Solar Photovoltaics - Details and Data
How does it work?
A rooftop solar photovoltaic (PV) system consists of multiple components. To begin, Solar Panels are made of multiple solar cells wired together. The panels produce direct current (DC) electricity when they are struck by sunlight. The total amount of electricity produced by a solar panel depends on the number of solar cells in the panel and how efficient the cells are in converting sunlight into electricity. Combining multiple solar cells in a solar panel increases the amount of energy produced. A solar cell is made of a semiconducting metal – usually silicone – which is covered with an anti-reflective coating so that the cell absorbs as much sunlight as possible. Photons – particles of light – then charge the semiconducting material, which then causes the metal to lose its excess electrons. The movement of electrons in this process is known the Photovoltaic Effect and is what induces a current flow.
Once the current is induced, the flow of energy is sent via conducting wires to an inverter, which converts the current from direct current (DC) flow to an alternating current (AC) flow where it can be used in households At the EcoCasa, three microinverters are wired to the solar panels and serve as Solar Meters which monitor solar energy production and send data to the inverter’s manufacturing company. The company houses a Data Monitoring site where users can monitor online how much energy their panels are producing.The microinverters are connected to an electrical disconnect which allows electrical workers to work safely by turning off the solar PV system during maintenance. The system is wired to an electrical subpanel in the house. This connects the electricity produced by the solar panels to the household electrical system to be used by lights and electrical appliances.
A solar PV system can operate independently or be connected to an electrical grid. Typically, grid tied systems have a net meter or bi-directional meter wired to them to monitor excess electricity that is sent back to the electrical grid. At the EcoCasa, excess electricity is distributed locally so that excess electricity produced by the EcoCasa’s solar panels can be used to power other buildings on the CECATI 118 campus. This banking and return of excess electricity is often discussed in terms of ‘net-metering’ or the practice of monitoring the amount of surplus energy produced. In the United States, net-metering policy varies by utility – in some cases households are able to receive retail discounts on their utility bills based on excess energy produced, while in other cases they are paid wholesale rates. In Mexico, the Federal Energy Commission generally allows for surplus energy to be discounted at the retail rate, although the circumstances are somewhat different given the CFE is a nationalized utility.
How much energy does it produce?
The amount of energy produced by the EcoCasa solar PV system varies by season, but the system produces electricity from the sun every month of the year. The monthly totals for energy production (starting in Mar 2019, the first full month of data collection) demonstrate that total monthly energy production is relatively consistent, with a small decrease in total monthly energy following the onset of the monsoon, and the clouds associated with these rainy days.
What factors affect when and how much energy is produced?
Solar PV panels need sunlight in order to produce energy, and the amount of energy produced increases starting in the morning, peaks around midday or early afternoon, and decreases as the sun moves across the sky before finally setting. Longer days in summer mean more hours where energy can be produced, but sunny days in the winter allow for substantial energy production as well. Weather events, and clouds in particular, can alter the amount of solar energy produced. Rainy (or even snowy) winter weather or monsoon storms on summer afternoons – will reduce the amount of solar energy produced during these weather events. Overall, the daily solar energy produced varies considerably based on the weather, including passing clouds and rain events, but the overall totals are relatively consistent in the southwestern United States and northwestern Mexico.
How much does a solar PV system cost?
Costs vary by the size of the solar PV system, which is related to the total amount of electricity needed to meet household demand, as well as whether the system is designed to provide 100% of the household’s electricity, or simply to supplement electricity provided via a grid connected household. The cost of solar PV technology has been declining in recent years, as the technology matures and becomes more widely available and as systems become more efficient, requiring smaller systems to meet household demand. International factors such as tariffs can disrupt these trends of gradual cost decreases. Battery storage costs are also declining, but batteries are still relatively expensive for household use. As batteries and storage become more cost effective, the feasibility of household scale solar will increase because these storage components allow households to store excess energy produced during peak solar hours, for use later in the day when solar energy production is less efficient, or overnight when solar power is altogether unavailable.
Even with decreases in overall, solar PV systems require a substantial investment for most households. In the U.S., typical household systems are estimated to cost between $6,000 and $20,000 (not factoring in tax credits). The material costs are similar in Mexico, but the labor and installation costs are typically less, resulting in somewhat lower overall costs, not to mention the role that government subsidies may play in offsetting costs for some households.
How much money does it save?
Despite their initial costs, solar PV systems can save money. The amount of savings depends on the price the household pays for electricity, the size of the system, and the amount of energy generated by the system. In the United States, state and federal tax rebate programs help offset the total cost of a household solar system. Many U.S. solar installers indicate that the break-even period is between 8-20 years depending on the factors indicated above, including state and federal rebates that offset the overall cost of the system. In Sonora, Mexico, the break-even period for local installations is estimated between 2-5 years for residential and 5-10 years for industrial, again depending in the size of the system, the level of household energy use, and whether the customer is able to qualify for subsidies or assistance provided by organizations such as FIDE (Fideicomiso para el Ahorro de Energía Eléctrica).
Solar PV systems may be particularly useful in offsetting household energy costs by limiting overall consumption of grid-based (utility provided) electricity. The cost of systems, and especially of battery storage systems, may limit complete replacement for household energy. However, when connected to utilities that utilize net metering to track surplus energy production, some households may be able to ‘break even’ on their monthly bills and others can reduce their bills by using solar to supplement their household energy needs. This also highlights a practical reality – off-grid systems, or systems installed in areas with no utility grids, can still provide intermittent electricity, even for households without the financial capacity to invest in expensive battery storage systems.
Benefits
- An alternative to energy produced by fossil fuels
- Excess electricity can be stored in a battery system or used to offset a utility bill (net metering)
- A source of electricity for communities without a developed or maintained energy infrastructure
Limitations
- Can only produce energy while it is sunny
- Battery storage is costly, not always efficient
- Energy produced from system may not cover entire household needs.
- Substantial initial cost
- Return on Investment (ROI) depends on local policies, size of system, installation cost, financing
Educational Activities
- Track energy consumption in household over a period of a week
- How much sun do you get? (Google's Project Sunroof, only works in US)