Two types of efficiency improvements give solar cooling a future

A solar air conditioner might be a sensible way to cool your home, if you can determine that the up-front costs will be recovered in reduced operational costs, or that the ecological benefits are worth the extra money.

In my original article on solar powered air conditioning I argued that the high costs of solar power made active solar cooling a much less sensible alternative than investing in a better building envelope, at least for anyone grid connected.

A solar air conditioner is basically a combination of solar electricity generation and air conditioning for the same building, such as a home.

The idea is that the hot summer sun is the main cause of unwanted solar heat gain in a building, so capturing some of that solar energy to generate electricity to run an air conditioner might make sense.

Also, since electricity prices are often highest when cooling demand is highest, a solar electric module can reduce your overall electricity bill most when it is used for cooling.

The trouble with investing in a solar air conditioner is that the up front costs are quite high: air conditioners consume a lot of power, so you’ll need a fairly extensive (and expensive) solar electricity module; and the air conditioner itself can be quite pricy.

There is a sweet spot between the efficiency of the solar air conditioner, and the capacity of the solar electricity module: the more efficient the air conditioner, the less solar generation capacity required, but unfortunately more efficient air conditioners tend to have a higher up front cost.

If solar electric system prices fall enough, then the need for an ultra-efficient air conditioner decreases. Photovoltaic system prices have indeed dropped steadily over the last few years. On the other hand, if air conditioner efficiency substantially increases over time, it may still be worth investing in a high efficiency unit in spite of reduced solar power costs.

And at some point, the combined reduction in solar power cost, and efficiency cost, may now be making a solar air conditioner a viable option for both businesses and homeowners.

Solar power cost changes

The cost per watt of solar generation capacity has dropped from roughly $7.50 in 1996, to $2 in 2008, to $1 in 2012. Note those last two dates: When I wrote my original solar air conditioner article, in November 2008, solar power cost was double what it is today in December 2012. So already the dynamic has changed considerably.

On the other hand, costs for solar power are not expected to drop nearly as quickly over the next decade as they have in the past 17 years, so if you are interested in setting yourself up with a solar air conditioner, waiting for prices to drop further is probably not going to help.

Air conditioner efficiency

Improvements in air conditioner efficiency over the past four years have also made the option of couping solar panels with an efficient air conditioner a better option. Efficiency has in some cases gone up dramatically due to new types of cooling technology.

At the lower level, consider the US DOE standards on cooling efficiency. The latest standard change was in 2006, when the Seasonal Energy Efficiency Ratio or SEER for home air conditioners was raised from SEER 10 to SEER 13, meaning a 30% increase in minimum mandated efficiency.

But actual average efficiency typically lags a change in standard by several years, because the new minimum standard is typically established by looking at the most efficient systems currently available, and setting a minimum efficiency a little below that.

As the new standard comes into play, manufacturers already at the standard get to sell more units, while others play catch-up; eventually competition drives everyone’s efficiency up, so there can be a lag of 2-5 years between a new standard and a major improvement in efficiency.

Three or four years ago when I looked into central air conditioners, the most efficient unit I could find was a very highly priced unit with an SEER 24 rating, while the most efficient unit produced in mass quantities had an SEER rating of 19.

We now have some manufacturers producing central air conditioners with SEER 40 efficiency, albeit at a much higher cost than an SEER 20 unit of equivalent cooling capacity., for instance, produces a line of air conditioners with SEER ratings up to SEER 40, that use an innovative alternative to the usual refrigerant cycle cooling technology found in most central and window air conditioners. They have actually demonstrated the feasibility of running their central AC system on a small photovoltaic system.

An example – my house

To illustrate the changing economics of a solar air conditioner, let’s consider my own house, which is a 1500 square foot home, including the basement. If I wanted to cool my home by 20F throughout the summer – say, from 90F down to 70F – here’s where things would have stood in 1996, 2008, and 2012 given the changing cost of solar power modules (expressed in $ per watt of generation capacity), and the improved efficiency of air conditioners (expressed in the standard EER or Energy Efficiency Ratio):

Year Cost per watt EER of unit Solar module cost
1996 $7.50 10 $9,296
2006 $2.00 13 $1,907
2012 $1.00 18 $689

In just the last six years, the cost has fallen by nearly two thirds, given the increased efficiency of air conditioner units and the decreased cost of solar power modules.

But I still question whether this is the most sensible investment for someone trying to save energy while staying cool. Before you install a solar air conditioner, you should address all of the following first:

Insulation: Make sure your home is properly insulated and sealed. Have a home energy audit done and address as many of the audit findings as possible. You may be able to decrease the capacity of the air conditioner you need (and therefore the solar electric panels) if you can make your home more efficient first.

Attic ventilation: Pay special attention to your attic. If you don’t have adequate attic ventiliation, your attic will create a huge amount of heat for the air conditioner to extract from the home. The attic should have vents at the base of the roof (such as in the soffits) and at the peak, to keep air flowing through it.

Attics can hit temperatures of 150F if not properly ventilated, and much of that heat will make its way down (even through insulation) into your home. That’s a lot of extra work for the air conditioner.

Solar panel siting: Make sure you have a suitable location for the solar panels. I’ve had too many neighbors tell me they’ve been talking with a solar panel salesperson to have a kit installed on their roof. I walk by their house and spot trees to the south, southeast, or southwest of the home. Shade on a solar panel for even part of the day will significantly decrease its output. Even the shadow of a twig on one 4×4″ cell can cut a huge amount of power from the panel the cell is a part of.

Look out for government subsidies

The other factor to consider is the availability of government subsidies for solar power generation. Many states and provinces have mandated purchase rates for small-scale solar power generation, where the government or utility guarantees you a premium on every kilowatt hour your solar electric panels generate. The rates can be up to $0.40, $0.60, or $0.80 per kilowatt hour, while the rate you pay for power you consume may be substantially lower.

If you are committed to having solar electric panels but can access a subsidy like the above, you will probably be financially better off feeding the power you generate into the grid, and pulling out cheaper electricity to run your air conditioner.

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