9.6. Solar Energy Systems
Solar is one of the fastest growing energy sources in the United States. Solar power which at its simplest is the raw energy created by the sun’s rays, and can be either active or passive. Solar technologies use the sun’s energy and light to provide heat, light, hot water, electricity, and even cooling, for homes, businesses, and industry. In fact, many believe that passive and active solar energy is the energy source of the future, for nowhere on the planet can we find any other energy source as powerful and without sacrificing our natural resources and environment.
9.6.1. Solar Options
There are many solar options today that can replace much of today’s regular energy needs, saving money, and benefiting the environment by cutting down on the use of fossil fuels. New technologies continue to be developed at such a rapid pace while at the same time being used to help us harness this enormous natural energy asset.
Active Solar Energy Systems
Solar electric systems are environmentally friendly because they do not generate emissions of greenhouse gasses or other pollutants, thereby do not have an adverse impact on global climate. Solar electric systems have proven to be reliable and are pollution-free. They make use of our most important renewable source of energy—the sun. And photovoltaic (PV) systems for homes and businesses are becoming more accessible and affordable.
Solar Photovoltaics
PV materials convert sunlight into useful, clean electricity. By adding PV to your home or office, you can generate renewable energy, reduce your own environmental impact, enjoy protection from rising utility costs, and reduce greenhouse gas emissions. Electricity is only one of many uses for solar energy. The sun of course is essential to your garden, and it can heat water very cost-effectively, but the most fundamental use of solar energy is in overall building design. Good design uses solar radiation to passively and/or actively heat your building and to help keep it cool. Solar energy is also increasingly being used for street lighting (Fig. 9.26). Building integrated photovoltaic systems (BIPVs) offer additional design options, allowing electricity to be generated by windows, shades and awnings, roofing shingles, and PV-laminated metal roofing, for example. BIPV options can be used in retrofits or new construction.
Figure 9.26 Example of solar power LED street lighting with automatic on–off, lasts for four to five nights after fully charged. Source: Hankey Asia Ltd.
As previously stated, solar energy is a renewable resource that is environmentally friendly, and unlike fossil fuels, solar energy is available in abundance and is free, and immune to rising energy prices. The many ways that solar energy can be used include providing heat, lighting, mechanical power, and electricity. It helps minimize the impact of pollution from energy generation which is considered to be the single largest contributor to global warming. Renewable energy could clean the air, stave off global warming, and help eliminate our nation’s dependence on fossil fuels from overseas. The recent upsurge in consumer demand for clean renewable energy and the deregulation of the utilities industry have spurred growth in green power—solar, wind, geothermal steam, biomass, and small-scale hydroelectric sources of power. This energy demand is further being served by the emergence of small commercial solar power plants around the country.
For decades, solar technologies in the United States and around the world have used the sun’s energy and light to provide heat, light, hot water, electricity, and even cooling, for homes, businesses, and industry. The types of renewable technologies available for a particular facility depend largely on the application and what sort of energy is required, as well as a building’s design and access to the renewable energy source. Building facilities can use renewable energy for space heating, water heating, air conditioning, lighting, and refrigeration. Commercial facilities include assembly and meeting spaces, educational facilities, food sales, food service, health care, lodging, stores and service businesses, offices, and warehouses.
In the LEED rating system, the on-site renewable energy credits are not always easily achieved, particularly in urban locations. Essentially, you need to generate 2.5–7% of the buildings electricity from wind, water, or solar energy, which due to the many site constraints in a city environment, leaves us with little more than solar energy to focus on.
9.6.2. Solar Electric System Basics
Solar electric systems, also known as PV systems, convert sunlight into electricity. When interconnected solar cells convert sunlight directly into electricity, they form a solar panel or “module,” and several modules connected together electrically form an array. Most people picture a solar electric system as simply the solar array, but a complete system consists of several other components. The working of a solar collector is very simple (Fig. 9.27). The energy in sunlight takes the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. This radiation from the sun heats a liquid which goes to a hot water tank. The liquid heats the water and flows back to the solar collector. The solar energy that strikes the earth’s surface at any particular time largely depends on weather conditions, as well as location and orientation of the surface, but overall, it averages approximately 1000 watts per 10 sq. ft. (equivalent to one square meter) under clear skies with the surface directly perpendicular to the sun’s rays. This solar thermal heat is able to provide hot water for an entire family during the summer. The collector size needed per person is just over 16 square feet (1.5 m2). An average family of four people therefore needs a collector of about 65 sq. ft. (6 m2). The most common component equipment generally used in on-grid and off-grid solar electric systems is listed below, although systems vary and not all equipment is necessary for every system type. Indeed, understanding the basic components of PV systems and how they function is not particularly difficult.
Solar Electric Panels: PV is the technical word for solar panels that create electricity, and PV material, most commonly utilizing highly purified silicon, converts sunlight directly into electricity. When sunlight strikes the material, electrons are dislodged, creating an electrical current which can be captured and harnessed. The PV materials can consist of several individual solar cells or a single thin layer, which make up a larger solar panel. Panels are usually mounted on either a stationary rack or a tracking rack that follows the movement of the sun (Fig. 9.28), and as they have no moving parts solar electric panels operate silently. Life expectancy of a typical system is 40–50 years. Panels are generally warranted for 20–25 years.
PV has over recent years been making significant inroads as supplementary power for utility customers already served by the electric grid. In fact, grid-connected solar systems now comprise a larger market share than off-grid applications. However, compared to most conventional fuel options, PVs remain a very small percentage of the energy makeup both within the United States and globally. Still, with increasing concerns of global warming, more and more individuals, companies, and communities are choosing PV for a variety of reasons, including environmental, economic, emergency backup, and fuel and risk diversification. The economics of a PV system for a home or business is not just the solar resource, but rather a combination of the solar resource, electricity prices, and local/national tax and other incentives.
Figure 9.28 Photo taken by Airman first-Class Nadine Barclay of the Nellis Solar Power Plant which is the largest PV power plant in North America. The 70,000 solar panels sit on 140 acres of unused land on the Nellis Air Force Base, Nevada, forming part of a solar PV array that will generate in excess of 25 million kilowatt-hours of electricity annually and supply more than 25% of the power used at the base. Source: Wikimedia Commons.
One of the critical aspects of solar design is siting. For example, solar panels like full sun, facing within 30° of south and tilting within 30° of the site’s latitude. A 1-kW system requires about 80 square feet of solar electric panels. Stationary racks can be roof or pole-mounted. Tracking racks are typically pole-mounted.
Inverter: An inverter converts the system’s direct current (DC) electricity produced by the PV modules into usable 120 Volt alternating current (AC) electricity which is the most common type for powering lights, appliances, and other needs. Grid-tied inverters are utilized to synchronize the electricity they produce with the grid’s utility grade AC electricity, allowing the system to feed solar-made electricity to the utility grid. Inverters are typically warranted for 5–10 years.
Array Mounting Rack: Mounting racks provide a secure platform on which to anchor the PV panels, ensuring that they are fixed in place and correctly oriented. Panels can be mounted on a rooftop, on top of a steel pole set in concrete, or at ground level. A PV array is the complete power-generating unit, comprising of one or more solar PV modules (solar panels) that convert sunlight into clean solar electricity. The solar modules need to be mounted facing the sun and avoiding shade for best results. Solar panels generate DC power which can be converted to AC power with an inverter.
Wiring: Selecting the correct size and type of wire will enhance the performance and reliability of the PV system. The size of the wire must be sufficiently large to carry the maximum current expected without undue voltage losses.
Battery Bank: This is used to store solar-produced electricity for evening or emergency backup power. Batteries may be required in locations that have limited access to power lines, as in some remote or rural areas. If batteries are part of the system, a charge controller may be required to protect the batteries from being overcharged or drawn down too low. Depending on the current and voltages for certain applications the batteries are wired in series and/or parallel.
Charge Controller: The main function of a charge controller is to protect the battery bank from overcharging. This is achieved by monitoring the battery bank, and when the bank is fully charged, the controller interrupts the flow of electricity from the PV panels. Modern charge controllers usually incorporate maximum power point tracking, which optimizes the PV array’s output, thereby increasing the energy it produces.
System Meter: They are used to measure and display several different aspects of a solar electric system’s performance and status, tracking how full the battery bank is; how much electricity the solar panels are producing or have produced; and how much electricity is in use.
Array DC Disconnect: The DC disconnect is used to safely interrupt the flow of electricity from the PV array. It is an essential component when system maintenance or troubleshooting is required. The disconnect enclosure houses an electrical switch rated for use in DC circuits, and if required, may also integrate either CBs or fuses.
Main DC Disconnect: Battery-based systems require Disconnect switches to allow the power from a solar electric system to be turned off for safety purposes during maintenance or emergencies. It also protects the inverter-to-battery wiring against electrical fires. A disconnect typically consists of a large, DC-rated breaker mounted in a sheet metal enclosure.
AC Breaker Panel: This is the point at which all of a property’s electrical wiring meets with the provider of the electricity, whether that is the grid or a solar electric system. The AC breaker panel typically consists of a wall-mounted panel or box that is normally installed in a utility room, basement, garage, or on the exterior of the building. It contains a number of labeled CBs that route electricity to the various spaces throughout a structure. These breakers allow electricity to be disconnected for servicing and also protect the building’s wiring against electrical fires.
Kilowatt-Hour Meter: Homes and businesses with a grid-tied solar electric system will often have AC electricity both coming from and going to the electric utility grid. A bidirectional KWH meter is able to simultaneously keep track of how much electricity flows in each direction which tells you how much electricity is being used and how much the solar electric system is producing.
Backup Generator: Off-grid solar electric systems can be sized to provide electricity during cloudy periods when the sun does not shine. But sizing a system to cover a worst-case scenario, like several cloudy weeks during the winter, can result in an unduly large system that will rarely be used to its full capacity. Engine generators can be fueled with biodiesel, petroleum diesel, gasoline, or propane, depending on the design. These generators produce AC electricity that a battery charger (either stand-alone or incorporated into an inverter) converts to DC energy, which is stored in batteries.
9.6.3. Types of Solar Energy Systems
Solar energy technologies use the sun’s energy and light to provide heat, light, hot water, electricity, in addition to cooling, for homes, businesses, and industry. Solar electric systems are attracting increasing attention because they are environmentally friendly and do not generate emissions of greenhouse gasses or other pollutants, thereby reducing global climate impacts. Solar panels reflect visible demonstration of concern for the environment, community education, and proactive forward thinking. The three most widely used types of solar electric systems are grid-tied, grid-intertied with battery backup, and off-grid (stand-alone). Each has distinct applications and component needs. However, the majority of households and businesses prefer to choose either a grid-connected or an off-grid system.
Grid-Tied Solar System (alternating current)—also known as on-grid, or grid-intertied PV systems, generates electricity for your home or business and routes the excess power into the electric utility grid. This type of solar electric system does not require storage equipment (i.e., batteries) because it generates solar electricity and routes it to the electric utility grid, offsetting a home’s or business’ electrical consumption and, in some instances, even turning the electric meter backward. Living with a grid-connected solar-electric system does not really differ from living with grid power, except that some or all of the electricity used comes from the sun. The crucial issue relative to the PV panels systems is the technical aspects of tying into the electricity grid. Applications of this type require the use of grid-tied inverters that meet the requirements of the utilities.
It is important that the systems do not emit “noise” which can interfere with the reception of equipment (e.g., televisions), switch off in the case of a grid failure, and retain acceptable levels of harmonic distortion (i.e., quality of voltage and current output waveforms). This type of system tends to be an optimum configuration from an economic viewpoint because all the electricity is utilized by the owner during the day and any surplus is exported to the grid. Meanwhile, the cost of storage to meet night-time needs is avoided, because the owner simply draws on the grid in the usual way. Also, with access to the grid, the system does not need to be sized to meet peak loads. This arrangement is termed net metering or net billing. The specific terms of net metering laws and regulations vary from state to state and utility to utility which is why for specific guidelines the local electricity provider or state regulatory agency should be consulted.
The Stand-Alone Grid-Tied Solar System with Battery Backup (alternating current) solar energy system is the same as the grid-tied system except that battery storage (battery bank or generator backup) is added to enable power to be generated even when the electricity grid fails. Incorporating batteries into the system requires more components, is more expensive, and lowers the system’s overall efficiency. But for homeowners and businesses that regularly experience utility outages or have critical electrical loads, having a backup energy source is invaluable. The additional cost to the customer can be quantified against the value of knowing that their power supply will not be interrupted.
The Stand-Alone Off-Grid Solar-Electric System without energy storage (direct current) is a configuration (i.e., without any energy storage device) that consists of a PV system whose output is dependent upon the intensity of the sun. In this system, the electricity generated is used immediately and therefore, the application must be capable of work on both direct current (DC) and variable power output. Stand-alone off-grid electric systems are most common in remote locations where there is no utility grid service. These systems operate independently from the grid to provide all the electricity required by a household or small business. The choice to live off-grid may be because of the prohibitive cost of bringing utility lines to remote locations, the appeal of an independent lifestyle, or the general reliability a solar-electric system provides. However, those who choose to live off-grid often need to make adjustments to when and how they use electricity, to allow them to live and work within the limitations of the system’s capabilities.
To meet the greatest power needs in an off-grid location, the PV system may need to be configured with a small diesel generator. This increases the capability of the PV system as it no longer has to be sized to cope with the worst sunlight conditions available during the year. The diesel generator can also provide the backup power, but its use is minimized during the rest of the year of the PV system, to keep fuel and maintenance costs to a minimum.
For any module with a defined peak power, the actual amount of electricity in kilowatt hours (kWh) that it generates will depend primarily on the amount of sunlight it receives. The electrical power output of a PV module is the current that it generates (dependent on its surface area) multiplied by the voltage at which it operates. The larger the module, or the solar array—the number of modules connected together, the more power is generated. A Linear Current Booster can be added to convert excess voltage into amperage to keep a pump running in low light conditions. An LCB can boost pump output by 40% or more. For safety considerations, PV arrays are normally earthed.
With respect to energy production, each kilowatt of unshaded stationary solar electric panels generates about 1200 kW-hours of electricity per year. A 1-kW, dual-axis tracking system will generate about 1600 kWh per year. Power is generated during peak daylight hours. Solar power exhibits a very good peak coincidence with commercial building electrical loads. Dual-axis tracking systems, where the panels follow the sun, will require periodic maintenance as would other systems.
Passive Solar Energy Systems
Passive solar heating and cooling represents an important strategy for displacing traditional energy sources in buildings and is an effective method of heating and cooling through utilization of sunlight. The sun’s energy arrives on earth in the primary form of heat and light. To be successful, building designs must carefully balance their energy requirements with the building’s site and window orientation. Buildings that are designed to collect, store, and distribute solar energy as heat is referred to as passive solar buildings. Such buildings maximize absorption of sunlight through south-facing windows and use dark-colored, dense materials in the building to act as thermal mass; the sunlight is stored as solar heat (light colors are less effective for heat storage). But in order to take the best advantage of solar gain, a passive-designed building should have an east–west axis, so that the front of the building is facing south.
The term “passive” indicates that no additional mechanical equipment is used, other than the normal building elements. Solar gains are generally introduced through windows and minimum use is made of pumps or fans to distribute heat or effect cooling. Passive cooling minimizes the effects of solar radiation through shading or generating air flows with convection ventilation.
Correct building orientation, thermal mass, and insulation are specified in conjunction with careful placement of windows and shading. The thermal mass absorbs heat during the day and radiates it back into the space at night. To do this, passive solar techniques make use of building elements such as walls, windows, floors, and roofs, in addition to exterior building elements and landscaping, to control heat generated by solar radiation. Solar heating designs collect and store thermal energy from direct sunlight in a manner that provides energy-efficient space and stable year-round temperatures, yet quiet, and comfortable.
Daylighting design is another solar concept that optimizes the use of natural daylight and contributes greatly to energy efficiency. The quantity and quality of light around us helps determine how well we see, work, and play. Light impacts our health, safety, comfort, morale, and productivity. Whether at home or in the office, it is possible to save energy and still maintain good light quantity and quality. But there are many benefits to using passive solar techniques including simplicity, price, and the design elegance of fulfilling one’s needs with materials at hand. Some of the advantages of passive solar designs include:
• At little or no cost, passive solar design can easily be designed into new construction and can in some cases be retrofitted into existing buildings.
• It pays dividends over the life of the building through reduced or eliminated heating and cooling costs.
• IAQ is improved through elimination of forced air systems.
• Sites with good southern exposure are most suitable.
• Retrofitting is rarely as effective as initially designing for this method.
LEED offers credits in its Indoor Environmental Quality section, Daylight, and Views. The intent of the credits appear to be to reduce electric lighting, increase productivity, and provide building occupants with a connection between indoor and outdoor spaces by incorporation of daylight and views into regularly occupied spaces.
LEED Requirements for these credits is to achieve daylight (through computer simulations) in a minimum of 75% or 90% of regularly occupied spaces, and achieve a daylight illuminance level of a minimum of 25 foot-candles and a maximum of 500 foot-candles in a clear sky condition on September 21 at 9.00 a.m. and 3.00 p.m. A combination of side lighting and/or top lighting may be used to achieve the total Daylighting Zone required which is at least 75% of all the regularly occupied spaces. Sunlight redirection and/or glare control devices may be provided to ensure daylight effectiveness. The provision of daylight redirection and/or glare control devices to avoid high-contrast situations should be provided to avoid impeding of visual tasks. Exceptions for areas where tasks would be hindered by daylight will be considered on their merits. It should be stressed that the USGBC Reference Guide or Website should be consulted for the latest updated requirements including possible exemplary performance credits.
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