Tesla claims to offer the lowest price on rooftop solar systems, but the company may be skewing the numbers a bit. Also, we take a look at some forest fire detection systems powered by innovative energy-harvesting devices.

Is the Tesla Solar Roof the Most Cost-Effective Solar Option?

In a recent blog post, Tesla claimed that it offers “the lowest-ever cost to go solar in the United States.” The post says that the company offers several standard-sized solar arrays (4, 8, 12 and 16kW), eliminating the need for a load analysis to determine how big of a system is required. Instead, the customer enters their zip code and average monthly electric bill, and the online design tool chooses from among the package deals. I can understand the need to standardize systems and automate as much of the design as possible in order to reduce costs, but there are good reasons for doing a site assessment, even when choosing off-the-shelf systems. But before I discuss that, let’s see how well the online design tool compares to my own calculations.

I entered the information for my house and Tesla suggested an 8kW solar array with a pair of Powerwall batteries for backup and storage, suggesting that the battery units (27kWh total) will provide six days of autonomy.

Based on our electric usage and local climate, the recommended array size is pretty close to what I calculated, and the two Powerwall batteries should keep the house energized overnight and during partly cloudy days, but in the event of a power failure or extended periods of cloudiness, 27kWh won’t power my house for six days. I suspect that the assumption is that the storage system will only power essential appliances during a power failure, but that’s not clear from Tesla’s website. 

Note that the incentives cut the original price almost in half. After looking at all the details, it’s clear that Tesla assumes that all available incentives will apply. Some of them, in fact, have limited funding and involve a lengthy application process. Expecting to receive all the available incentives is optimistic, if not misleading.

The benefit of a standard-sized system is that you pay less for the “soft costs,” since the system sizing is automated, but there’s more to a solar design than just estimating the system size. A good site assessment includes a load analysis to determine ways in which the customer can increase efficiency, as it’s almost always less expensive to conserve power than to generate more. Additionally, a shading analysis will determine how much of the solar resource actually reaches the rooftop. In my case, it would show that there are several trees that will severely limit solar electricity production. While removing them would be better for the array, it would also add nearly $10,000 to the bottom line and increase my cooling needs in the summer. And from an environmental standpoint, we’d also be losing natural carbon scrubbers and wildlife habitat. None of those issues are addressed with Tesla’s online design tool.

Furthermore, a solar designer will discuss battery options with the customer. Although the prices are dropping, energy storage is still quite expensive. Most states continue to offer net-metering, which allows the user to put excess solar energy onto the grid, effectively using the grid as “virtual storage.” This eliminates the battery cost, but it doesn’t provide backup power if there’s a utility failure. Nonetheless, $17,000 is a lot to spend on a backup system, so a good solar assessor will present the pros and cons of net-metering versus behind-the-meter storage.

All in all, it appears that Tesla’s online design tool conducts a worst-case analysis when sizing up the array (but not the battery, which is more expensive) and a best-case analysis on the price estimate. It’s possible that taking everything into account, Tesla might offer the lowest-priced option, but customers should be prepared for a higher bottom line than the estimator suggests.

Self-Powered Early Warning Systems to Detect Forest Fires

If a tree burns in the forest and there’s nobody there to see it, can it still cause a devastating forest fire? Of course! (You didn’t think I was getting all Zen, did you?)

But what if sensors could detect a small fire and send an electronic warning to fire-prevention agencies before it develops into a full-fledged forest fire? They can—and quite inexpensively—but remote sensors and transmitters require power (usually in the form of a battery), which isn’t very practical when you need many devices scattered throughout a forest. Two teams of researchers addressed this issue by developing battery-free self-powered warning systems.

One group from Michigan State University powered a carbon monoxide detector and temperature sensor using kinetic energy produced by the movement of tree branches in the wind. The energy is harvested by a triboelectric nanogenerator (TENG)—a device that generates power in the same way that rubbing a balloon against your hair produces static electricity. The TENG is made of two cylinders—one inside of the other—with the inner cylinder suspended by a spring. As the branch moves with the wind, the cylinders rub against one another, generating electricity. The resulting energy is stored in a supercapacitor, which then powers the sensors. The image below shows the output of a TENG-powered carbon monoxide (CO) detector, with the peaks and valleys on the oscilloscope corresponding to the presence and removal, respectively, of a CO source.  

Another team of scientists at Renmin University and Beijing University of Chemical Technology solved the problem with a minimalistic approach, using an inexpensive, paper-based thermoelectric generator (TEG) that serves as both a heat sensor and a power source. Thermoelectric generators produce power when exposed to a temperature differential, so a tree-mounted TEG will generate electricity when fire heats the bottom side more than the top.

The innovative TEG consists of a 5cm-long strip of paper sprinkled with two dissimilar ionic liquids in the center and electrodes at the ends. When exposed to temperature differences as low as 35K, the TEG, which costs a mere $0.04 to make, produced enough power to drive a microcontroller and transmitter. Although its design is simple and very inexpensive, I suspect that the power would only be sufficient for short-range transmissions, where the TENG, which constantly accumulates and stores kinetic energy, would have a farther reach. Either way, as climate change leads to droughts that often trigger forest fires, early warning systems like these will save lives and property.

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