Is PV solar more cost effective locally produced or at a remote "plant"?
October 24, 2013 10:19 PM   Subscribe

ASU (Arizona State University) as made a huge investment in PV solar: My friend and I are arguing about the costs of ASU's solar program.

One of us insists it would be cheaper to engineer and build the PV panel farms out in the desert and then back-haul the energy. The back of the napkin cost estimation of doing locally is 2 to 3 time the total project cost. The point made is installing the panels at ASU on the buildings, building shade for the parking parking structures, etc requires extra engineering and cost because the mounting structures are not uniform (all the varied surfaces) and they have need to be 'extra' engineered for civilian safety.

The other person says the costs are probably the same cost, maybe even lower for locally mounted PV panels. Since the energy is local there is not cost for power transmission over a long haul. Land does not need to be leased for this purpose (you are reusing 'wasted' space). The shade from solar panels might lower energy costs further for buildings AND the cars being shielded from the sun. The workforce for locally maintained panels might be cheaper over the long haul. Finally, building a solar plant has it's own costs that are maybe high.

Who's right?
posted by cowmix to Technology (16 answers total) 3 users marked this as a favorite
It's very efficient to transmit electricity over distance. This is mainly due to the fact that the resistance (heat loss) is proportional to the square of the current, and since you can decrease current for the same power by increasing the voltage (P=IV). This source quotes approximately 3.5% loss per 1000km.

However, there is significant up-front cost associated with building the transmission network (if you had to install all 1000km of cable to connect your consumer with your power source, that would probably not be economical) If you can connect to an existing grid this is greatly mitigated.

Assuming you can easily connect the PV installation with the grid, I would think the biggest variable by far is the average sunlight intensity of the panel site compared to the cost of installing transmission lines. If you had to put a site 500km away but it received an average 40% more sunlight year round, it would be providing 38% more electricity after it gets transmitted back - assuming that 40% more sunlight can be converted to 40% more power.
posted by spatula at 10:41 PM on October 24, 2013 [1 favorite]

Efficient power transmission requires that it be stepped up to a high voltage. That extra equipment has a cost too, apart from the cost of the transmission line.

Also, cowmix, your friends should keep in mind that solar panels on a roof will likely reduce heating of the building under direct sun, which would have an impact on cooling costs. Building shade for the parking structures may also be desirable in its own right, to decrease heating of the concrete, thereby mitigating some of the heat-island effect.
posted by Good Brain at 10:49 PM on October 24, 2013 [2 favorites]

Response by poster: @spatula it might be efficient to transmit power but I wonder what the cost is to build power lines, towers, and other equipment to pull that off.
posted by cowmix at 10:52 PM on October 24, 2013

The costs for mounting on buildings are generally lower than for mounting systems on solar "farms". When I worked in the solar PV industry, our rule of thumb was that a farm was 15% more expensive for a small/medium sized system, about 10% for a larger installation, negligible for a super-large (like, 0.5MW) installation

Mounting systems for roofs are a bit like lego - they are standard pieces that you put together to suit the particular roof, and the panels are clipped or clamped on. You don't do a spearate engineering certification on every mounting system you install - you rely on the manufacturer's certification that the system is safe if installed as per their instructions. One cost you need with a building is a lift to get everything up onto the roof and mounted, and safety equipment for people working on the roof.

For a solar "farm", typically a custom designed metal frame is constructed, attached to a concrete footing. Then a roof-mount system will be attached to this frame, and the solar PV panels attached to that as per a roof. You don't need a scissor lift and safety equipment, but you do need earthmoving equipment to dig the footings, a concretor to pour the footings, and you have to transport a lot of steel (for the frame) to the site. You also need an engineerin to certify the footings and frame, because they are bespoke.

For a super large installation, the cost of the ground frames becomes negligible compared to the cost of the rest of the stuff.

An advantage of on-site solar generation is that you do avoid the transmission losses. However, the average losses for converting from DC electrity (out of a solar panel) to AC electricity (which you can use) from a commercial inverter are between 5% and 15%. That is, they are larger than the transmission losses - the inverter is the weak link in the system.

My advice to clients was always to put it on a building, because they could have it quicker, with fewer contractors - and to put more any spare money into better inverters, not more panels.
posted by girlgenius at 11:21 PM on October 24, 2013 [4 favorites]

Using existing infrastructure also avoids the environmental cost of digging up the desert.
posted by carsonb at 12:05 AM on October 25, 2013 [1 favorite]

Those systems are all kinds of awesome and very nice to see. A couple of points not mentioned yet:

- There might be several different meters on each campus and while I'm not up on Arizona's net-metering rules, it might be better to have several smaller systems tied into each meter than have one mega-system.
- Like Good Brain mentioned, in addition to transmission lines, large ground mount systems also need transformers to tie into which are also expensive.
- Ground mount systems require security - fences, motion lights, cameras, etc. to prevent theft and vandalism.
- Large ground mount systems can use larger inverters which typically have slightly higher efficiencies so that's good. On the flip side, if those larger inverters are near people, there needs to be some kind of wall built to act as a sound dampener.
- The ground mount systems they already have in place have single axis trackers which can break down while roof mounted systems generally have no moving parts and require less maintenance.
- In addition to keeping the roofs cool, the modules will also protect the roof from UV damage, extending the life of the roof.
- To periodically clean the modules you will need a water source which may or may not be readily available for a ground mount system.
- Those engineering costs as percentage of the total system cost are actually pretty low.

I feel it's highly likely that the various PPA providers chose the system with the best value for each situation which is why there are several roof mounted systems but also some large ground mount systems in the mix.

On preview, carsonb makes a good point too.
posted by snez at 12:17 AM on October 25, 2013

Desert-based farm systems are indeed an ecological nightmare sometimes. An example would be the major solar farm that is being built right now just north of Desert Center in California, halfway between Phoenix and LA off of I-10. There's a great deal of concern that it will really damage habitat for some endangered species, like a tortoise species that is already greatly at risk.

Here's an article about a similar project in the Mojave desert and some of the issues it has faced.

On the other hand, I can't speak to the logistics of ASU's rooftop and parking lot strategy, but I do know this- the campus has had an immensely positive response to things like the new shade structures in the lot next to Sun Devil Stadium. The Tempe campus could use covered parking for lots like these, keeping cars cooler and reducing some of that awful heat island effect that happens when you pave over huge swaths of land with bare asphalt.

So, put my vote in the "use existing structures or urban spaces" camp.
posted by Old Man McKay at 12:27 AM on October 25, 2013 [1 favorite]

For those who would like visuals, here is what ASU recently installed to cover a couple of huge open asphalt lots.

They've also done this for some parking structures.

Plus, plenty of installation on roofs, etc.
posted by Old Man McKay at 12:37 AM on October 25, 2013

The UK subsidy levels were initially set so that small-scale domestic installations recieved a subsidy about 50% higher than large scale solar farms. These figures were calculated to allow roughly similar rates of return.

My understanding is that the bigger plants get fairly substantial economies of scale from the non-panel parts of a PV installation. I think I'm right in saying that this has meant that larger farm economics have also got better faster as the cost of PV panels has come down in the last 7 years.

Having panels on you roof will mean some loss reduction. Systemic losses in the UK are typically 7-9% though this can rise to 11% at peak times. in terms of the economics of running a domestic system this is reflected in the money you save when you generate your own power rather than take from the grid. (This paragraph is not strictly relevant to the question but I thought might be of interest.)
posted by biffa at 2:29 AM on October 25, 2013

cowmix: "The shade from solar panels might lower energy costs further for buildings AND the cars being shielded from the sun."

This is a significant benefit (lower cooling costs, less damage to shaded materials). The first is easily quantifiable the second is of course more subjective but I'd bet students able to park in the shade will greatly appreciate it.

Also universities have a mandate to educate and by installing solar on campus they normalize it for everyone who goes there. A solar farm off the beaten path out in the desert somewhere doesn't have that same educational component.
posted by Mitheral at 2:39 AM on October 25, 2013 [2 favorites]

Getting a connection to move large amounts of electricity around Arizona is expensive. The last company I worked for was quoted approximately 90% of the capital cost of the project (→ so almost double) to get a very large system connected to APS's HV wires.

Also, the desert thing. I can't remember which is which, but one of the (county, state) wants the surface preserved as much as possible, and the other wants it graded. Flat. Devoid of life. The two don't want to agree.

Also, if you generate power (and shade!) locally, you can use it to avoid your power costs. Remote power effectively offsets it, which is a much more complex financial transaction.
posted by scruss at 4:40 AM on October 25, 2013

Not the right comparison. You wouldn't build PV in the desert. You would build a solar thermal power station in the desert. Lower cost of infrastructure - less energy that goes into the construction - PV cells require a large percentage of the power they will produce in the manufacturing process.
posted by three blind mice at 7:21 AM on October 25, 2013

Lots of great points in defense of local installations.

Campus electrical distribution systems would also factor into this. Some campuses (I don't know about ASU) might have an internal loop connecting all buildings with one or two connections to their utility company/power generation. Others may connect each building directly to the utility company. In the latter case, you'd have to include the cost of building an internal loop to distribute electricity from a remote solar farm.

As others have mentioned, there are also great educational, PR, and pro-environmental behavior reinforcement benefits from having solar on campus.
posted by JackBurden at 8:12 AM on October 25, 2013

tbm: The energy payback for domestic PV is estimated by NREL to be 1-4 years with the variation dependent on the kind of panel but it should be noted that this is based on ten year old data. The EPIA suggest similar figures, again for rooftop installation, and suggests that the typical payback time has declined significantly over time due to technology improvements. The Fraunhofer Institute's PV report at the end of 2012 suggested an energy payback period of less than 1 year for Southern European solar conditions. Since the solar conditions in Arizona are typically slightly better than say Seville, Spain you would expect to also see PV also have an energy payback under a year there.

There is less data on the energy payback of concentrated solar power (CSP) but this report suggests 2-3 years.

I think you would also be looking at less risk with the PV than the CSP due to the greater experience with it.
posted by biffa at 8:41 AM on October 25, 2013 [1 favorite]

A factor that needs to be taken into consideration as far as the environmental problems of installing in the desert is water. This can offset ecological damage - to what degree may be up for debate. First, power generation uses a lot of water. So there's that. But then we have the Central Arizona Project. Moving that amount of water that kind of distance, and uphill to boot, requires a tremendous amount of power. The Navajo Generating Station near Page supplies this power, and it's a coal fired plant that has historically been releasing a lot of bad emissions. A third of that power is going offline to help the station comply with EPA mandates. That's power coming off the grid. The Mojave Generating Station also went offline several years ago. Four Corners is taking 25% of its power offline. So we're losing a lot of power generation and demand is growing. The CAP is going to take up a big chunk of Navajo's output for decades to come. We need to replace that power. Large solar installations can be a part of this plan, in combination with expanded rooftop solar. Smart locations with the sites can help. For example, the San Manuel mine closed some years back and there is a LOT of land where the plant was. That's a perfect location for a large solar installation. Davis-Monthan AFB has a lot of empty land on base that they are installing large scale solar on. There are plenty of places that utility scale installations can be placed that would not further damage the environment.
posted by azpenguin at 4:17 PM on October 25, 2013 [1 favorite]

Yes, tbm, nobody's touching solar thermal these days. Too complex and can't compete with PV. If there was a contract with a really aggressive time-of-delivery bonus it might make sense, but these days, single-axis polycrystalline PB is the only game in town.

There's a metric crapload of PV in the desert just west of Phoenix; most of it wheeling west to California. Most of Palo Verde's output will go west post CA's 2018 (or so) stringent RPS requiement.
posted by scruss at 5:10 PM on October 25, 2013

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