# which is efficient on solar electricity by photo voltaic or thermal

As we know solar energy used to generate electricity in two major ways: photovoltaic and thermal energy. The most small applications or personal use are mostly using solar cells. But when it comes to large scale (government) electricity generation using solar energy those are preferring the thermal energy option.

I want to know which is producing more electricity per square meter?

And why is thermal energy used on large scale electricity generation?

• One could argue that fossil fuels and hydro are also solar energy... As for the relative efficiencies of your named methods you haven't put much effort into even sketching out the relevant parameters for each and how they might change from, e.g., small (one meter square) to very large (square kilometer) installations. – Jon Custer Jan 4 '16 at 15:52
• That's why I mention Personal and government. Simply I want to know which is more energy harvesting per squire meter? – sugunan Jan 4 '16 at 15:55
• Well, the answer depends on various parameters. Note that Bill Gates can probably afford a personal molten salt plant, so personal vs government is not relevant. And efficiency can be described in many ways, from absolute theoretical thermodynamic efficiency do $/kWh... – Jon Custer Jan 4 '16 at 16:03 • This might be more of an economic question. Wholesale electricity prices have always fluctuated throughout the day, and photovoltaic solar is pushing the midday prices down. Thermal storage of solar heat allows the production of electricity at night, when prices are higher. For personal use, prices are usually quoted up front and constant throughout the day. – MSalters Jan 4 '16 at 16:26 • Everything engineering related in the end boils down to economics. Physics is just a means to satisfy the economics requirements. Why would anybody build thermal plants? Because they can get more money for the energy off the solar peak than on-peak. There is also a strong engineering base for thermodynamic power plants, and those folks like a piece of the pie, too. – CuriousOne Jan 4 '16 at 19:32 ## 2 Answers Most utility-scale (large-scale, the thing you've referred to as "government") generation is photovoltaic. Photovoltaics work on any scale, from watts to gigawatts. Whereas concentrating solar thermal generation needs to get a mass of fluid up to hundreds of degrees celsius, in order to drive a turbine. It's absurdly inefficient (in energy terms and economic terms) to do this at small scale, so solar thermal plants tend to be of the order of tens to hundreds of megawatts. It's still a young technology, and we may yet see gigawatt-scale plants. You also need direct rays for concentrating solar power (CSP), which limits it to only those locations that get the very best direct sunshine. Note that power generation per square metre is almost always irrelevant. It's almost never an important measure, because there's way more than enough space to meet all our energy demands from renewables, and sunlight is free. The one exception is rooftop systems: where the system owner is trying to maximise generation (and thus revenue) for reasonable cost, but has a highly constrained amount of surface area to work with. For what it's worth, at the best locations for solar power stations, a well-designed PV system will typically generate more watts per square metre than a well-designed CSP system. Ivanpah CSP seems to be around$10W/m^2\$, which is well below what we'd get for PV at that location. But land is cheap and plentiful, so it really doesn't matter.

• So what is the reason "CSP" still used even it is low efficient? – sugunan Jan 18 '16 at 5:07
• It's a new technology, still being developed. People will continue to invest as long as it shows the potential to be economically viable. It's very inefficient only at small scale: at larger scale (10s-100s of megawatts) it's efficient enough to be potentially economically viable. – 410 gone Jan 18 '16 at 7:35

Ok, I've got some experience with both solar thermal applications and PV panels. The answer to this question cannot be simple because it involves a wide range of variables. First of all, when comparing such technologies you have to keep in mind that the potential of any solar application depends on the region where it is installed. For example, CSP (concentrated solar power) can be installed only in regions characterised by very high solar irradiance (e.g. Spain, USA), whereas PV farms can be installed also in regions with a limited solar irradiance (e.g. Germany). So if you were in Germany the answer would simply be "PV farms produce much more energy and are much more cost-competitive than solar thermal". In fact, there are no CSP plants but many PV plants. Instead, if you are in a region characterised by a very high solar irradiance, you can install both PV and CSP and have a good result. Which one is better? In this case the answer depends on the specific technological solution chosen; for example, CSP can be realised with 4 different solutions: Linear Fresnel, Parabolic Trough, Solar Tower or Solar Dish, each having different pros and cons and performances. Most diffused ones are Solar Tower and Parabolic Trough; at the current state of the art they both are far from being cost-competitive with fossil fuels without incentives. Their LCOE is also very variable depending on the location (shifting from Seville to Las Vegas would cause a big drop in LCOE). As for the power, there are functioning plants up to 50 MW, and considering all the losses the overall plant efficiency is in the range 15-18%. Major problems are related to storage and to the selection of a proper heat transfer fluid. If you choose to install PV panels, again you can select monocristallyne silicon, policristallyne silicon, thin film, or the promising Concentrated Solar PV with multi-junction cells. Existing PV farms are 99% mono or poly silicon; the efficiency of this technology can vary from 15% to 30% (recent record), but the balance of plant must include other efficiency related to auxiliary components (inverter etc.). One important point is that PV cells perform better in cold climates, as they are affected by temperature. Anyway, PV can usually get a better LCOE performance. New Concentrated PV Multi-Junction technologies can reach 46% efficiency (current record), but they need additional costly components (concentrators, tracking system) and are still not so diffused in the market.