Thermal Storage Walls

 A thermal storage wall is a passive solar heating system in which the primary thermal storage medium is placed directly behind the glazings of the solar aperture. The outer surface of the massive wall is painted a dark color or coated with a selective surface to promote absorption of solar radiation. Solar radiation absorbed on the outer surface of the wall is converted to heat and conducted (or convected in the case of the water walls) to the inner surface where it is radiated and convected to the living space. Heat transfer to the living space is sometimes augmented by the addition of circulation vents placed at the top and bottom of the mass wall. These vents function in the same manner as the vents in a TAP system except that only a portion of the solar heat delivered by the system passes through the vents.

A thermal storage wall provides an effective buffer between outside ambient conditions and the building interior; night time heat losses are reduced during the cold winter months, and during the summer, unwanted heat gains are limited. This moderating effect generally enables thermal storage walls to outperform direct gain systems. There are many types of thermal storage walls distinguished by the type of storage medium employed.

 

Trombe Wall. A Trombe wall is a thermal storage wall that employs solid, high density masonry as the primary thermal storage medium. Appropriate thicknesses range from 6 to 18 inches depending on the solar availability at the building site. Sunny climates require relatively thicker walls due to the increased thermal storage requirements. The wall may be vented or unvented. A vented wall is slightly more efficient and provides a quicker warm up in the morning but may overheat buildings

containing little secondary thermal storage mass in the living space.

 

Concrete Block Wall. Ordinarily, a thermal storage wall would not be constructed of concrete building blocks, because solid masonry walls have a higher heat capacity and yield better performance. However, where concrete block buildings are very common they may offer opportunities for passive solar retrofits. The south facing wall of a concrete block building can be converted to a thermal storage wall by simply painting the block a dark color and covering it with one or more layers of glazing. Walls receiving this treatment yield a net heat gain to the building that usually covers the retrofit costs rather quickly. The relatively low heat capacity of concrete block walls is offset somewhat by the large amount of secondary thermal storage mass usually available in these buildings. Concrete floor slabs and massive partitions between zones help prevent overheating and otherwise improve the performance of concrete block thermal storage walls. Concrete block thermal storage walls may also be introduced during the construction of new buildings. For new construction, however, it is advisable to take advantage of the superior performance of solid masonry walls by filling the cores of the block in the thermal storage wall with mortar as it is erected. This process is inexpensive and the resulting performance increment covers the increased cost. The design procedures developed herein are applicable to 8-inch concrete block thermal storage walls with filled or unfilled cores.

 

Water Wall. Water walls are thermal storage walls that use containers of water placed directly behind the aperture glazings as the thermal storage medium. The advantage over masonry walls is that water has a volumetric heat capacity about twice that of high density concrete; it is therefore possible to achieve the same heat

capacity available in a Trombe wall while using only half the space. Furthermore, a water

wall can be effective at much higher heat capacities than a Trombe wall because natural convection within the container leads to an nearly isothermal condition that utilizes all of the water regardless of the wall thickness. The high thermal storage capacity of water walls makes them especially appropriate in climates that have a lot of sunshine.

Transpired collector - Energy-Saving Mechanism

 A transpired collector reduces the load on a building’s heating system by heating intake air with solar energy. It preheats the ambient air by up to 40°F, reducing all or a portion of the load on a heating system during daylight hours. Although the transpired collector itself may not be able to achieve the required indoor air temperature on cloudy days or when the outside temperature plummets, it still provides useful energy and reduces utility bills.

The dark-coloured transpired collector is a large solar collector, absorbing the solar energy striking it. The wall captures between 60% and 75% of the available solar energy, making it one of the most efficient solar collectors designed to date. In addition to capturing direct solar radiation, the transpired collector collects the indirect, scattered, and reflected sunlight known as diffuse solar radiation. Typically, diffuse solar radiation, which includes a portion of the radiation on clear days and all the radiation on overcast days, makes up about 25% of the total annual radiation at the Earth’s surface.

The dark, corrugated metal sheets that make up the wall are 0.8-millimeter (mm)-thick and are typically manufactured from aluminium or galvanized steel. The perforations through which the air flows are 1.6 mm in diameter and are placed at regular intervals. The total percentage of the collector made up of these holes is referred to as the collector porosity.

Transpired collector operation during the winter months.

Transpired collector operation during the summer months.

Heat transfer fluid contained in the collector loop

 Heat transfer fluid contained in the collector loop are :

 

WATER. As a heat transfer fluid, good quality water offers many advantages. It

is safe, non-toxic, chemically stable, inexpensive, and a good heat transfer medium.

Two drawbacks include a relatively high freezing point and a low boiling point.

Excessive scaling may occur if poor quality water is used.

 

GLYCOLS. Propylene or ethylene glycol is often mixed with water to form an

antifreeze solution. Propylene glycol has the distinct advantage of being nontoxic,

whereas ethylene glycol is toxic and extreme caution must be used to ensure that it is isolated from any potable water. For this reason, uninhibited USP/food-grade propylene glycol and water solution will be specified for any solar preheat system that requires an antifreeze solution.

Linear Fresnel Solar Collector

 

Solar thermal energy collection system

A solar thermal energy collection system is defined as a set of equipment that intercepts incident solar radiation and stores it as useful thermal energy to offset or eliminate the need for fossil fuel consumption. Four basic functions are performed by a typical solar system.

COLLECTOR SUB-SYSTEM. The collector sub-system intercepts incident solar

radiation and transfers it as thermal energy to a working fluid. It is defined as the solar

collector, the hardware necessary to support the solar collector, and all interconnecting

piping and fittings required on the exterior of the building housing the system.

 

STORAGE SUB-SYSTEM. The storage sub-system retains collected thermal

energy for later use by the process load. It is defined as the storage tank and its fittings

as well as other necessary supports.

 

TRANSPORT SUB-SYSTEM. The transport sub-system delivers energy from the

collectors to storage. This sub-system is defined to include the heat transfer (or

working) fluid, pump(s), the remaining system piping and fittings, an expansion tank,

and a heat exchanger (if required).

 

CONTROL SUB-SYSTEM. The control sub-system must first determine when

enough energy is available for collection. It must then activate the entire system to

collect this energy until it is no longer available as a net energy gain. The control

subsystem thus consists of electronic temperature sensors, a main controlling unit that

analyzes the data available from the temperature sensors, and the particular control

strategy used by the controller.

Sunspaces

 There are many possible configurations for a sunspace but all of them share certain basic characteristics. Sunlight enters the sunspace through south facing glazing that may be vertical or inclined or a combination of the two and is absorbed primarily on mass surfaces within the enclosure; the mass may be masonry or water in appropriate containers and is generally located along the north wall and in the floor. The massive elements provide thermal storage that moderates the temperature in the enclosure and the rate of heat delivery to the living space located behind the north wall. Operable windows and circulation vents in the north wall provide for heat transfer by thermal convection from the sunspace to the living space. The north wall may be an insulated stud wall placed behind containers of water or a masonry wall through which some of the heat in the sunspace is delivered to the building interior by thermal conduction as occurs in a Trombe wall. A sunspace may be semi-enclosed by the main structure such that only the south facing aperture is exposed to ambient air, or may be simply attached to the main structure along the north wall of the sunroom, leaving the end walls exposed.

COLLECTOR EFFICIENCY AND PERFORMANCE

 

Collector efficiency is defined as the fraction of solar energy

incident upon the face of the collector that is removed by the fluid circulating through the collector. Several parameters are defined as follows:

Ti = heat transfer fluid inlet temperature

Ta = ambient air temperature

I = solar irradiance on the collector

Ac = solar collector surface area

FR = collector heat removal factor, a dimensionless parameter describing the ratio

of actual energy gained by the collector to that which would be gained, in

the limit, as the absorber plate temperature approaches the fluid inlet

temperature. This value is similar to a conventional heat exchanger's

effectiveness.

UL = overall heat loss coefficient. This factor describes the cumulative heat

transfer between the collector and the ambient surroundings.

t = transmittance of the glazing.

a = absorption coefficient for the absorber plate. Note that this value varies with

wavelength. A selective surface is one that absorbs short wavelength solar

radiation very well while emitting longer wavelength thermal radiation poorly.

Thermosiphoning Air Panels

 Thermosiphoning Air Panels. Thermosiphoning air panels (TAPs) are also appropriate for use on metal buildings either as retrofits or in new construction. Two configurations occur in practice and the first, which is referred to as a frontflow system. Again there are one or more glazing layers over an absorbing metal surface but, in this case, the metal panel is insulated on the back side. Heat

transfer to the interior occurs via circulation vents cut through the metal panel and its insulation at the upper and lower extremes. Solar radiation absorbed on the the outer surface of the panel is converted to heat and convected to the adjacent air which then rises due to buoyancy forces and passes through the upper vent into the living space. The warm air leaving the gap between the inner glazings and the absorber is replaced by cooler air from the building interior that enters through the lower vents. In this manner, a buoyancy driven loop is established and sustained as long as the temperature in the air gap exceeds that in the living space. Passive backdraft dampers or manually operated vent closures must be employed to prevent reverse circulation at night. Backdraft dampers are usually made of a lightweight plastic material suspended above a metal grid such that air flows freely in one direction but is blocked should the flow attempt to reverse.

The second type of TAP configuration is called a backflow system. In a backflow system, the flow channel is behind the absorber plate rather than in front of it. An insulated stud wall is constructed a few inches behind the metal panel and vents are then cut at the top and bottom of the wall. Air in the flow channel thus formed is heated by convection from the back of the absorber panel and a circulation loop is established in the same manner as in a front flow system.

TAPs have thermal storage requirements similar to those of direct gain and radiant panel systems. Generally speaking, the best performance will be obtained from passive solar systems associated with high heat capacity structures. Although a backflow TAP performs slightly better than a comparable system in the frontflow configuration, the difference is not significant and construction costs should govern any choice between the two. Both TAP configurations outperform radiant panels and direct gain systems with comparable glazings and thermal storage mass. This performance edge is due to the low aperture conductance of TAPs, which can be insulated to arbitrary levels, thereby limiting night time heat loss.

Radiant Panels for Homes

 Radiant Panels

Radiant panels are simple passive solar systems that are inexpensive and well suited as retrofits to metal buildings. Note that the solar aperture consists of one or more layers of glazing material placed over an uninsulated metal panel. The metal panel would ordinarily be a part of the building shell so that a retrofit is constructed by simply glazing an appropriate area on the south side of the structure. Any insulation or other poorly conducting material should be removed from the inner surface of the glazed portion of the metal panel to facilitate heat transfer to the interior.

 

Solar radiation is absorbed on the outer surface of the metal panel after passing through the glazings. The panel becomes hot and gives up heat to the interior by radiation and convection. Thermal mass must be included inside the building shell as with direct gain systems. Usually, only a concrete slab will be available before retrofitting a metal building and it may sometimes be necessary to add water containers to achieve the desired thermal capacitance. Radiant panels perform on a par with direct gain buildings and are likely to be less expensive when used as retrofits to metal buildings.

Transpired collectors

Transpired collectors use solar energy to preheat ventilation (outdoor) air as it is drawn into a building. The technology is ideally suited for buildings with at least moderate ventilation requirements in sunny locations with long heating seasons. Transpired collector technology is remarkably simple. A dark, perforated metal wall is  installed on the south-facing side of a building, creating approximately a 6-inch (15-cm) gap between it and the building’s structural wall. The dark colored wall acts as a large solar collector that converts solar radiation to heat. Fans associated with the building’s ventilation system mounted at the top of the wall draw outside air through the transpired collector’s perforations, and the thermal energy collected by the wall is transferred to the

air passing through the holes. The fans then distribute the heated air into the building through ducts mounted from the ceiling. By preheating outdoor air with solar energy, the technology removes a substantial load from

a building’s conventional heating system, saving energy and money.

A transpired collector is installed on all or part of a building’s south-facing wall, where it will receive the  maximum exposure to direct sunlight during the fall, winter, and spring. The size of the wall varies depending on heating and airflow requirements and climate, but in many applications, the transpired collector will cover the maximum south-facing area available. The amount of energy and money saved by a transpired collector depends on the type of conventional fuel being displaced, occupant use patterns, building design, length of heating season, and the availability of sunlight during the heating season. In general, each square foot of transpired collector will raise the temperature of 4 cubic feet per minute (cfm) by as much as 40°F, delivering as

much as 240,000 Btu annually per square foot of installed collector.

In addition to the metal sheeting that captures solar energy, the transpired collector heating system includes air-handling and control components that supply the solar heated air. The ventilation system, which operates independently of a building’s existing heating system, includes a constant-speed fan to draw air through the

transpired collector and into the distribution duct. Engineers typically use a 3-horsepower, 32-inch blade fan with about 10,000-cfm capacity.

Source: CED Engineering

 Where to Apply

The following are the most common transpired collector applications:

• Manufacturing plants

• Vehicle maintenance facilities

• Hazardous waste storage buildings

• Gymnasiums

• Airplane hangars

• Schools

• Warehouses requiring ventilation

 

What to Avoid

The following is a list of general applications and conditions that preclude the cost effective use of transpired collector technology:

• Outdoor air not required

• Shaded or insufficient south-facing wall area

• Buildings with existing heat recovery systems

• Locations with short heating seasons

• Multiple-story buildings (because of possible problems with fire codes).

SOLAR COLLECTORS and TYPES

 SOLAR COLLECTORS

A solar collector is a device that absorbs direct (and in some

cases, diffuse) radiant energy from the sun and delivers that energy to a heat transfer

fluid. While there are many different types of collectors, all have certain functional

components in common. The absorber surface is designed to convert radiant energy

from the sun to thermal energy. The fluid pathways allow the thermal energy from the

absorber surface to be transferred efficiently to the heat transfer fluid. Some form of

insulation is typically used to decrease thermal energy loss and allow as much of the

energy to reach the working fluid as possible. Finally, the entire collector package must

be designed to withstand ambient conditions ranging from sub-zero temperatures and

high winds to stagnation temperatures as high as 350 degrees F (177 degrees C).

 

COLLECTOR TYPES. The three major categories that have been used most

often are flat-plate glazed collectors, unglazed collectors, and evacuated tube

collectors. A general description of each collector type and its application is given

below.

FLAT-PLATE. Flat-plate solar collectors are the most common type used and

are best suited for low temperature heating applications, such as service water and

space heating. These collectors usually consist of four basic components: casing, back

insulation, absorber plate assembly, and a transparent cover. The absorber panel is a

flat surface that is coated with a material that readily absorbs solar radiation in the

thermal spectrum. Some coatings, known as "selective surfaces", have the further

advantage of radiating very little of the absorbed energy back to the environment.

Channels located along the surface or within the absorber plate allow the working fluid

to circulate. Energy absorbed by the panel is carried to the load or to storage by the

fluid. The absorber panel is encased in a box frame equipped with insulation on the

back and sides and one or two transparent covers (glazing) on the front side. The

glazing allows solar radiation into the collector while reducing convective energy losses

from the hot absorber plate to the environment. Similarly, back insulation is used to

reduce conductive energy loss from the absorber plate through the back of the collector.

UNGLAZED. Unglazed collectors are the least complex collector type and

consist of an absorber plate through which water circulates. This plate has no glazing or

back insulation. These collectors are often made of extruded plastic because they are

designed to operate at relatively low temperatures. Since they are not thermally

protected, these collectors should be operated only in warm environments where lower

thermal losses will occur. Swimming pool heating is the most common use of unglazed collectors.

EVACUATED TUBE. Evacuated tube collectors are best suited for higher

temperature applications, such as those required by space cooling equipment or for

higher temperature industrial process water heating. Convective losses to the

environment are decreased in this type of collector by encapsulating the absorber and fluid path within a glass tube that is kept at a vacuum. Tracking mechanisms and/or parabolic solar concentrating devices (simple or compound) are often used, resulting in somewhat higher equipment costs.

COLLECTOR ENERGY BALANCE.

 The collector parameters described

above allow an energy balance to be expressed as:

Energy Collected = Solar Energy Absorbed - Thermal Energy Losses to the

Environment

The energy balance can be written in a simple equation form using the efficiency

parameters described above:

Energy Collected = (FRta)(I)(Ac) - (FRUL)(Ac)(Ti - Ta) (Eq. 1)

Equation 1 shows that heat losses to the environment are subtracted from the net solar

radiation transmitted into, and absorbed by, the collector. Assuming that the efficiency

parameters are fixed for a given collector model, the main factors that affect the amount

of energy collected are I, Ti, and Ta. The geographical location and the season dictate

the weather variables I and Ta. The type of process load and system configuration

determines the relative circulation fluid temperature, Ti.

Typical collector efficiency curve

Typical solar collector efficiency plots

Solar cooling systems

Solar cooling systems are attractive because cooling is most needed when solar energy is most available. If solar cooling can be combined with solar heating, the solar system can be more fully utilized and the economic benefits should increase. Solar cooling systems by themselves, however, are usually not economical at present fuel costs. Combining solar heating and cooling systems is not easy because of the different system requirements.

ABSORPTION COOLING. 

Absorption cooling is the most commonly used method of solar cooling. An absorption refrigeration machine is basically a vapor-compression machine that accomplishes cooling by expansion of a liquid refrigerant under reduced pressure and temperature, similar in principle to an ordinary electrically operated vapor-compression air conditioner. Two refrigerant combinations have been used: lithium bromide and water, and ammonia and water. There have been a number of proposed solid material absorption systems also.

In the absorption cooler, heat is supplied to the generator in which a refrigerant is driven from a strong solution. The refrigerant is cooled in the condenser and allowed to expand through the throttling valve. The cooled, expanded refrigerant receives heat in the evaporator to provide the desired cooling, after which the refrigerant is reabsorbed into the cool, weak solution in the absorber. The pressure of the resulting strong solution is increased by pumping and the solution is available to repeat the process.

The performance of the system is governed largely by the temperature difference between the generator and the condenser and absorber units. Since the generator temperatures in solar driven systems are only moderate, it is important to keep the condenser and absorber temperatures as low as possible. The LiBr system is preferred over ammonia systems for solar energy applications because of the lower generator temperatures required. Permissible generator temperatures for a water-cooled LiBr system range from 170 deg. F to 210 deg. F (76 deg. C-99 deg. C) compared to the 205 deg. F to 248 deg. F (95 deg. C-120 deg. C) temperatures required for a water-cooled ammonia absorption system. Most, if not all, of the commercially available absorption units use LiBr and water as the absorbent-refrigerant fluid pair. Because the LiBr will crystallize at the higher absorber temperatures associated with air cooling, these units must be water cooled. A prototype ammonia-water unit, amenable to direct air cooling, has been built by Lawrence Berkeley Laboratories.

Parabolic trough systems concentrate solar radiation, specifically direct normal insola(DNI), onto a receiver tube located along the focal line of a single-axis tracking parabolically curved, trough-like reflector. Heat transfer fluid flowing through the receiver tube absorbs the thermal energy. The heat is collected and used to generate steam which is produces electricity by a Rankine cycle turbine-generator. Troughcan be hybridized (natural gas can be burned to produce steam when the sun isn’t shining) or can use thermal storage to dispatch power to meet utility peak load requirements.

The operating temperature of trough plants is limited by the thermal property of the heat transfer fluid (HTF) that is suitable for pumping through miles of piping in the solar fIn typical applications, oil flowing through the receiver tube is heated to about 390°C and used to boil water to produce steam. The resulting steam is used in a Rankine power cycle and expanded through a turbine connected to an electric generator. As with any steam cycle, the exhaust steam is cooled and condensed back to liquid water to be recirculated in the cycle. The condensers can be either water-cooled or air-cooled, or a hybrid combination.

Solar collector- Parabolic Trough

 

Parabolic trough system schematic

 

Common Acronyms in Energy

 AC alternating current

AGC automatic generation control

BA balancing area

Btu British thermal unit

CAISO California ISO

CREZ Competitive Renewable Energy Zones

CSP concentrating solar thermal power plants

DC direct current

EIA U.S. Energy Information Administration

ELCC effective load-carrying capability

EPACT 1992 Energy Policy Act of 1992

ERCOT Electric Reliability Council of Texas

EWITS Eastern Wind Integration and Transmission Study

FERC Federal Energy Regulatory Commission

FOA Funding Opportunity Announcement

HVDC high-voltage direct current

Hz Hertz

ISO Independent System Operator

kW kilowatt

kWh kilowatt-hour

LMPs locational marginal prices

LOLE loss-of-load expectation

LOLP loss-of-load probability

MISO Midwest ISO

MW megawatt

MWh megawatt-hour

NERC North American Electric Reliability Corporation

NYISO New York ISO

PJM Pennsylvania, New Jersey, and Maryland RTO

PV photovoltaic

RE Futures Renewable Electricity Futures Study

ReEDS Regional Energy Deployment Systems

RSG reserve-sharing group

RTO Regional Transmission Organization

WWSIS Western Wind and Solar Integration Study

Indian Firms turn to Chineese vendors for Solar PLI scheme

World's largest floating solar power plant

China is now the world's No. 1 investor in renewable energy, and the largest-ever floating solar plant has recently gone live in the country. Located on an old coal mine in the eastern province of Anhui, the $151 million Three Gorges project can power 94,000 homes at full capacity