Posts: 9398
Joined: Mon Sep 19, 2011 1:49 pm
Location: East side of San Francisco Bay

Re: GCC: Kia and Hyundai to introduce solar roof charging on selected vehicles: HEV, BEV and ICE

Fri Nov 02, 2018 4:31 pm

RegGuheert wrote:
GRA wrote:
RegGuheert wrote:This type of system is a total waste on any vehicle which is parked in a garage during the daytime.

Of course, and it's also a total waste at night. So?
Let me be more clear: solar panels should not be installed on cars because in many cases they will not be usable. Many, many cars are parked in garages or other parking structures, particularly in the high-density (read: crowded) cities which you recommend that people live in. Solar panels should be installed in places where they will be hit by the sun. Otherwise we are simply damaging the planet for no reason, i.e. we are virtue signalling.

On top of that, even cars parked in the sun will largely squander the resources which went into the manufacture of the solar panels. This is because the 12V battery is likely to be full much of the time, so the panels will not produce at a full rate. Also, the panels cannot be oriented in a manner which optimizes their production.

Or, we could let customers, who know what conditions their cars will experience, decide whether or not the option makes sense for them.

Let's look at what's likely to be a typical system, as I'm informed as well as willing/able to run the math ;) . I measured the flat roof area of my Forester this morning, and it's about 6' x 3' (the roof curves down at the front to meet the windshield, and I didn't count that area, although I could have). The Niro's about 4" shorter overall than my Forester, and I don't know what the usable width is, so let's be conservative and call the Niro's flat roof area 90% of 18 ft.^2, or 16.2 ft.^2. Converting to metric, that's 1.505m^2, call it 1.5m^2. Peak sun is defined as 1,000W/M^2 at sea level, although in many metropolitan areas actual peak insolation is more like 800W/m^2 owing to air pollution; OTOH, at altitude or in deserts it can be 1,200W/m^2 or more. As of 2015, the best research cell efficiencies were (organic, perovskite, and dye-sensitized cells) 10.6-12.6%; thin films, 13.6-23.3%; crystalline Si cells, 21.2-27.6%; and multijunction cells (with concentrators), 31.6-44.4% (NREL 2015: "Best research-cell efficiencies". They're a bit higher now: Naturally, commercial modules are lower efficiency.

Assuming single-junction crystalline modules with an efficiency of at least 10% (12-16% is typical for commercial single-junction mono- or poly-crystalline modules) we can expect 80-120Wp/m^2, or for this panel area 120-180Wp. However, as the panels are flat rather than tilted at something close to the latitude, we have to apply a correction factor. For Northern California (Lat. say 38 deg.), NREL designated a 7/12 (30 deg.) roof pitch as 1.00, the best year-round production. For a flat installation (where the orientation is irrelevant) the multiplier is .89, so that gives us 106.8 - 160.2 Wp., call it 107-160Wp. This is enough to power most aux. loads while underway, and replace much if not all of the daily vampire drain on PEVs if parked in the sun, as well as lowering the internal temp when needed. Using more efficient modules and/or the hood area as well will increase the output.

Summarizing, while expecting a car-roof-mounted PV system to be able to recharge the drive battery significantly is not realistic*, handling most or all of the aux. loads and/or reducing or eliminating vampire drain is, if the car is regularly parked and/or driven in the sun. I live in the city, my car sits on my parking pad unshaded for 6 or more hours a day for a week or more at a time, and on road trips the sun's usually shining, I'm often at altitude, in the desert or both, and I'm often parking at trailheads with no shade. For someone whose situation is similar to mine, such a panel might make sense, depending on the price and how much it improves the mpg/range. For someone living in Dublin and/or who regularly garages their car during the day, almost certainly not.

Toyota already offers a solar roof on the Prime, but currently in their domestic market only, possibly because their current version can't meet FMVSS standards for roll-over crashes. An alternative reason is that it apparently costs the yen equivalent of $3k, which is way too expensive for what you get.

*On further reflection and calcs, let me modify that statement. If I thought it reasonable to drive a 3,000+ lb. car (instead of riding a 30 lb. bike) the 8.4 miles to work and back, it would sit in an essentially unshaded parking lot (the only possible shade would be partial and intermittent from the shadow of a few scattered light poles) for 8 hours a day. In summer, we get about 6 hours or so of peak sun equivalent. Max. driving speed enroute is limited to about 35 mph (avg. including stops for signals is under 20 mph), so I could expect at least 5 (and maybe 6) miles/kWh assuming no A/C use. Given the above facts, such a PV set-up should be able to provide all the energy I'd need to drive the 4.2 miles home for several months of the year (assuming no energy used for anything else).

Wondering how much PV on the hood as well would add, I measured my Forester's. Max. width was about 57 inches, but it tapers a bit and I figured 54" could be counted on. Avg, length after the round down to the bumper was 36", minus a square inch or two for the windshield washer nozzles. The hood slopes downwards slightly along its entire length before the round-down. Call it 13.5 ft.^2, or after converting to metric, another 1.25m^2, for a total (slightly less than flat plate) PV area of 2.75m^2. As I have a reflective, insulated windshield sunshade, when parking in the sun I normally face the car south or somewhat west of south when I have a choice, so the front of the hood and roof slopes would actually boost the useful PV gain - however, I'll assume it's all horizontal for this calc. Six hours of peak sun equivalent insolation would produce (10% efficient modules) 1.65 kWh x .89 (flat tilt modifier) = 1.4685 kWh, quite possibly allowing me to do the entire round trip commute off the car's PV. 12% or better PV and it would be pretty easy.

Of course, the ideal solution would be to cover the parking lot with a PV roof, but until the property owner does so (voluntarily or through government mandate), this would be an option.
Guy [I have lots of experience designing/selling off-grid AE systems, some using EVs but don't own one. Local trips are by foot, bike and/or rapid transit].

The 'best' is the enemy of 'good enough'. Copper shot, not Silver bullets.

Return to “Solar”