is by using a plastic blanket… A thick plastic sheet with small air bubbles (like a more durable bubble-wrap) will float right at the surface of the water, and dramatically reduce evaporative cooling (and less significantly reduce convective cooling) when you aren’t using your hot tub.
When you want to use the hot tub, you take the blanket off and turn the heat back up… but assuming reasonable insulation within the walls of the tub, you should be able to retain most of your heat.
The cost of such a blanket is probably ~$20-$30, though I’ve never priced such a thing. That’s likely 1% of the premium cost of a so-called high efficiency hot tub.
It is conceivable that some energy might be saved by careful positioning of the air jets, so as to minimize evaporation loss due to aeration while the tub is in use while maintaining the experience for those using the tub… but I’d suspect the potential gains here to be in the <1% range, and I'd also suspect the premium on such a tub would be enormous.
Beyond that, heating is heating. It's 100% efficient energy transfer, but it's going to take energy.