Not really. The headline above is a little tongue-in-cheek. Readers of this blog will know our fans are designed to be run at night, when cool outdoor temperatures can be drawn indoors. Since photovoltaic cells can’t generate electricity at night, directly powering a whole house fan with solar generated electricity isn’t possible—unless, of course, one wants to run their fan during the day, which would heat the home rather than cool it!
We do, however, frequently receive calls from homeowners wanting to know if they can run a whole house fan directly off of their home solar array. We wish they could. Because they’re a more efficient substitute for mechanical air conditioning, whole house fans greatly compliment solar by reducing the size of array needed to provide the same cooling. But again, photovoltaics don’t work at night.
To operate a whole house fan with electricity generated by a solar array, homeowners need some sort of energy storage device that would allow them to store electricity they generate but do not use during the day. Then, this stored electricity could be drawn down through the night to power the whole house fan.
In short, they need a battery—which is why we’ve followed Tesla’s recent announcement of their new Powerwall home battery system with great interest. Home battery systems have been available for some time now. But they have never been as prominent in the popular discourse as they are now, makings this the ideal moment for a
blog post “back of the envelope” analysis about using a home battery to operate an AirScape fan.
For the purposes of illustration (and search engine optimization), we’ll use the Powerwall system as our test case. Please don’t take this as an endorsement; like we said above, home battery systems have been available for some time now, just not from Elon Musk & Co. Also, using the Powerwall as an example has the advantage of making it easy for us to find the information we need at Tesla’s website (www.teslamotors.com/powerwall). Here is the salient information about the Powerwall’s performance:
- The Powerwall is available in two models: a 7 kWh unit for daily cycling, and 10 kWh unit for back-up applications. During the cooling season, a whole house fan will need to be run every, or nearly every day. Thus, from here on in, we’ll consider only the 7 kWh version.
- While the petite Powerwall stores 7 kWh, the homeowner can only use a portion of that amount because the battery isn’t 100% efficient (no battery is). According to Tesla, the 7 kWh Powerwall’s round-trip efficiency is 92%. Since we’re only concerned with the battery’s performance after it has been charged, we need to find a one-way efficiency. Assuming the battery’s inbound efficiency and outbound efficiency are equal, its one-way efficiency would be about 96%. This means the Powerwall’s 7 kWh will only discharge about 6.7 kWh.
- Furthermore, the Powerwall only discharges direct current (“DC”) electricity, whereas whole house fans and every other household appliance only run off of alternating current (“AC”). Thus, an AC-DC inverter is required to convert the battery’s DC output into AC. And since no inverter is 100% efficient, some additional electricity will be lost here as well. Since we power our facility here at AirScape using an on-site photovoltaic array, we have some experience with AC-DC conversion and think 97% is a reasonable assumption for an inverter’s efficiency. Under this assumption, the Powerwall provides about 6.5 kWh of usable electricity.
- Finally, the Powerwall is rated to discharge 2.0 kW continuously, and 3.3 kW in a single peak. Since a whole house fan is designed to be run continuously through the night, we’ll only concern ourselves with the former figure.
These specifications define the constraints within which our whole house fan must operate in order to not exceed a single Powerwall’s performance limits: First, the fan cannot draw more than 2.0 kW at a time. Second, the fan cannot use more than 6.5 kWh in a single sitting. The most watts any of one of our fans draws is 810 (by our new 5.0e model on its highest setting). Since 810 watts < 2.0 kW, all our fans meet the first constraint.
To see if they meet the second constraint, we need to make some assumptions for how long the fan is run and at what speed settings. We recommend our customers run their fans on high for a couple hours in the early evening to quickly cool down their homes, and then lower the speed to low and run the fan through the night to “pre-cool” the house ahead of the next day’s rise in temperatures. Let’s assume this means the fan is run for 3 hours on high and then for an additional 9 hrs on low.
The chart below shows each of our fan models, the watts they draw on their highest and lowest settings, the length of time they are operated in this back of the envelope model, and the resulting total power consumption. We publish our fans’ specs in watts; for the sake of clarity, we’ve converted watts to kilowatts here.
As you can see, under this model’s assumptions, all of our fan’s use much less than the maximum 6.5 kWh. Moreover, our assumptions here are arguably quite conservative (homeowners will probably run their fans for fewer hours on high, and for fewer hours in total). We therefore find it completely possible that a homeowner could use a solar array to charge a Powerwall in order to operate a whole house fan at night.
Of course, it is important to remember there are many more appliances in a home than its whole house fan. Refrigerators, televisions, and lighting, to name just a few, all use electricity after the Sun sets. Whether or not someone could power a whole house fan and every other appliance in their home is beyond the scope of this blog post, and we don’t want to imply they can. On the other hand, up to nine 7 kWh Powerwall units can be linked together to provide up to 63 kWh of storage.
Also, just because a homeowner can store a lot of electricity with a home battery system, doesn’t mean that doing so is worth the money. A 7 kWh Powerwall costs $3,500, and that’s before the costs of an inverter and installation. To the best of our knowledge, this price is very competitive vis-a-vis other battery systems. But even so, given the economics of net metering and off-peak electricity pricing, we’re pretty sure that home battery systems aren’t worth the investment, at least not yet. Over at Catalytic Engineering they have a blog post that explains this as well as we ever could; we highly recommend it.
The one application we can think of where a home battery system would make sense is an entirely “off the grid” house. Of course, even the modest electricity consumption (compared to A/C) of a whole house fan is a lot compared to no electricity consumption at all. Given the high cost of generating and storing electricity piecemeal, if someone truly wants to live off the grid, passive cooling techniques (e.g. shade) and behavioral adaptions (e.g. siestas) are by far the most cost-effective.
That said, we love to talk to anyone about whole house fans, so give us a call if you’d like to know more about whether one of our fans is right for your particular application.