See this video: The Future of Heat Pumps is Underground (and other places, too!)
As I really want a concise, data-driven economical and environmental summary of different energy efficiency projects I can consider, I decided I would do some analysis and go ahead and start writing my analyses. Below are my notes on researching less inefficient clothes dryers. More to come!
I don’t have any of these appliances, and this is a preliminary bit of research. Our dryer is in our garage, so the air temperature there is generally somewhere between outside ambient and about 68° (inside) temperature. We’re currently using a conventional, vented electric dryer and paying around $0.10/kWh for much of our electricity.
I’m using dollar amounts as a way to decide whether to pick something and which option to pick and as a proxy for environmental impact.
From an economic perspective, if I treat a purchase of some energy efficient appliance as an investment, I’ll compare it against a 9% annual return. The appliance will cost some amount, $X, and when lower energy costs save me $X over some period of time, I now have ‘doubled’ my investment: I have saved the original purchase price and I have the appliance itself to continue saving more money. I’m grossly simplifying by assuming the appliance will last forever. A 9% interest rate will double in
$X*2 = $X*1.09^t or
2 = 1.09^t or
log(2)/log(1.09) = 8 years, so purely on the economics, anything that can pay for itself in less than 8 years is probably a good bet, and the shorter the return period, the better.
Heat Pump Clothes Dryer
- Some can be 110v, so can be placed anywhere
- Because it uses lower temperatures, it is gentler on clothes.
- Can be 3-4x as expensive (to buy) vs a conventional vented dryer
- Loads can take 1.5hrs to dry instead of 45m
- Removed moisture has to be drained/dumped
Electric dryer baseline:
- Assume 7 cu ft capacity that directly correlates with load size
- Assume 4 kWh/load * $0.10/kWh = $0.40/load
- Assume 5 loads/wk * 52 weeks * $0.40/load = $104/year
Heat pump comparison:
- Assume a marginal purchase cost of $1000 for the heat pump dryer
- Assume 1 kWh/load * $0.10/kWh = $0.10/load
- Assume 5 loads/wk * 52wks/yr * $0.10/load = $26/yr
- ROI is: marginal purchase cost $1000 divided by ($104-26) = $78/yr savings is 12.8 years
- There might be EnergyStar rebates to reduce the purchase price
- In the winter, since no conditioned air is vented outside, there may be extra savings thanks to keeping the garage a closed system
Condenser Clothes Dryer
There are also condenser dryers that are cheaper up-front but twice as energy intense as heat pump dryers. They seem to be a good middle-ground between conventional vented and heat pumps, though they are also often smaller in size (~4 cu ft versus conventional and heat pump 7 cu ft).
- Assume the marginal purchase cost is $500
- Assume load size is (4/7) and thus we’re doing (7/4) as many loads
- Assume 2 kWh/load * $0.10/kWh = $0.20/load
- (5*7/4) loads/wk * 52wks/yr * $0.20/load = $91/yr
- ROI is: marginal purchase cost $500 divided by ($104-91) = $78/yr savings is 38.5 years
- Rebates and extra savings points from heat pump dryer also apply here.
Spin Dryer intermediate step?
Using a spin dryer, this site claims that a five minute cycle at 400W can reduce the drying cycle time by half.
- 400W would be low enough to run on a 110V outlet, so it could be placed anywhere
- Reduced drying times
- In some situations, could dry clothes ‘well enough’ and obviate the need for the other dryer
- Extra step involved
- Extra appliance needed
- Assuming that’s correct, a first pass might cost 0.4 kW * (5/60) hr = $0.033/load to spin.
- This is also 5 loads/wk * 52wks/yr * 0.033/load = $8.67/yr.
- Assuming a purchase price of around $160 (for example, with this product)
- Assuming heat pump is used for the second step, annual cost is halved to $13, so combined annual cost is $21.66.
- The ROI is now ($1000 heat pump + $160 spin dryer) divided by ($104-21.66)/yr = 14.1 years
- Assuming condenser dryer is used for the second step, annual cost is halved to $45.50, so combined annual cost is $54.17
- The ROI is now ($500 heat pump + $160 spin dryer) divided by ($104-54.17)/yr = 13.2 years
- Assuming only a spin dryer is used with a conventional dryer, the annual cost is dropped from $104 to $52.
- The ROI is now $160 spin dryer divided by 52 = 3.1 years
In summary, a spin dryer has a really attractive ROI if you’re willing to put up with having the extra appliance occupying space and the extra step involved.
For environmental purposes, a heat pump dryer seems much better than a condenser dryer. The additional drying time might be a problem, though this could be mitigated by first spinning.