Solar Water Heating
Water heating accounts for a substantial portion of energy use at many residential, commercial, institutional, and Federal facilities. Nationwide, approximately 18% of energy use in residential buildings and 4% in commercial buildings is for water heating. Solar water heating systems, which use the sun’s energy rather than electricity or gas to heat water, can efficiently serve up to 80% of hot water needs—with no fuel cost or pollution and with minimal operation and maintenance (OM) expense. Solar water heating currently represents less than 1% of the potential water heating market (about 1% of residential buildings have solar water heating, fulfilling about two-thirds of each building’s water heating requirements).
Solar water heating systems can be used effectively throughout the United States at facilities that have an appropriate near-south-facing roof or nearby un-shaded grounds for installation of a collector. A variety of building types can take advantage of solar water heating systems including swimming pools, residences, hotels, laundries, hospitals, prisons, and kitchens. Solar water heating systems are most cost effective for facilities with the following characteristics:
- Water heating load is constant throughout the year (not vacant in summer)
- Water heating load is constant throughout the week (use solar heat every day)
- Cost of fuel used to heat water is high (examples include electricity, which represents 46% of the water heating market, and propane, which represents 2% of the market in remote locations)
- A sunny climate (helpful but not required—in 2003, the three largest markets were Florida, California, and New Jersey).
This overview is intended to provide specific details for Federal agencies considering solar water heating technologies as part of a new construction project or major renovation.
A solar water heating system is made up of several key components including:
- Solar collectors
- Thermal storage
- System controls/controller
- A back-up, conventional water heater.
Solar water heating is a reliable and renewable energy technology used to heat water. Sunlight strikes and heats an absorber surface within a solar collector or an actual storage tank. Either a heat-transfer fluid or the actual potable water to be used flows through tubes attached to the absorber and picks up the heat from it (systems with a separate heat-transfer-fluid loop include a heat exchanger that then heats the potable water.) The heated water is stored in a separate preheat tank or a conventional water heater tank until needed. If additional heat is needed, it is provided by electricity or fossil-fuel energy by the conventional water heating system.
Thermal storage is generally required to couple the timing of the intermittent solar resource with the timing of the hot water load. In general, 1 to 2 gallons of storage water per square foot of collector area is adequate. Storage can either be potable water or non-potable water if a load side heat exchanger is used. For small systems, storage is most often in the form of glass-lined steel tanks.
Active systems have a delta T (temperature difference) controller to start and stop the pumps. If the temperature in the solar collector outlet exceeds the temperature in the bottom of the storage tank by a set amount, such as 6°C, or 42.8°F, the controller starts the pump. When this temperature difference falls below another set value, such as 2°C, or 35.6°F, the controller stops the pumps. The controller will also have a high-limit function to turn off the pumps if the temperature in the storage tank exceeds a third setting, such as 90°C, or 194°F. Due to the simplicity and low cost of a delta T controller, it is wise to keep controls independent of any whole-plant energy management system, although it is desirable to include some indication of system performance, such as output from a Btu meter or preheat tank temperature sensor in the building control system.
Solar water heaters save energy by preheating water to the conventional heater. Solar domestic hot water systems are usually designed to meet 40% to 70% of the water heating load. A back-up, conventional heater is still needed to meet 100% of the peak hot water demand generally, especially for cloudy days or for when the solar system is down for service.
Types of Collectors
Although solar water heating systems all use the same basic method for capturing and transferring solar energy, they do so with three specific technologies that distinguish different collectors and systems. The distinctions are important because different water heating needs in various locations are best served by certain types of collectors and systems.
Materials and components used in solar water heating systems vary depending on the expected operating temperature range.
Low-temperature systems (unglazed) usually operate at low temperature, up to 18°F (10°C) above ambient temperature, and are most often used for heating swimming pools. Often, the pool water is colder than the air, and insulating the collector would be counter-productive. Low-temperature collectors are extruded from polypropylene or other polymers with ultraviolet stabilizers. Flow passages for the pool water are molded directly into the absorber plate, and pool water is circulated through the collectors with the pool filter circulation pump. As of 2004, swimming pool heaters cost 10 to 40/ft².
Small sample of an unglazed, low-temperature solar collector showing flow passages and header pipe.
Small sample of mid-temperature flat plate collector showing cover glass, insulation, copper absorber plate, and flow passages.
Mid-temperature systems produce water 18°F to 129°F (10°C to 50°C) above outside temperature, and are most often used for heating domestic hot water. However, it is also possible to use mid-temperature solar water heating collectors for space heating in conjunction with fan-forced convection coils or radiant floors.
Mid-temperature collectors are usually flat plates insulated by a low-iron cover glass and fiberglass or polyisocyanurate insulation. Reflection and absorption of sunlight in the cover glass reduces the efficiency at low temperature differences, but the glass is required to retain heat at higher temperatures. A copper absorber plate with copper tubes welded to the fins is used. In order to reduce radiant losses from the collector, the absorber plate is often treated with a black nickel selective surface, which has a high absorptivity in the short-wave solar spectrum, but a low-emissivity in the long-wave thermal spectrum. As of 2004, mid-temperature systems cost 90 to 120/ft² of collector area.
When deciding whether solar water heating systems are a good match for a particular construction project, several factors must be considered. Solar water heating systems are viable in many applications throughout the country, but special consideration should be given to projects where:
- The avoided cost of energy is high (gas is not available, electricity rates are above
Operation and Maintenance
OM costs of each solar water heating system are estimated at half of 0.5–2% of the initial cost per year, depending on the system type and design. Oamp;M is similar to that required of any hydronic heating loop and may be provided by site staff, with experts called in if something should fail. Regularly scheduled maintenance includes:
- Checking the solar collectors and frames for any damage and noting the location of broken or leaking tubes for replacement
- Examining the proper position of all valves
- Inspecting and maintaining the pipe insulation and protective materials to minimize losses and maintain freeze protection
- Checking the tightness of mounting connectors and repairing any bent or corroded mounting components
- Determining if any new objects, such as vegetation growth, are shading the array and move them if possible
- Cleaning the array annually with plain water or mild dishwashing detergent (do not use brushes, any types of solvents, abrasives, or harsh detergents)
- Checking all connecting piping for leaks and repairing any damaged components
- Examining plumbing for signs of corrosion
- Observing operational indicators of temperature and pressure to ensure proper operation of pumps and controls
- Ensuring that the pump is running on a sunny day and not at night
- Using an insolation meter to measure incident sunlight and simultaneously observe temperature and energy output on the controller faceplate. Compare these readings with the original efficiency of the system (See ASHRAE handbooks for more tests).
- Checking status indicators of the controller faceplate and comparing indicators with measured values
- Documenting all OM activities in a workbook and making that workbook available to all service personnel
- Flushing the potable water storage tank every year to remove sediment
- Flushing-and-filling the heat transfer fluid every 10 years
- Flushing the system to remove scaling due to poor water quality, as required (only potable water portions of the system)
- Replacing the sacrificial anode in the storage tank as needed.
Additional maintenance can include replacing temperature sensors that have been disconnected, replacing pump capacitors and motors, repairing leaks or damage from freezing, and replacing glass broken by hail or vandalism. At some point in time—typically in excess of 10 years—the storage tank may need to be replaced.
For large facilities, active, indirect systems are most frequently used. For smaller facilities in mild climates with a modest freeze threat, passive direct or indirect systems are also a viable option.
The Federal Energy Management Program (FEMP)’s Guide to Integrating Renewable Energy in Federal Construction has more information on assessing renewable energy options.
Monetary savings from installing a solar water heater depend on a variety of factors, including climate, how much hot water is used at the location, conventional fuel costs, the water temperature required, and system performance. On average, however, the installation of a solar water heater will decrease water heating bills by 50% to 80%.
A general rule of thumb for Federal facilities is that a solar water heating installation will pay for itself within 10 to 15 years when installed against electricity. As specified by the Energy Independence and Security Act of 2007, the expected life of a solar water heating system used for life-cycle analysis is 40 years, which means a facility can look forward to as much as 30 years of free energy.
New construction systems usually have better economics than retrofit projects because of reduced installation expenses. The new low-cost plastic solar water heating kits have significantly reduced installation costs, but they do not perform as well as some traditional systems under high-temperature, high-volume conditions. For Federal facilities, a renewable energy installation should pay for itself within its system lifetime including the time/value of money for it to be cost effective. The key parameter is savings-to-investment ratio. A savings-to-investment ratio of greater than 1.0 would be cost effective. Federal life-cycle cost analysis standards are outlined in regulation 10 C.F.R. § 436.
Agencies can often improve system economics and access additional incentives when alternate project funding mechanisms are used. Among the renewable energy project funding options are energy savings performance contract and utility programs. FEMP has established indefinite quantity contracts under which any Federal agency can issue delivery orders for solar water heating systems in an energy savings performance contract. Several utilities offer rebates, leases, or other solar water heating programs.
FEMP’s Guide to Integrating Renewable Energy in Federal Construction provides further information about renewable energy project funding for Federal construction projects.
A complete listing of incentives is provided in the Database of State Incentives for Renewables Efficiency (DSIRE). Contact the local utility company for more details.
Assessing Resource Availability
Several factors are involved in determining whether a site has a good resource for solar water heating. The first is the amount of solar radiation that the site receives. The first map shows the basic solar radiation available in the United States. As noted previously, many sites with an average solar radiation rate above 4.5 kWh/m² per day should be carefully considered for solar water heating.
But, even a site with a less attractive solar resource can have good potential for solar water heating if the energy rate it offsets is high enough or incentives are available. To depict this, the National Renewable Energy Laboratory (NREL) has put together a series of maps that combine solar resource with an assumed system cost and depict factors needed to make a system cost effective. These maps are available for systems that will offset electricity use and for those that will offset natural gas.
As an example, the two maps below depict the electricity rate needed to make a solar water heating system cost effective. One map assumes an installed system cost of 75 per square foot of collector area (likely for a larger, commercial system) and the second assumes a cost of 150 per square foot (smaller system). The first map indicates that much of the country could cost-effectively use solar water heating at 75/ft² if the offset electricity cost is above
The special considerations to consider when designing and installing solar hot water systems include solar access, solar rights, and relevant codes and standards.
Solar Access and Solar Rights
Solar access laws protect a consumer’s right to install and operate solar energy systems on a home or business, including the property’s access to sunlight. Access to sunlight refers to the ability of one property to continue to receive sunlight across property lines without obstruction from a nearby home or building, landscaping, or other impediment. The most common type of solar access laws are solar easement and solar rights.
Solar easement grants the owners of solar energy systems the right to continued access to sunlight without obstruction from a neighbor’s property and prevents future property developments that could restrict solar access. Solar easement agreements must be in writing and are subject to the same recording and indexing requirements as other property interests. The majority of solar easement agreements stipulate the following elements:
- Description. Dimensions of the easement, including vertical and horizontal angles and hours of sunlight required, during which nearby buildings, vegetation, or other structures cannot obstruct direct sunlight to a solar energy system.
- Restrictions. Limits placed upon landscaping and vegetation, structures, and other objects that would impair or obstruct the passage of sunlight through the easement and affect solar energy system performance.
- Terms. The terms and conditions, if any, under which the easement may be revised or terminated.
Solar rights provide protection for homes and businesses by limiting or prohibiting private restrictions (e.g., neighborhood covenants and bylaws, local government ordinances, and building codes) on the installation of solar energy systems. About a dozen states have passed solar rights laws that limit the restrictions that neighborhood covenants and/or local ordinances can impose on the installation of solar equipment. The laws vary in their provisions for protection of solar equipment, types of buildings covered, applicability to new versus existing construction, and enforcement of rights. Vague or absent provisions in solar rights laws have led to lawsuits and delays in a number of states.
.06/kWh. Available incentives would improve this even more.
Relevant Codes and Standards
The use of solar water heating is consistent with administration directives:
- Executive Order 13693, Planning for Federal Sustainability in the Next Decade
- Energy Policy Act of 1992 (EPAct) directs agencies to:
- include renewable energy [such as solar water heating] along with energy efficiency measures (Section 542 of the National Energy Conservation Policy Act),
- demonstrate new technologies, and include environmental benefits such as reduced greenhouse gas emissions in the criteria by which demonstration technologies are selected (Section 549),
- include recommendations for cost-effective renewable energy projects (Section 550).
Install all solar water heating equipment in conformance with industry standards, including:
- American Water Works Association (AWWA)
- AWWA C651 Disinfecting Water Mains
- ASHRAE 90003 Active Solar Heating Design Manual
- ASHRAE 90336 Guidance for Preparing Active Solar Heating Systems Operation and Maintenance Manuals
- ASHRAE 90342 Active Solar Heating Systems Installation Manual
- ASHRAE 93 Methods of Testing to Determine the Thermal Performance of Solar Collectors
- UFC 3-440-04N Solar Heating of Buildings and Domestic Hot Water
- FM Approval Guide
- NFPA 70: National Electrical Code®
- SRCC OG-300-91 Operating Guidelines and Minimum Standards for Certifying Solar Water Heating Systems
Solar Thermal Systems
Solar thermal systems convert solar radiation to thermal energy. These systems differ from PV systems, as PV systems convert solar radiation to electricity, not thermal energy.
The main components of a solar thermal system are solar collectors and a hot water tank. Solar collectors, like solar panels, are installed on the roof of a building. Solar collectors convert solar radiation to heat, which is then transferred to a hot water tank through a heat transfer fluid. The heat transfer fluid is comprised of either water, ethylene glycol, or a combination of the two liquids. There are two types of solar collectors: flat-plate and evacuated-tube. The question of which collector is preferred for your system depends on a number of factors: the roof of the building in question, your budget, the climate of your location, and the type of system you want to design.
Once the hot water tank is heated, hot water can be dispensed throughout your home or transferred to a boiler. If your solar thermal system does not generate enough heat to the hot water tank, a backup system will kick in.
Types of Collectors
1] Flat plate collectors: These are the most basic types of collectors used in the solar thermal industry. They are designed for low-temperature operations in the temperature range of 60oC to 100oC. Their dimensions generally have an area of around 1.5 to 3 sq. m and are very simple in construction and assembly. Flat plate collectors are used for water heating and space heating.
They consist of a box which has an insulating layer on the bottom and sides of the box. A dark coloured absorber plate with attached tubes is layered on top of the insulation and glazed with a transparent material such as plastic or glass. When solar radiation falls on the absorber plate transmitting through the glass/plastic cover plate, the absorber plate which has tubes attached it heats the liquid inside them. Usually, water is used. The water that is heated is then transferred from the tubes either by the force of gravity, a pump or even due to the pressure of the liquid being used. The efficiency of the heating capacity of the collector heavily depends on the ambient temperature and the availability of solar radiation.
Collectors underperform in cold temperatures but are effective during hot temperatures. Another important design parameter that can affect the performance of the collector is the glazing material. The function of such a material is to effectively transmit radiation and minimize losses associated with heat. Therefore, as mentioned before, materials such as glass are commonly used because they can transmit 90% of short-wavelength radiation. Plastics are also used because they also possess high transmittance rates for values as high as 0.4 but their disadvantage is temperature limitations because, at high temperatures, they are prone to some form of deterioration.
2] Evacuated Tube Collectors: These were developed to overcome some of the limitations of flat plate collectors such as low output during cloudy or cold days and internal regression of system components due to weathering or moisture. Evacuated tube collectors consist of solar collector tubes with heat pipes inside the tubes which is vacuum-sealed. There are many pipes which are connected and laid out in parallel. These tubes have a metal inner tube which acts as an absorber. These metal tubes are attached to a fin which helps in minimizing heat loss due to radiation.
The metal tubes are surrounded by a glass outer tube and the space between the inner and outer tube is vacuum-sealed. The heat transfer fluid that is used in these collectors is liquid-vapour phase change materials to transfer heat. These phase change materials can change their phase from liquid to vapour and back to liquid as it undergoes an evaporating-condensation cycle. An example of this fluid is methanol. The cycle involves evaporating the phase change material and converting it to vapour form when solar radiation absorbed. The vapour passes through the heat sink and condenses during which it releases its latent heat. The latent heat is passed through the heat exchanger where it gets collected by either water or glycol which can then be utilized or stored. Meanwhile, the condensed liquid repeats the process.
3] Concentrators: The third type of collector is known as concentrating collectors or concentrators. They can either be stationary or moving collectors. They’re usually used for high-temperature applications because they make use of a concave reflecting surface to direct the sun’s radiation beam to a focal point which increases the radiation flux. There are 2 types of concentrating collectors:
- Non-imaging collectors such as Compound Parabolic Collectors: In this type of technology, the concentrators are unable to image the sun to one focal point and instead direct the beam to 2 focal points. The concentrators should be employed in such a way that there is a gap or space between the reflector and the receiver to inhibit the absorber from directing the heat away leading to heat losses. The gap shouldn’t be too big as well because that can also affect the performance of the concentrator.
- Imaging collectors such as Parabolic Trough Collectors, Parabolic Dishes, Central Receivers and Fresnel Collectors: These collectors can image the sun’s radiation to one single point which helps in achieving very high temperatures. They help in reducing thermal losses by supporting a small absorber area with a large aperture area. In parabolic concentrators, the geometry of the reflector is used to determine the incident angle and the distribution of the angle across the parabolic shaped reflector. The intensity of the radiation and the width of the sun imaging plays a role in the effectiveness of the concentrator. In Fresnel lens concentrators, a single lens is broken down into concentric annular sets of lens. Fresnel lens helps in reducing the production costs because of the flatness of this type of concentrator. Another type of concentrators that are classified as imaging is heliostats and central receivers. These concentrators have sun-tracking mirrors which are called heliostats. These direct the sun’s radiation to a central point on a tower. They operate at high power and temperature levels in the range of 500MW and around 800 o C respectively
Types of Thermal Energy Storage Systems
Solar thermal storage systems can be classified as sensible heat storage, latent heat storage and thermochemical storage.
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1] Sensible Heat Storage Systems: These systems store thermal energy through the heating and cooling process of a medium such as water, rocks and molten salts. This system is quite popular due to its economic feasibility and non-toxicity. This system depends on the specific heat of the medium, the temperature difference and the amount of storage media being used. Sensible heat storage systems can be classified as:
- Underground storage: Heat that is absorbed through solar radiation is absorbed by underground media such as clay, rocks and sand. Heat is pumped to these media using pipes and boreholes positioned at suitable intervals. The charging and discharging rate depend on the rate of heat transfer and the array area of the pipes used.
- Water tank storage:This is one of the most widely used storage technologies wherein a solar collector heats the cold water coming from the storage tank using solar insolation. The heated water is then transferred back to the stratified tank. The tank is also thermally insulated to prevent heat losses.
- Packed-bed storage:In this storage system, the particulate material is packed in a unit and thermal energy is stored by the bed through fluid circulation. The fluid that’s commonly used is air. When heat is added to the system, the fluid moves downward and when the heat is being removed, fluid flows upward. In such a system, heat addition and removal cannot be performed simultaneously. This system is highly stratified which is an advantage to efficiency. The packed bed of matter is heated as a temperature front is transferred from one side of the bed to the next.
- Molten Salt Storage:When considering solar thermal technology, one of the biggest challenges is storage. For solar thermal collectors and even concentrated solar collectors, one of the major storage options that are commonly employed is molten salt technology. It enables the use of molten salt both as a heat transfer fluid and as a storage medium for solar thermal technologies. It can be used in medium to high-temperature operations for storage and transportation of thermal energy. Their major advantage of water as a heat transfer fluid is its reduced risk of freezing during colder temperatures. Water tends to freeze when it gets too cold which can be an operational hazard by clogging the tubes. Molten salt has an improved anti-freeze property which minimizes this risk. Molten salt also has a wider operational temperature range, increased safety and durability. They also have reduced risk of corrosion and are cost-competitive especially by reducing maintenance costs. Therefore, the selection of the heat transfer fluid can significantly affect system performance.
2] Latent Heat Energy Storage Systems: This type of storage system is classified as phase change system because they absorb and release thermal energy during the medium’s change of state/phase. Therefore, the material used for this storage system is known as phase change materials (PCMs). At first, heat is absorbed sensibly as input temperature increases but soon it will reach a point where the heat is absorbed or released at a constant temperature. A further supply of heat will not increase the temperature of the material but instead will absorb this heat and this is known as latent heat. If a PCM has a large latent heat, then better is the material quality. The point at which heat gets absorbed at constant temperature is usually at the melting point of these materials. Similarly, as the material solidifies, it will release the heat to the surroundings which can be harnessed accordingly. These materials are classified as organic materials such as paraffin, inorganic materials such as salt hydrates and eutectic materials which are a mixture of organic and inorganic substances. These materials have high energy density and take less volume which is why PCMs have been the subject of interest and research in recent years.
3] Thermochemical storage systems: These systems store solar radiation in the form of chemical energy. The heat or thermal energy is used to conduct reversible endothermic reactions by storing energy as a chemical potential. The energy is stored in chemical bonds and when the energy is to be released, reverse chemical reactions take place. There are 2 types of thermochemical storage systems. The first one is a direct system where heat from the solar receiver is directly transferred to the reactor where endothermic reactions take place. The second type is the indirect system where heat is transferred to the reactor from heat transfer fluids. This type of energy storage has higher energy density than sensible heat storage or latent heat storage. These systems also reduce costs by reducing the volume needed to occupy in storage tanks because of the chemical potential
Active and Passive Systems
Solar thermal systems can also be classified as active systems or passive systems. Active systems are those systems which use controllers, valves and pumps to conduct heat transfer fluids through the collector. They are more efficient and expensive than passive systems and can operate even during power loss. They’re easier to install as well. Active systems can be classified as open-loop (direct) systems or closed-loop (indirect) systems. Open-loop systems use the same hot water that is pumped through the systems for use in the application. They can be operated either manually or automatically but they are not suitable for temperatures in the sub-zero range where freezing is common. Closed-loop systems are suitable for use in freezing temperature conditions and pump the heat transfer fluid through the water heater.
Passive systems are those systems which do not use external mechanical systems such as pumps or controllers to conduct heat transfer fluid through the collector. They rely on natural mechanisms of conduction, convection and radiation. They use thermal mass mediums such as rocks, water, air, etc. for heat management
2] Solar Distillation and Desalination: Another application for solar thermal systems is for water purification through solar distillation and desalination. Distillation is one of the oldest methods that are commonly used to purify a substance by filtering out the components based on their volatilities. It involves evaporating a solvent in one location and condensing the solvent vapour in another location. This purifies the solvent and when the energy supplied to facilitate this process is provided by solar radiation, it is termed as solar distillation. Conventionally, distillation occurs in constant conditions of temperature, pressure and flow rate but solar distillation is dependent on the solar insolation available with the highest performance shown during maximum irradiance. It also varies throughout the year with it showing better performance during the warmer months as compared to the colder months. The major advantages of opting for solar distillation are because the need for regular operation and maintenance is minimized due to the absence of moving parts. The use of solar radiation also completely avoids burning of fossil fuels and hence has zero greenhouse gas semissions. The versatility in being able to install these systems in remote locations also provides an added advantage. Water desalination using solar thermal energy is a classification of solar distillation which uses passive systems to minimize dependency on construction, operation and maintenance. Using a solar collector significantly reduced thermal losses and increased efficiency because of the small surface area of the absorber and because of the lack of need for extra components.
3]Food Drying: The final application of solar thermal systems is food drying using an indirect passive system. Drying food crops is one of the highly practised methods to remove moisture from food items. Moisture often provides a medium for bacteria and fungi to grow which consequently leads to the food being spoiled. This greatly affects the farmers who harvest these crops and at the same time affect the nation’s economy if not managed effectually. Preventing moisture from seeping into the food helps to retain the flavour and nutritional value of the food. The major factors that affect food drying are airflow, humidity and temperature. The use of passive solar systems uses the process of convection to keep the foods dry by using solar radiation. It is more economical and helps in managing the temperature, humidity and airflow conditions during the different stages of drying
Overall, it may be concluded that solar thermal energy systems are one of the most promising fields alongside alternating technologies, help to power the society. This article inferred the basic understanding of solar thermal systems, the commercial systems that support solar thermal energy from flat plat collectors to concentrating systems. The article also helped in explaining an important storage technology-molten salt, that’s used in these collector systems which operate considerably better than other heat transfer fluid materials. The article finally explained some of the applications using active and passive solar thermal systems which perform more efficiently than conventional technologies. Therefore, we hope that this article has provided you with added knowledge as to why solar and alternative technologies have to be considered for the future!
Republic Of Solar
Insights, Resources and Opportunities.
Novel CSP Design Combines the Solar Receiver with Thermal Energy Storage
Among the many international researchers inventing improvements in Concentrated Solar Power (CSP), a Chinese-Finnish team has designed an interesting innovation. They propose integrating energy storage in rocks and fan-recirculated hot air into the solar receiver itself.
Normally in CSP the thermal energy storage of the solar heat is in molten salts in tanks on the ground, far from the solar receiver at the top of the tower where the heat is collected – or in Trough CSP, far from the receivers in the solar field (How CSP works, trough, tower etc).
Instead, this team proposes combining both solar receiver and storage in the same unit, and placing it on the ground, under a beam-down optical reflecting system.
With no need to build a separate thermal energy storage tank (How CSP storage works) the researchers see potential for cost-cutting in their combination receiver with storage, and the particular design also allows for additional efficiencies.
“The advantage of our novel integrated receiver storage system is a more cost effective compact structure, and it also has higher charging efficiency and absorber efficiency compared to the conventional receiving systems with separated receiver and storage units,” explained co-author Song Yang, a doctoral candidate at China’s Southeast University School of Energy Environment.
Fig. 8. Isothermal diagrams to the vertical-section of the packed bed after 5, 15, 25 charging cycles using a Gaussian (GD) and uniform distribution (UD) as the boundary condition. IMAGE@Yang et al
A novel idea
Although the idea builds on earlier research, this solar receiver design incorporating storage in rocks and fan-driven hot air in a single unit is the first of a kind.
“The integrated receiver/storage idea is not initially proposed by us; Slocum et al have done a similar prototype, but in their design they used a liquid solid heat transfer; molten salt. We just make use of air from the surroundings and rocks in the packed bed,” said Yang.
“Second we propose a reheating device for recirculation and this is very new. Air is not very good at heat transfer, so by using reheating fans to recirculate the air we can enhance very much the heat transfer performance of the packed bed, so we have somewhat overcome the drawback.”
Due to the novel design of directly radiative heat transfer as well as circulation flow, their design facilitates a higher temperature process, with a good solar to exergy conversion ratio (how much thermal energy can be converted to power) of 52% and charging and discharging efficiencies well beyond 99% and 92% at 770°C.
The rocks/heated air combination allows for a much wider working temperature range than current CSP. Molten salt storage, the state of the art commercially, is good at transferring heat, but it has a working temperature range between from 290°C – 560°C from its coldest to hottest (it mustn’t get hotter or it becomes unstable about around 600°C).
Because the rocks are solar heated from outdoor air temperature (so from say 10°C to around 700°C) the result is a much greater working range, resulting in a much higher efficiency, making for lower cost electricity production.
“Rocks can suffer much higher temperatures than molten salts, up to 800°C,” Yang pointed out. “Why not use rocks? They are cheap and easy to access. So our new design can probably make a high temperature dispatchable CSP possible.”
The team will now demonstrate the design at half megawatt scale in China
After completing a stint working with colleagues in Australia, Yang has just returned to his home university in Nanjing where the 450 kWth demonstration plant will be built and tested. Lamps will simulate 900 suns of reflected sunlight, bounced up to the reflector and beamed down onto their receiver/storage underneath.
The test is indoors, because like Finland; Eastern China, where the university is located lacks the solar resource for CSP operation. But Yang said that if the demo results, which he expects to see in two years, are promising, then the next step will be outdoor demonstration, probably in Western China. Of course, once proven, Yang believes that the design would be applicable globally in every high DNI region suited to CSP.
The container of rocks will be sunk into the ground, to prevent ratcheting, which can happen when any solid objects are heated in containers. If the rocks are heated up they might expand more than the tank can, causing the tank walls to deform, resulting in tank failure. So the team plan to sink the packed bed of rocks into the ground, so the lateral earth pressure prevents the buried tank wall from expanding outward during the heating phase.
One module of the 50 MW Yumen beam down system during construction IMAGE@CSPFocus
Advantages and challenges of a beam down CSP system
In beam down, a heliostat field like one for tower CSP reflects beams of sunlight from a solar field of mirrors or heliostats up to a raised central “bowl” of hyperbolic mirrors. These secondary mirrors refocus the beams down onto a focal point underneath where the solar receiver is positioned.
Beam down is itself a very new type of CSP, only prototyped in the UAE at Masdar. and a few other international testing labs. Although it is very novel, the world’s first industrial scale 50 MW beam down project just began operation in 2019 at Yumen in China, as part of its pilot CSP program. Completed in just 3 years, it comprises 15 modules and stores thermal energy for 9 hours.
“Beam-down – or so-called secondary reflecting systems – is naturally suited to our integrated receiver storage. That’s mainly because they can concentrate the sunlight downward to the ground instead of somewhere up in the air. So the heavy structure of our integrated receiver storage can be installed easily,” explained Yang.
However, as a new CSP technology, manufacturing issues and cosign losses in beam down’s secondary mirrors bring as yet unresolved challenges that are still being worked on at the RD level by Yang and other researchers in the international SolarPACES network.
“The biggest drawback of the double reflecting system is that the optical accuracy for the central reflector, usually a hyperboloidal mirror, is very demanding due to a high sensitivity to overall optical efficiency and concentration ratio. This is somewhat difficult to manufacture at the industrial scale; in the other words, not cost-effective,” he explained.
Secondary reflection itself also loses almost 10% in optical efficiency. “But I’m confident the whole other research community internationally can increase the optical efficiency,” Yang concluded. “We are just trying to find a new way, a fundamental method, a mathematical way to design the segmented mirror.”
Glazed flat plate
Flat plate collectors (FPC) are essentially insulated boxes that have a flat dark plate absorber that is covered by a transparent cover (Figure 3.13). The solar energy heats the absorber and heat is carried away by a heat transfer fluid that flows through riser tubes that are connected to the absorber. The riser tubes are attached to the absorber in a parallel pattern or they meander from one side to the other.
The cover (usually a sheet of glass) is held in place by a frame above the absorber. The frame also seals the collector at the sides and at the back. It must provide mechanical strength and rain tightness, and must be designed to enable simple roof- and facade attachment or even integration into these building elements. The back and sides of the collector are insulated. Flat plate collectors are usually installed in stationary systems, i.e., they do not rotate to follow the path of the sun. The advantages of flat plate collectors are their simple, robust, low-maintenance design, and their large and effective aperture area.
Flat plate collectors are most commonly used for commercial or residential domestic hot water systems. These collectors generally increase water temperature to as much as 160 °F (71 °C). Special coatings on the absorber maximize absorption of sunlight and minimize re-radiation of heat. These collectors are prone to freezing and in climates where this can occur a mixture of about 60% water and 40% polypropylene glycol is used as the collector fluid (heat transfer medium).
Flat plate collectors similar to today’s design have been manufactured for over 30 yrs and experience has been gained as to the proper materials to use for best performance and long life. The casing is typically made of aluminum. The absorber plate is made of copper or aluminum; steel is seldom used. To maximize the absorption of the solar energy the absorber plate is typically coated with black chrome, which is a selective covering providing good absorption and weak reflection of solar radiation.
Copper is normally used as the flow channel (tubing) through which the heat transfer fluid flows. It must be well bonded to the absorber plate for good heat transfer. The tubes are commonly placed in parallel rows (as shown in Figure 3.13) where the flow is released in a header at the top of the collector and is collected at the bottom.
Another tube arrangement is for the flow to meander across the surface of the absorber in a back and forth serpentine fashion. In this case the volume of heat transfer fluid spends more time on the collector surface and a greater temperature increase occurs. To obtain proper heat transfer from the absorber to the collector fluid the spacing between the runs of tubing cannot be too great and a tube interval of 4 to 5 in. (102 to 127 mm) is typical. In all cases, the tubes in a collector need to be placed so that the fluid can completely drain from the collector by gravity.
The housing around the absorption plate is mainly to minimize the heat loss to the environment and to provide a weather tight enclosure to prevent corrosion and other types of deterioration. Behind the absorption plate, rock or glass wool, or an insulating foam may be used an the insulating material. Typically a depth of 1-1/2 to 3 in. (38 to 76 mm) of insulating material is used. The insulating material must have the thermal stability to withstand the high temperatures that occur during times of collector stagnation. A glass cover is placed above the absorption plate that allows the solar radiation to pass through while limiting heat loss. Plastic covers deteriorate over time and are not recommended. Double pane glass covers retard the transparency to the solar radiation and thus are not commonly used. For sealing materials, EPDM or silicone rubber type materials should be used as the seal between the casing and the glass cover; adhesives should be silicon based and openings for pipes should be sealed with silicon based products.
Flat plate collectors are used mainly for producing domestic hot water and, in some cases, where building space heating is also accomplished. Standard flat plate collectors typically perform best providing hot water below 160 °F (70 °C). There are high performance flat plate collectors (those with a double, anti reflective cover) that perform well providing up to 200 °F (93 °C) hot water. These are seldom used due to their high cost. Above that temperature, the efficiency drops significantly due to the higher temperature difference between the collector fluid and the ambient air.
It is possible to reduce the thermal heat losses by avoiding convective losses such as by using vacuum tube collectors. The following section discusses this option.
Evacuated tube collectors (Figure 3.14) can be designed to increase water/steam temperatures to as high as 350 °F (177 °C). They may use a variety of configurations, but they generally encase both the absorber surface and the tubes of heat transfer fluid in a vacuum sealed tubular glass for highly efficient insulation. Evacuated tube collectors are the most efficient collector type for cold climates with low level diffuse sunlight.
There are three types of evacuated tube collectors: (1) direct flow, (2) heat pipe, and (3) Sydney tube type. The direct flow type has the heat transfer fluid flowing through copper tubes attached to a absorber plate mounted inside the evacuated tube. The heat pipe type uses a heat pipe attached to the absorber plate. The heat pipe transfers the heating energy to the condensing section of the heat pipe where the collector fluid is warmed. This occurs in the header where the evacuated tubes are connected. The last type has an evacuated tube called a Sydney tube (Figure 3.15) that encapsulates a heat conductor sheet (absorber) with heat transfer fluid carrying tubes. The Sydney tube slides over the absorber section and locks into the collectors header forming a tight seal. Within the Sydney tube the space between inner and outer glass tube is evacuated. The selective coating is sputtered onto the outside of the inner glass tube. A heat conductor/transfer sheet is located inside the inner glass tube that conducts the heat from the glass into the U-form tubes carrying the heat transfer fluid. The Sydney tube type collector’s performance can be enhanced through the use of a compound parabolic concentrator located behind each tube. This device will reflect the solar radiation that passes between each evacuated tube back to the underside of the cylindrical absorber in the collector tubes. There are various other construction methods like flat or round absorber, and single- or double-walled glass.
All evacuated tube collectors have the following in common:
- A collector consists of several evacuated glass tubes positioned in parallel and are joined by an insulated manifold at one end for the supply and removal of the heat transfer fluid (Figure 3.16).
- Due to the vacuum insulation (pressure 10-2 Pa) heat loss caused by conduction and convection are minimal.
- The upper end of the tubes is connected to the “header.”
- The tubes are circular to withstand the outside pressure.
Evacuated tube collectors have only insulated tubes and a pipe header to which the evacuated are connected. The collector fluid tubes use copper and typically black chrome is used as the selective absorber coating. The pipe header is insulated and has a protective cover.
This type of collector is used when there is a need for hotter water than would be necessary for domestic hot water heating. Hotter water is needed for applications that have cooling in the summer as a requirement and in some cases where building heating is a major need. Solar assisted cooling uses an absorption or adsorption chiller, which requires hot water temperatures in the range of 130 to 350 °F (55 to 180 °C).
An evacuated tube type collector may also be chosen as an alternative for a flat plate collector in areas where winter time freezing occurs. In this case, water would be used as the heat transfer fluid in the collector and warm water would be pumped into the outside piping and collector when freezing of those components is threatened. This would required a small amount of heated water due to the insulating quality of the evacuated tubes. As a result, the cost and inferior heat transfer characteristics of a water glycol mixture is avoided. Also the a hotter water could be produced in the collector providing a lower heat transfer fluid flow thereby reducing distribution pipe and storage tank sizes. Also, the heat exchanger between the collector and the storage tank could be avoided thus reducing the required leaving collector temperature. As a total system, the evacuated tube collector could have a total cost competitive with a flat plate collector system. The use of evacuated tube type collectors obviates most of the stagnation concerns associated with an anti-freeze heat transfer fluid.
These collectors use curved mirrors to FOCUS sunlight onto a receiver tube (sometimes encased in an evacuated tube called CPC or compound parabolic collectors) running through the middle or focal point of the trough (Figure 3.17). They can heat their heat transfer fluid to temperatures as high as 570 °F (299 °C). Such high temperatures are needed for industrial uses and for making steam in electrical power generation. Because they use only direct-beam sunlight, parabolic-trough systems require tracking systems to keep them focused toward the sun and are best suited to areas with high direct solar radiation like the desert areas of the Southwest United States. These collector systems require large areas for installation, so they are usually ground mounted. They are also particularly susceptible to transmitting structural stress from wind loading and being ground mounted helps with the structural requirements.
Parabolic-trough collectors generally require greater maintenance and supervision and particularly benefit from economies of scale, so are generally used for larger systems. Because of their higher cost and greater maintenance needs this type of collector is not recommended for US Army heating needs in their standard buildings.