Sunday, September 10, 2017

A Solar-Powered Electric Motorhome? Not Exactly

This recent article reported about the “e.home” concept motorhome being displayed by German leisure vehicle manufacturer Dethleffs.

Many casual readers might interpret this story (which is NOT in fact an actual product announcement, but only the debut of a “concept vehicle”) as suggesting that it is a “solar powered RV.” I hope to communicate here the reality of what is and is not possible regarding solar power and an electrically-propelled RV.

As an owner of a small diesel-powered motorhome and having exclusively driven electrically-powered vehicles as a “daily driver” for the past four years, I’ve frequently done hobby number-crunching to understand the realities of living with a motor vehicle which carries less stored energy than a single gallon of gasoline. I’ve also had to calculate our energy requirements when “dry camping” - going for days “off the grid” depending entirely upon the power stored in what amount to a couple of large automotive-style batteries.

The "home" part of a motor home has very modest power requirements - a couple of hundred watts worth of PVs (photovoltaic panels) can indefinitely maintain "house batteries" to allow frugal power use for lighting and modest ventilation, water pumping and communications/computing requirements. Heating and cooling the cabin beyond 20F differentials between ambient conditions and target interior temperatures requires thousands of watts of power. The Dethleffs e.home features a couple of nice ideas to address cooler climates (like Germany): a way of storing daytime solar warmth in the form of phase-change materials (like the resuable medical heat packs that can be activated on demand, and “reset” by melting their crystallized contents in a microwave oven) for release during cool evenings; and electrically-generated heat delivered to the users via radiant heat - as through warmed floors - which provides comfort in cool climates without attempting to directly heat the cabin air.

Our “small” 11,000 pound diesel-powered Class-C motor home is capable of 20mpg at 60mph - quite good in a category where similar RVs can get single-digit mileage. Driving electric cars around the streets of Los Angeles, we’ve averaged 4.5 miles/kilowatt-hour - this fuel efficiency can be represented as “151 MPGe.” “MPGe” is the U.S. Environmental Protection Agency’s unit of measurement to allow consumers to compare the efficiencies of gasoline-powered internal combustion engines with those of alternatively-fueled vehicles. So the suggestion here is that our BMW i3 has been using stored electrical energy as efficiently as a gasoline-powered vehicle that could go 151 similarly-driven miles on a single gallon of gas.

The article mentions that the 3,000 watts of solar panels “help provide power to its electric drivetrain.” That’s a generous allowance, if potentially misleading. Pushing a small Class-C motorhome through the air at cross-country travel velocities using electochemical batteries is an ugly energy use proposition. The claimed battery capacity for the e.home is 228 amp-hours - about 4 times the capacity of today's typical 80-100 mile EVs, and about the same as a mid-sized Tesla Model S or X battery. Such a battery pack would likely weigh and cost 4 times as much (3K-5K pounds, and $25K-$40K) as typical EV battery arrays.

I estimate that pushing the e.home through the air on level ground at 60mph requires about 40kW (54bhp), which is using stored power over 40 times the rate at which it can charge from sunlight. (Aerodynamic loading is exponential, meaning that the power requirements square with the speed. So if it takes 10 horsepower to push the shape through the air at 30mph, it takes 40 hp to push it at 60mph.)

Three kilowatts (3kW) of photovoltaic panels aren't really going to have much charging impact upon an estimated 72 kW/hours of propulsion battery storage. Even if the e.home were parked in the sun, in the summer, close to the equator, in front of a mirrored wall to expose all the panels at once, it would take at least two days (72kWh/3kW = 24 hours) to restore a fully depleted battery. In real life, days are partly-cloudy, one lives in an arbitrary region below the Arctic Circle and less than half the panels are exposed to the sun at any given time, and then at inefficient, oblique angles. During a 12 hour day, the average yield of 3 kilowatt array would be little more than 1kW. For every hour driven at 60mph, the e.home would require 40 hours of sunlight to replenish in those conditions. That would result in a very leisurely travel schedule. If the e.home were driven to full depletion (all modern battery systems actually preserve a significant proportion of the cells' actual charge to prevent damage), then stopped somewhere in the German countryside to allow the PV panels to top off the propulsion batteries in "mostly sunny" conditions, I guess that the the propulsion pack would achieve 100% charge about Day 8 (maybe Day 11 or 12 if it were short winter days , and maybe 14-16 if you were trying to heat the cabin very slightly with electricity).

The article does refer to a “plug-in motor home,” but again, this may be misleading. In the EV community, “plug-in” refers to motor vehicles which can be fueled by connecting them to the electric utiltiy. However, even this paradigm has its limiations. If a commuter drives 40 miles round-trip to work and home in a typical EV, the vehicle can be charged from any household outlet (so-called “Level 1” charging) in about 10 hours with no special equipment. Installing a “Level 2” EVSE (Electric Vehicle Supply Equipment) at one’s home or office allows charging at an increased rate so that the same 40 miles of battery use can be replenished in just two hours. Even at the higher Level 2 rate which is the most widely available public charging infrastructure, the e.home RV’s large 228Ah battery pack - similarly sized to the mid-sized batteries available in Tesla Model S and X cars - would take most of a day to fully replenish (~14-15 hours from depleted to full). (Most campgrounds have 30-50A power service at each RV site, allowing for similar or shorter EV charging times.) So even with plug-in charging rates 6-7 times higher than its solar panels can achieve, the e.home must plug in for at least 14 hours every 100 miles. (There is a faster commercial-only Level 3 charging infrastructure, but it is not widely distributed, and would almost certainly NOT exist along travel corridors conducive to camping.) If the e.home was plugged into a typical 6-7kW L2 charging station during a long summer day in the sunshine, the ~1kW contribution from its solar panels could shorten the charging time from depleted to full by an hour or so.

So this concept RV shouldn't be interpreted as a "solar powered vehicle" (the title of this article appropriately calls this a "solar-assisted" concept vehicle). Unfortunately, many casual readers won’t perceive that. There's a reason that those cockroach-shaped solar-powered vehicles you see college engineering students "racing" (at 50-60mph) across the Australian outback ride on bicycle tires, can be picked up (gently) by two people and cost hundreds of thousands of dollars. We reached a technological threshold some years ago beyond which it's been possible to build a vehicle which can propel a human at 60mph on level terrain using the sunlight that falls upon the approximate surface area of a conventional motor vehicle. That said, accomplishing this feat requires that such a machine be made of exotic, expensive materials to manage weight and reduce aerodynamic drag, and utilizes the most efficient photovoltaic panels and electric motors, with impracical cost implications for consumer products. The World Solar Challenge takes place across thousands of miles of sunless, hot Australia, and also exposes the drivers to vicious and even dangerous interior temperatures, yet no competitor risks adding performance-robbing weight with refrigeration systems. These vehicles also provide no specific protection in a collision with a conventional vehicle, which weighs 10 to 20 times as much.

So can an RV be propelled by sunlight generated by its own photovoltaic panels?

Photovoltaic panels for residential use currently cost ~$3/watt and achieve efficiencies of 8-10 watts per square foot. So a fantasy solar-PROPELLED RV equipped with 100kW of PVs (so that the 80kW motor can operate in overcast conditions, and can have some reserve power to accelerate and climb hills, in addition to a 40kW cruise demand) would cost at least $300K for the PV panels alone (not including physical support infrastructure, plus interconnecting wiring and control circuitry, or batteries for load surges). The PVs would take 10,000 square feet of surface area. Figure 20% loss in area to framing, so that 12,000 sq ft of area is necessary to mount the solar panels. The surface area of a 53-foot semi-trailer is 450 square feet, so 26.6 trailers - we’ll round that up to 27 semi-trailers worth of PVs would be required to support the 100 kilowatt array. Unfortunately, the mass added by those 27 trailers exceeds the power produced by the 80 kilowatt motor in the RV, which can no longer move the $2M, 1/4 mile long solar road train. Point is, a self-contained solar-powered RV isn't in our future. For that matter, we won't likely be driving cars that are directly powered by the sun. Electrical energy storage will always be a part of the EV model, whether chemical, mechanical or otherwise.
It's certainly possible to travel entirely on _stored_ energy generated by sunlight - we do that when we use gasoline and diesel fuels, which store years of sunlight collected by living plants and allow us to release that energy in a fraction of the time. In fact, wind and hydroelectric energy production are ultimately driven by our Earth's solar-powered weather system. Only nuclear power (which uses radioactive material produced billions of years ago) is a non-solar power source we currently utilize to propel our motor vehicles (in some municipalities, including ours).
So, like many such announcements in our our current age, and appropriate to “concept vehicles,” the Deffleths e.home concept RV is more of a promotional idea than a product. All RVs could benefit from many of the efficiency features touted in the showcase vehicle. But the dream of an electrically-propelled RV that can recharge from sunlight on a self-contained system will remain a fantasy.
I welcome the future in which a electrically-powered RV might exist. I've explored the idea as a Thought Experiment many times, and have commented that it will be a real landmark in EV battery, propulsion and charging infrastructure technology when and if it becomes economically viable to perform long distance leisure travel with the weight loads of an RV (in current practice, RV design goals of utility and comfort increase vehicle weight from 3 to 10 times as much as a passenger vehicle).

I hope to see that.

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