Thermal storage is widely regarded as the future for the renewable energy campaign because, unlike many intermittent renewable resources such as wind energy, it offers a “zero-emissions” technology with firm capacity and dispatchability characteristics. The thermal storage system provides an added benefit: allowing the plant to be designed to optimize the electricity load profile to meet specific market needs. A plant can be designed, for instance, to maximize electricity production during a period of peak demand or to continue to produce electricity after the sun goes down.

Figure 1 (left) illustrates how the thermal storage system can be utilized to “shift” electricity production to the peak demand period. Solar energy is collected when the sun begins to shine, but electricity is produced approximately 6 hours later in order to generate electricity during a period of peak demand. The red line represents direct solar irradiation, the solid blue line represents the production curve without storage and the dotted blue line represents the production curve with 6-hr storage.

Technology Description

Thermal storage technology uses a solar “power tower” design, which generates power from sunlight by focusing energy onto a tower-mounted central heat exchanger or receiver.
As shown in Figure 2 (above), a field of sun tracking mirrors called heliostats is used to reflect and concentrate the solar radiation onto the receiver (Step 1). At Solar Reserve’s Solar Two facility, molten salt is circulated through tubes in the receiver, collecting the energy gathered from the sun (Step 2). The hot molten salt is then routed to an insulated hot thermal storage tank where the energy can be stored with minimal energy losses (Step 3). When electricity is to be generated, the hot molten salt is routed to a heat exchanger (or steam generator) and used to produce steam at high temperature and pressure. The steam is then used to power a conventional steam turbine, generating electricity (Step 4). After exiting the steam generator, the molten salt is sent to the cold salt thermal storage tank (Step 5) and the cycle is repeated.

The salt is a combination of sodium and potassium nitrate, with a melting temperature of 460°F. In the liquid state, molten salt has the viscosity and the appearance similar to water. “In solar applications, molten salt is used for a number of practical reasons,” says Terry Murphy, Chief Executive Officer for SolarReserve, who along with others helped develop the molten salt technology at Rocketdyne. “Molten salt is a heat storage medium that retains thermal energy very effectively over time and operates at temperatures greater than 1000°F, which matches well with the most efficient steam turbines. Second, it remains in a liquid state throughout the plant’s operating regime, which will improve long-term reliability and reduce O&M costs. And third, it’s totally ‘green,’ molten salt is a non-toxic, readily available material, similar to commercial fertilizers.”

A primary advantage of molten salt central receiver technology is that the molten salt can be heated to 1050°F, which allows high energy steam to be generated at utility-standard temperatures (1650 psi minimum, 1025°F), achieving high thermodynamic cycle efficiencies of approximately 40 percent in modern steam turbine systems. This high cycle efficiency is maintained while allowing the use of dry cooling towers, which is important in arid states with the best solar potential. The molten salt heat transfer loop through the receiver is isolated from main steam temperatures and pressures, resulting in cost savings through the use of low-pressure salt piping. Finally, the system is designed to minimize the length of the molten salt loop to less than 2,500 feet, which is heat traced to prevent ‘freezing.’

Thermal storage systems using molten salt have been identified for use with other solar technologies, such as parabolic trough systems, which have been the dominant solar thermal technology installed to date. Trough plants will require an additional heat exchanger to transfer the energy from the working fluid to storage and to transfer the energy in storage back to the steam system. It is estimated that the additional heat exchanger required for a trough plant causes a loss in cycle efficiency loss of up to 7 percent. In addition, a trough facility that can only achieve a hot working fluid temperature of 700°F will require approximately 3 times the thermal storage volume to generate a given amount of electricity as an integrated thermal storage system which stores energy at 1050°F.

The high cycle efficiencies and flexibility available with a central receiver system and integral thermal storage provides a compelling offering to the renewable energy purchasers.

Source: Renwable Energy World

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Even without the expected decrease in demand in 2009, technology revenues would be lower than the US $20.4 billion (€14 billion) of 2008 as cell and module prices are around 40% below 2008 levels. Figure 1 (shown below) provides technology revenues from the manufacturer to the first point of sale in the market from 2003 through 2013. For 2009 and 2010, an estimate of revenues for the recession forecast has been provided. With technology prices at the current level, even growth in sales volume, which is highly unlikely, would result in lower revenues in 2009.

Figure 1. Worldwide module revenue volume for recession, conservative and accelerated growth models 2003-2013.

Accelerated growth in the photovoltaic industry continued in 2008, with 79% market growth over the previous year to 5.5 GW. Unfortunately, the market was significantly oversold in 2008, stranding around 2 GW of product in supply side inventory at the beginning of 2009.

Most of the overselling was into Spain, which with a market volume coming in at 2.3 GW in 2008, represented 42% of total photovoltaic system sales worldwide. Along with high prices for modules and PV systems, quite a few instances of poor module product and poorly constructed systems, and permit speculation, the oversold market led the Spanish government to alter its support programme. The new decree capped the market, lowered the feed-in tariff and effectively closed Spain to new product sales for perhaps two years, or more.

Other than Germany, the PV industry currently has no other global market capable of accepting a volume of sales remotely similar to Spain. Moreover, the global recession and financial crises have further hobbled an industry that had been enjoying accelerated growth since 2004. For these reasons, the PV industry is set to experience its first decrease in demand in more than 30 years — and not just flat growth, but a decrease in sales volume of perhaps 30%, or even more.

Figure 2, (below), provides data for 35 years of PV industry growth, from 1974 through 2008, while Figure 3, (below), reveals three forecast scenarios for 2009, which are on based on assumptions related to recession, conservative and accelerated growth for the sector.

Though the PV industry enjoyed accelerated growth from 2004 through 2008, this rate will not continue in 2009, and accelerated growth is unlikely into 2010. In 2009, lending from the international debt markets continues to be depressed. Meanwhile, the loss of a major market — Spain — is having a deleterious effect on growth, inventories remain high, and global economies remain in recession.

Figure 2. Photovoltaic industry history 1974-2008 (CAGR = compound annual growth rate).

Furthermore, although market development is underway, Germany remains the only market capable of consuming more than a gigawatt of product, and other markets, such as Italy, are underperforming. Japan, South Korea, the United States and others continue to experience slow growth. The good news is that module prices in the soft market seen in 2009 continue to decrease significantly.

As previously noted, while the PV industry has experienced slow or flat growth so far this year — to July 2009 — it is an industry that has not experienced negative growth in 35 years or more. Conversely, years of significant strong growth of more than 70% include: 1975 at 150%, 1976 at 141%,1977 at 87%, 1978 at 112%, 1980 at 128%, 1983 at 88%, and 2008 at 79%.

Since 1974, the PV industry has only experienced three years of soft growth, defined here as demand growth of less than 10% in a given year: 1986 at 8%, 1993 at 3% and, 1994 at 10%.

The Incentive Driver

Historically, the PV industry has enjoyed strong growth, though at much lower volumes than today. The strong growth that the PV industry enjoyed since 2004 was driven by incentives, in particular, the feed-in tariff laws in Europe, and even more specifically, Spain’s generous programme. Though for countries in Europe (in general) there is no reason to assume that feed-in tariff programmes will stop altogether, the problems experienced in Spain (overselling, fraud and poor quality products among them) are having a sobering effect on government incentive planning in other EU countries. The support programmes of the future will need to include mechanisms that manage growth along with stimulating it.

The incentives that the industry relies on come with downward price pressure, which is a significant constraint. However, given the goal of grid parity, there is literally nowhere for price to go but down. Grid parity, nonetheless, is a complex subject, differing in most global markets. Moreover, grid parity provides a level competitive playing field for solar (a worthwhile goal on its own), but does not ensure success.

The industry also needs an increasing number of highly trained installers, sales personnel, engineers and such like, and this comes at a cost. Lower costs and prices are necessary for the continuation of incentives and, therefore, demand. For accelerated growth to continue, and for the eventual slowing of demand to happen gradually, unlike the expected steep decline in 2009, the PV industry must learn to manage its demand. It must develop incentives with triggers to control demand when it accelerates too quickly.

The industry must also control its supply chain from expensive raw material, to consumables, and through to the end user, and must participate with balance of systems (BOS) manufacturers to innovate and develop inexpensive and robust BOS. All raw materials, consumables and machinery are more expensive at this point because of the higher price of oil, which is necessary for transportation.

Other caveats to limitless growth are the high price of PV systems, and the availability of less expensive alternatives, including conventional energy sources such as natural gas and coal. In recent years, the current high volume of industry demand, coupled with raw material shortages, threw the industry into a panic. Instead of the technology standard, ‘if we build it they will come,’ the new mantra became, ‘they are coming and we can’t build it.’

The industry reacted by buying silicon feedstock and cell futures, and by raising component (module) and system average prices, globally. These long-term contracts for raw material, wafers and cells are proving unsupportable and in many cases, are being rewritten or ignored.

Figure 3. Recession, conservative and accelerated forecast scenarios for grid-connected PV, 2008-2013.

The Past Can Inform the Future

It is useful to study specific periods in the PV industry’s history, in terms of growth and drivers for growth, to see what can be learned from these periods which can be useful in understanding the direction of this still young industry. Figure 2, offers compound annual growth rates for the PV industry for specific periods, 1974–1984, 1984–1994, 1994–2004 and 2004–2008.

During 1974–1984, strong compound annual growth of 84% was due to utility and government-backed grid-connected demonstration projects. During this period, the grid-connected application was 30%–50% of total demand, though from annual totals less than 20 MW. Following this decade-long period of significant growth, lower compound annual growth of 13% for 1984–1994 was due to an almost complete cessation of these projects. During this period, grid-connected applications (primarily unsubsidized or incentivized) was less than 10% of annual demand.

Stronger compound annual growth of 33% during 1994–2004 reflects the beginning and continuation of the strong incentive programmes that continue to drive PV industry growth. Specifically in Europe, the feed-in tariff model has proven to be the most successful incentive model. Japan’s residential rooftop programme in the late 1990s, a capacity subsidy, built a sustainable market for solar roofs in that country. In the US, incentives in California created the most significant market in that country.

The 2004–2008 period also managed to encompass two significant events for the sector: the PV industry’s greatest raw material (silicon feedstock) shortage and its strongest period of sustained accelerated growth.

During this period, demand for large field grid-connected applications in Europe, largely driven by the feed-in tariff model of incentives, created the largest global market (79% in 2008) for solar systems. However, the solar-grade silicon raw material shortage that had pushed up prices for crystalline silicon modules also created an entry point for thin-film technologies, which had previously been viewed as risky. The industry’s compound annual growth for this period was 51%.

Grid-connected Growth Drivers

Like it or not … strong growth in the PV industry comes with strong growth in grid-connected applications. Off-grid (remote) applications show slow, steady growth over time, but have not driven the industry into gigawatt sales. It is the grid-connected applications (residential, small, medium and large commercial, large field commercial and utility) that dominate the market for photovoltaic modules. Indeed, at 94% of total sales in 2008, the volume of grid-connected installation leaves very little module product available for off-grid applications.

The grid-connected application remains driven by government subsidy/support programmes (Europe’s feed-in tariffs, US rebates, for example). Without such programmes the market for grid-connected PV products would decrease dramatically. The significant decrease in demand in 2009 is a lesson to the industry about the significant changes that could take place in demand, revenues and profitability when markets are abused, and when so-called ‘black swan’ events, such as the global recession, alter the playing field and force reactive market and price setting.

Figure 3 (shown above) offers an aggregate five-year forecast for grid-connected applications. The recession forecast is presented in Figure 3, but is considered a two-year anomaly. Meanwhile, Figure 2 excludes off-grid applications. However, at more than 90% of the total market demand, the volume of grid-connected applications effectively represents the total industry volume.

All is not doom and gloom, however, with encouraging current market developments in the US and some other countries. There is continued progress in lowering manufacturing costs so that a reasonable margin can be maintained along with lower system prices. We see progress in increasing efficiencies for all technologies, and business model innovations, meaning that accelerated growth will resume for the PV industry. Certainly, at this stage in PV industry development (which could be likened to its preadolescence) there is room to grow and much to learn before a stable, sustainable level of annual growth settles in. Until then, exciting, and sometimes painful times remain ahead.

Source: Renewable Energy World

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