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!

Preface

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

Pros

  • Ventless
  • Some can be 110v, so can be placed anywhere
  • Because it uses lower temperatures, it is gentler on clothes.

Cons

  • 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

Cost analysis

Electric dryer baseline:

  • Assume 7 cu ft capacity that directly correlates with load size
  • Assume 4 kWh/load[1] * $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[2] * $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

Example dryer

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).

Cost analysis

  • 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[3] * $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.

Example dryer

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.

Pros

  • 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

Cons

  • Extra step involved
  • Extra appliance needed

Cost analysis

  • 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

Opinion

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.