Fossil fuels have
long been the standard for producing heat for industry. They are cheap and available
and produce heat suitable enough for medium and high-temperature applications. According
to a 2021 paper in the Journal of Cleaner Production, the viability of solar
thermal for process heat is dependent on several factors, both technological
and economical:
“The integration of solar thermal energy systems with the
industrial processes mainly depends on the local solar radiation, availability
of land, conventional fuel prices, quality of steam required, and flexibility
of system integration with the existing process.”
Solar thermal energy
converts sunlight into heat while PV solar converts sunlight into electricity. Solar
thermal requires solar collectors, which can be of several different types. Low-temperature collectors may be glazed, unglazed, Trombe walls, or solar roof
ponds. Quotes are from Wikipedia.
“Glazed solar collectors are designed primarily for
space heating. They recirculate building air through a solar air panel where
the air is heated and then directed back into the building.”
“Unglazed solar collectors {aka. solar walls} are
primarily used to pre-heat make-up ventilation air in commercial, industrial
and institutional buildings with a high ventilation load.”
“A Trombe wall is a passive solar heating and ventilation
system consisting of an air channel sandwiched between a window and a
sun-facing thermal mass.”
“Solar roof ponds for solar heating and cooling were
developed by Harold Hay in the 1960s. A basic system consists of a roof-mounted
water bladder with a movable insulating cover.”
Solar air heat collectors in buildings are more popular in
the U.S. and Canada than liquid heat collectors since the buildings have
existing ventilation systems for heating and cooling.
Solar process
heating systems are designed to provide large quantities of hot water or space
heating for nonresidential buildings. Medium-temperature collectors can be used
for water heating. Drying, cooking, steam distillation, and sterilization. High-temperature collectors may utilize parabolic mirrors and are used in
concentrated solar thermal (CST) systems.
Collectors for
water heating include flat plate collectors, evacuated tube collectors, and
some other types. One of the slides from the webinar (see below) lists these
and their characteristics.
Concentrated or concentrating solar thermal (CST) or CSP) can generate high temperatures for process heat. This technology is suitable for very high temps and can be used in mining, petroleum, minerals processing, chemical processing, petrochemical processing, and desalinization.
American Solar Energy Society Webinar Review
This was a
fascinating webinar presented by Bill Guiney of Artic Solar, for the Solar
Thermal Division of the American Solar Energy Society. Guiney is a 42-year
veteran of the industry.
High-temperature
solar thermal applications include power generation, air conditioning, and industrial
process heat. Medium-temperature applications include hot water and air
conditioning.
Low-temperature apps include things like pool & spa
heating. Solar thermal can be utilized and optimized in a number of ways. Solar
can be used to elevate groundwater temps for things like boilers, heaters, and
heat pumps to reduce energy consumed.
For facilities
that use hot water the consumption in gallons/day is needed. Unfortunately, he
says, there is no federal tax credit for solar thermal swimming pool heating.
Hotels use natural gas, electricity, and propane, but could use rooftop solar
for electricity and process heat. Schools can utilize solar thermal. Jails are another
possibility. He notes that private jails seem to have no incentive to save
energy. Flat plate collectors for hot water for these facilities. Greenhouses
and district heating are also possibilities to integrate solar thermal.
Guiney covers several
project case studies showing the diversity of solar thermal process heat
applications. Air & space heating, heat, dehydration, drying & cooling
are the main processes. Air heating is needed for many projects. Dairies use
lots of heat for both heating and cooling. They must get 110 degrees milk from cows
lowered down to 37 degrees quickly which requires cooling. Sanitization and
sterilization are also needed for dairy. These require heat. Solar thermal with
its high temperature can purge their piping and heat exchangers resulting in more
bacterial removal and higher quality milk. He mentions a project pumping honey and
another pumping bitumen (tar). In both cases, the heating is to make it thin
enough to flow. Steam for a distillery is another application. Solar thermal provides
heat for boilers in a number of projects. Enhanced oil recovery projects
require heat. Solar thermal can also assist in evaporation and distillation of
oilfield water in water treatment projects. Meat processing and other food
processing facilities use lots of process heat for sanitizing processes.
He touches on
solar cooling, noting that it is often misunderstood, but doesn’t really elaborate.
He lists four kinds of solar cooling: 1) ammonia absorption, 2) lithium-bromide, 3) dehumidification, and 4) solar-assisted heat pump technologies. Waste heat from dehumidification can be regenerated back into the system.
Solar thermal
for assisting boilers must be low temperature, less than 100 degrees C because you can’t
inject steam into a boiler, the water must be in liquid form. But it can be
used to get the water most of the way there. Boilers can blend heated water and
condensation return to optimize heat exchange. When evaluating projects, it is
important to calculate all the requirements as well as the relative fuel costs and
the relative GHG emissions.
Guiney notes
that a solar thermal system is simple, uncomplicated, and usually easily managed.
He likes to add a 2nd circulation pump to his projects, to run the
system with dual pumps so that if one goes down the system can still run at half
pump rate. He also likes to size his systems low, at 70%, preventing the
possibility of overheating. That seems unusual to me. It seems like there
should be some other surefire way to prevent overheating, such as a digital management
system with sensors and switches that can prevent overheating.
He notes that there
are several solar thermal performance simulators, including a free app from
NREL called System Advisor Model (SAM). However, he also noted that he can basically
do his own performance modeling.
The last subject
was subsidies, which are very good for solar thermal, especially the increase
due to the IRA, which raised the federal tax credit from 26% to 30% with an
additional 10% for a total of 40%. Tax credits are also saleable as are renewable
energy credits (RECs). There are companies that buy tax credits. Non-profits
can participate but have requirements. The USDA offers RECs, loan guarantees
loans to 75%, and 50% USDA grants to agricultural and rural businesses, which
often also include businesses in small towns and small cities in rural areas. It
can mean as much as 90% subsidization! A project can see positive cash flow in
the first year on those projects. While that may be amazing to the beneficiary,
I have argued that is unfair to those who don’t qualify.
A New ‘Thermal Trap’ Breakthrough Uses Solar
Thermal to Create a Temperature Over 1000 deg Celsius
Researchers at ETH
Zurich have made a thermal trap to achieve temperatures for process heat above
1000 degrees C. This means that it can make high-temperature heavy industry process
heat from low-carbon sources. This may eventually have implications for decarbonizing
the heavy industry sector. It was a significant breakthrough, exceeding
previous attempts by about 6 times.
“{CSP} plants typically operate at up to 600 degrees. At
higher temperatures, heat loss by radiation increases and reduces the
efficiency of the plants. A major advantage of the thermal trap developed by
ETH Zurich researchers is that it minimizes radiative heat losses.”
The technology can also improve the efficiency of CSP plants.
References:
Solar
Thermal Divisions Webinar: Solar Thermal Applications for Process Heat. American
Solar energy Society. Webinar video. October 25, 2024. Solar Thermal Divisions
Webinar: Solar Thermal Applications for Process Heat
Concentrating
Solar-Thermal Technologies for Industrial Process Heat. Dr. Kamala C. Raghavan.
Solar Energy Technologies Office. U.S. Dept. of Energy. Office of Energy
Efficiency and Renewable Energy. May 2024. Concentrating
Solar-Thermal Technologies for Industrial Process Heat
Solar
thermal energy technologies and its applications for process heating and power
generation – A review. Ravi Kumar K., Krishna Chaitanya N.V.V., and Sendhil
Kumar Natarajan. Journal of Cleaner Production. Volume 282, 1 February 2021,
125296. Solar
thermal energy technologies and its applications for process heating and power
generation – A review - ScienceDirect
Harnessing
the Sun: Innovative Thermal Trap Reaches Over 1000 °C Using Sunlight. Fabio
Bergamin, ETH Zurich. SciTech Daily. May 28, 2024. Harnessing
the Sun: Innovative Thermal Trap Reaches Over 1000 °C Using Sunlight
Solar
for Industrial Process Heat Analysis. DOE. NREL. Solar
for Industrial Process Heat Analysis | Energy Analysis | NREL
Solar
thermal energy. Wikipedia. Solar thermal energy
- Wikipedia
Solar thermal
collectors. Wikipedia. Solar thermal
collector - Wikipedia
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