In 2012, Johnson Controls Inc. published a white paper I wrote titled “Solar Thermal Energy: The Time Has Come.” In it, I stated, “Solar heating, often overshadowed by photovoltaic systems, is the most cost-effective, on-site renewable energy resource. It presents a vast opportunity for public and private organizations to save on fossil fuels, cut costs and reduce carbon emissions.” What’s changed? These facts are truer today.

In the white paper, I discussed the technical potential for solar water heating in the United States; valued at about one quadrillion Btu of energy savings per year, worth billions of dollars in energy cost savings and 50 million to 75 million metric tons of carbon dioxide emissions.

These figures may be merely theoretical, but they illustrate the vast possibilities for solar thermal technology to displace fossil fuels, counteract climate change and save building, business owners and industrial processes money (hundreds of thousands of dollars). 

For those of you who only know solar energy as photovoltaics (PV), there is another solar technology that has the potential to significantly reduce greenhouse gas (GHG) emissions often referred to as the other solar “white meat”; basically ignored by most of the state and DOE, Federal Energy and R&D programs except solar thermal electric generation (CSP). Common uses of the other “white meat,” or I should say the other solar technology include: swimming pool heating, boiler water preheating, domestic water and space heating, air conditioning and high temperature heat for a wide range of commercial and industrial processes such as Industrial Process Heat (IPH). 

A recent study titled, “Initial Investigation into the Potential of CSP Industrial Process Heat for the Southwest United States,” by Parthiv Kurup and Craig Turchi of the National Renewable Energy Laboratory, looked at the technical potential and the applications of the different CSP technologies based on solar delivery and facility temperature requirements. The assessment for California indicates a technical thermal energy potential of almost 23,000 TWhth/yr., significantly more than the estimated demand of about 48 TWhth/yr., for the industrial sectors in California that utilize mostly natural gas for IPH. The report validates the contributions and opportunities for commercial solar industrial process heat (SIPH) plants, which is becoming a growth industry and opportunity to re-establish the contributions of solar thermal heating.

The report also states, “After significant interest in the 1970s, but relatively few deployments, the use of solar technologies for thermal applications, including enhanced oil recovery (EOR), desalination and industrial process heat (IPH), is again receiving global interest. In particular, the European Union (EU) has been a leader in the use, development, deployment and tracking of SIPH plants. In the nonresidential sector, users of solar thermal technology include hotels, hospitals, prisons, restaurants and cafeterias, government buildings, universities and schools, athletic facilities, manufacturing plants, and laundries.” 

These are all growth markets that are not limited to Europe and the southwest U.S. These applications are in every community and city in the nation. The industry, and universities, like the University of California Merced Solar Lab and National laboratories, continue development of high temperature concentrating technologies that are capable of temperatures from 230 F to 350 F or higher and advancements in storage for CSP. The U.S. DOE is also investing in R&D for advanced concentrating solar power (CSP) for technologies producing 1200 F or more.

The important fact is solar thermal technologies capable of temperatures reaching 350 F for SIPH plants are currently economical and available today in the marketplace.

In today’s renewable energy market, solar thermal collection systems provide lower a levelized cost of energy (LCOE) than any other solar energy technology due to technological efficiencies and cost advantages, therefore making a better business case than any other technology for broader market acceptance. When the LCOE, the relatively low U.S. market penetration and manufacturing demand needs of the solar thermal market are collectively considered, a tremendous investment opportunity is revealed. 

SIPH applications require high-temperature thermal technologies, which may require large surface areas and tracking systems adding significant hardware and operating and maintenance costs. These concentrating technologies need direct beam radiation, which limits their applications to the desert and dry environments and generally have significant production losses in diffused radiation, which makes up the majority of the U.S. and all of the Caribbean. New external concentrating parabolic collector (XCPC) systems are non-tracking, use direct and global horizontal (diffused) radiation and can be applied in any climate. 

At present, solar thermal technology faces some headwinds, but longer-term trends appear to work in its favor. For the time being, the price of natural gas — the main fuel solar heating displaces — are at low levels as hydraulic fracturing (fracking) operations dramatically increase domestic supplies. 

Fuel and commodity prices are cyclical by nature. In 2008, prior to the great recession, natural gas prices stood near historic highs. Prices may rise again as the U.S. exports more gas, as utilities add gas-fired peaking power plants and replace older, polluting coal-fired power plants used for base load with smaller gas turbines. Also with potential to tip the scale are the increased production of liquefied natural gas (LNG) for export and wider acceptance of compressed natural gas (CNG) as a fuel for buses, taxis, cars and a wide assortment of fleet vehicles.

Growing numbers of states and utilities offer incentives and rebates for renewable energy installations. In addition, renewable portfolio standards (RPS), or some kind of renewable requirements, have been passed in about 40 states, requiring utilities to derive specified percentages of their power from renewable sources. Of those, about 16 allow solar thermal to meet the goals.  

Electric utilities, municipalities and some state legislatures have developed incentives and marketing campaigns using photovoltaics to meet RPS requirements. As I just said, of the 16 states that include solar water heating, 13 states allow solar space heating, and 11 states include solar industrial process heat to qualify for the RPS. Much more could be done to develop solar thermal incentive programs for residential, commercial and industrial applications. It is apparent the industry and the industry association has let the PV industry dominate the discussions at the state and national legislatures and utility commissions. It is clear, the prospective users and regulators of solar thermal energy may not fully understand it or appreciate its versatility and value. It is a different industry today. It’s not about heating water; it’s about industrial process heat, clean water and air-conditioning.

The California Energy Commission, again leading the way to energy independence, renewable energy and energy efficiency to reduce carbon emissions, has issued a RFP for advanced water heating system demonstrations, advanced HVAC and building envelope demonstrations, integrated natural gas system demonstrations and applied research strategies for appliances, zero net energy buildings, and codes and standards.

More opportunity than meets the eye

The most widespread solar thermal application is water heating. On average, for each such system installed in place of an electric water heater, 0.5 kW of peak demand is deferred from the utility’s load. When a utility solar water heating program like Hawaii’s has thousands of solar water heaters installed displacing electricity, the demand reduction is measured in megawatts. 

A commercial solar water heating system will displace the hot water generated by a natural gas-fired boiler. On a larger scale, commercial SIPH plants will provide the necessary energy for:

• Industrial process heat 
• Desalination
• Food and beverage processing
• Solar cooling and refrigeration systems 
• Oil and gas extraction
• ORC or Stirling electric generation

Space heating

Similar to solar water heating systems, these systems generally use more solar collectors, larger storage units and more sophisticated designs. Concentrating or tracking solar thermal technologies are required to meet space heating loads. For the higher temperatures needed for hydronic forced air heating systems (180F) temperatures, flatplate collectors and most evacuated tube systems cannot consistently operate at those temperatures. 

Cooling

Here, solar heating systems are coupled with absorption chillers and use a thermal-chemical sorption process or ammonia to produce air-conditioning without electricity. The process is like that of a refrigerator except that there is no compressor. The absorption cycle is driven by a thermal transfer fluid — heated water or glycol mixture — from the solar collector. Water cooled to about 44 F runs through copper piping, and forced air passing over the piping produces air conditioning. Options include replacing electric chillers or injecting chilled water generated by a solar absorption chiller into a building with a large cooling load.  

Despite the high potential, solar thermal capacity in the United States lags behind much of the world. For example, on a per-capita basis, the nations’ ranking has dropped from 35th to 50th globally in solar water heating (excluding swimming pools).

Megawatt-scale solar thermal applications for district heating and solar heating and cooling in the commercial and industrial sector is a growing market. The two largest solar thermal systems are in Denmark and supply heat to district heating networks. The two largest solar cooling systems are in Singapore and the United States. And, the world’s largest solar process heat system is installed in Chile at a copper mine. 

Catching the sun

Many organizations fail to benefit from solar thermal energy largely because they do not know the many possibilities it offers. There are three basic levels of solar thermal energy:

• Low-temperature (80 to 100 F) for purposes such as swimming pool heating.
• Medium-temperature (100 to 160 F), largely for domestic/service hot water heating and space heating.
• High-temperature (180 to 350+ F) for industrial processes and air conditioning and hybrid HP desalination.

When is solar thermal attractive?

Solar water heating systems can be highly cost-effective in facilities that have constant or even intermittent hot-water demands. Other forms of solar heating can be economical in a wide range of settings, depending on heat requirements, local climate and sun conditions and other factors.

Advances in thermal or seasonal storage will have significant impacts on net-zero energy buildings. There are low cost methods to integrate seasonal storage into buildings which will improve the economics and significantly reduce the energy required to heat buildings and homes in the winter. Years ago, heating buildings with large solar thermal systems in the winter caused problems in the summer. Too much heat, stagnation and over-heating were common problems. Drainback system designs solved the over-heating issues, but the economics were upside down, using the system only in the winter. Today, new technologies allow us to heat the building in the winter and drive absorption chillers for cooling or ORC electric generators in the summer. These systems can easily couple to thermal storage if necessary so the economics for year-round solar thermal systems are significantly improved but unfortunately under-utilized. 

Solar thermal is a highly cost-effective way to deploy renewable energy, reduce long-term operating costs and make progress toward sustainability, reducing GHG or green building goals.