Electric power can come either from the electrical grid or renewable energy systems, such as solar electric, wind electric, wind and solar hybrid, or microhydropower. Choosing between the electrical grid and renewable energy systems is driven by environmental concerns, economics and cost, total electrical consumption, and codes and regulations. Renewable energy systems can be either grid-connected or stand-alone. Grid-connected systems generate power from a renewable energy source, allowing excess power to be sold to electrical companies. Grid-connected systems can also access power from the grid when the renewable energy system underperforms. Alternatively, stand-alone systems are entirely independent and are typically combined with a gasoline generator for backup power when the system fails or underperforms.
The following content focuses on off-grid systems with a backup generator and the option to connect to a 30-50 AMP plug (RV Hookup). Designing an off-grid system consists of five parts: calculating your energy consumption, making the power, storing the power, controlling the system, and supplying your electrical load.
Disclaimer: The system described here incorporates Schneider Electric Connext SW solar electric equipment. All parts are designed to create a fully integrated system, per Schneider Electric's design specifications. The below system components are consequently all Schneider Electric products, only compatible for their integrated system. Otherwise, a system comprised of parts by varying manufacturers may require monitoring devices for each system component.
Another Disclaimer: Solar electric system design and installation could be dangerous and potentially fatal. The information below is for general purposes only. You should consult an electrician and solar electric system specialist for specifications that best suit your needs.
Below are a few variables and definitions commonly used in calculating energy consumption and sizing solar panel arrays and battery banks:
Wattage (W) is the unit of power used to quantify the rate of energy transfer in watts: W = V*A
Voltage (V) is the measure of electrical potential difference quantified in volts: V = W/A
Amperage (A) is the strength of an electric current in amperes: A = W/V
Watt Hour (Wh) is the measure of power consumption equal to one watt for one hour: Wh = W*1hour
Kilowatt Hour (kWh) is a derived unit of power calculated every 1000 watts: 1 kWh = 1 Wh*1000
Daily Watt Hours (dWh) is the daily power consumption of electrical devices: dWh = Wh* number of hours per day
Total Energy Consumption (TEC) is the summation of dWh of all electrical devices: TEC = dWh1 + dWh2 + dWh3...
Derate Factor (DF) is the factor of energy loss in wiring distribution efficiency: DF = 48%
Solar Panel Power Rating (StC) is the published power rating of a solar panel under standard testing conditions (StC): StC = per solar panel manufacture
Lowest Solar Insolation (sHd) is the lowest peak of the sun's electromagnetic radiation per unit area: sHd = look up solar irradiances for your area
Days of Autonomy (DA) is the number of days a solar electric system can supply power without another means of charging the batteries.
Depth of Charge (DoD) is the limit of energy withdrawal given in percentage of total capacity.
Battery Derate Factor (bDF) is the factor of performance yield when the battery is exposed to colder temperatures.
CALCULATING ENERGY CONSUMPTION
To calculate energy consumption, two general quantities must be known: the number of watts per hour that an electronic device uses and the number of hours per day that electronic device is in use. Most devices have a "wattage label" either on the device itself, on its packaging, or in the owner's manual; using wattage labels, the power consumption of each device can be computed. Alternatively, a kill-a-watt monitor can be used to measure the power consumption of each device connected to a power strip. Once the data is gathered for each device, an excel calculator can be used to compute the total watt hour usage per day. This calculator by the altEstore could also be used for computing.
In the TINY LIFE WIKI Case Study 1 House, the calculation yielded a daily wattage consumption of 5000 watts.
MAKING THE POWER
To determine the number of solar panels a system requires, simply divide the total daily energy consumption by the solar panel wattage. However, for a more precise measurement, system inefficiency should be considered along with the amount of solar radiation the tiny home location receives. A general rule of thumb for system inefficiency is about 48%, a value known as the "derate factor."
Solar panel wattage is measured per hour. For example, a 290-watt solar panel will collect 290 watts of power per hour in optimal conditions. To calculate for the least optimal conditions (the hours during the day when the tiny home location is receiving the lowest available solar radiation during sun-hours) solar irradiance data for the tiny home location is required. The lowest set of data should be used. Once this information is gathered, the following formula can be used to calculate the necessary number of solar panels to support the system.
daily wattage consumption + 48% derate factor ÷ lowest optimal sun hours for your area ÷ solar panel wattage = number of solar panels.
In the TINY LIFE WIKI Case Study 1 House, the wattage consumption calculation resulted in the necessity of 5 solar panels.
STORING THE POWER
In a solar electric system, power is typically stored in an array of batteries known as a battery bank. To optimize system power and ensure that power is not depleted when sun exposure is low, the size of the battery bank must be properly calculated. Batteries are the most expensive part of any solar electric system, and one way of increasing their life span is to not entirely discharge them of power. Most manufacturers recommend that depth of discharge not drop below 50%; this value will be used when sizing the battery bank. Temperature is another factor that effects the size and health of a system's battery bank; high temperatures can shorten battery life, while low temperatures can reduce battery capacity. Manufacturers recommend an ambient temperature of 77° for optimal battery performance. The size of the battery bank must account for the reduced efficiency due to temperature; use this table to calculate for the lowest ambient temperature the battery bank will be exposed to.
In addition to battery discharge and temperature, other factors the must be considered for power storage are days of autonomy, system voltage, and inverter efficiency. Days of autonomy for a solar electric system accounts for the number of days the sun will not shine but power will still be needed from the system without reliance upon a generator or a grid connection. A battery bank is typically designed for 3-5 days of autonomy; the greater number of days of autonomy desired, the greater the number of batteries. In the TINY LIFE WIKI Case Study 1 House, 3 days of autonomy were calculated due to cost savings. System voltage is another factor that could affect the cost of a system setup. Low voltages will require high currents because amperage(current) = wattage ÷ voltage; systems with high currents will require larger cables and fuses, which are very expensive. In the TINY LIFE WIKI Case Study 1 House, a 24-volt system was implemented. Finally, efficiency of the system's inverter must be considered, as some power is lost when converting DC to AC. Inverter efficiency is typically provided by the manufacturer in the product specification sheet. Once all of the information is gathered, the following formula can be used to calculate the battery bank amp hours:
daily wattage consumption ÷ inverter efficiency in decimal (90% = .9) × days of autonomy ÷ depth of discharge in decimal (50% = .5) × battery temperature derate factor ÷ system voltage = battery bank amp hours.
In the TINY LIFE WIKI Case Study 1 House, the above calculation resulted in 1652.78 amp hours.
CONTROLLING THE SYSTEM
The control and monitoring of a solar electric system requires the following components: a charge controller to control the flow of electricity to batteries, an inverter to convert DC to AC, a battery monitor to monitor the battery bank state of charge, and a system monitor panel to provide system statistics. Each component is described in greater depth in SYSTEM PARTS below.
SUPPLYING THE LOAD
The final aspect of the solar electric system setup involves the combiner box and the circuit breaker box. The combiner box is usually located close to the solar panel array and combines all of the wires from each solar panel, then sending them to the charge controller. The circuit breaker box is supplied by the inverter and is usually equipped with breakers supplying every part of the house with electricity.
Energy for a solar electric system is acquired through radiant light and heat from the sun. Technologies such as solar heating, photovoltaics, solar thermal heating, solar architecture, and artificial photosynthesis have been developed to harness such energy. The solar electric system described here uses photovoltaics (PV) as its energy generation source. A photovoltaic system uses an array of solar panels, each comprised of an array of solar cells, which generate electrical power. Manufacturers of PV panels provide a wide range of panels to choose from, and factors that should be considered when searching for the right manufacturer include efficiency, durability, cost, location of manufacturing plant, warranty, and panel specifications (size, weight, solar cell array, construction material). SOLARNATION is a excellent platform for reviewing manufacturers, as they provide an in-depth review guide to selecting an appropriate panel.
Reliable solar panel manufacturers, such as CandianSolar, Recom, SolarWorld, and YingliSolar, are generally comparable, differing only slightly in pricing. In the TINY LIFE WIKI Case Study 1 House, CandianSolar panels were chosen because they were manufactured in Canada and were the most cost effective per watt hour.
The charge controller is a battery management device that regulates the flow of electrical current in both directions between the power generation source, battery, and power load. To optimize battery life span and long-term associated costs, the charge controller either prevents the system from completely draining (deep discharge) the battery by disconnecting the electrical flow (low-voltage disconnection, LVD) or prevents the system from overcharging by disconnecting the electrical flow (high-voltage disconnection, HVD).
The power inverter converts direct current electricity (DC) to alternating current electricity (AC). For renewable energy systems (solar, wind, hydro) completely disconnected from the electrical grid, off-grid inverters should be used. However, off-grid inverters could also be used for a grid-connected system to provide backup power when grid power fails. In a solar electric system, an inverter/charger (inverter with built-in AC charger option) is recommended, as it provides a generator connection to charge batteries when solar exposure is low.
Safety features protect stand-alone and grid-connected small renewable energy systems from being damaged or from harming people during events, such as power surges, lightning strikes, or when equipment malfunctions.
There are three types of safety disconnects that a solar electric system must incorporate:
A Direct Current switch, which disconnects the supply of direct current electricity from the solar array to the solar electric system. CivicSolar offers detailed information on how to size the DC switch.
A Battery Bank Disconnect switch, which turns off the flow of electricity to the battery bank for temporary system maintenance.
An Inverter Disconnect switch, which turns off the flow of electricity to the Inverter for temporary system maintenance.
In order to protect a solar electric system from an electrical surge, such as those caused by a lightning strike or a wire shortage in the system, a surge protector must be installed. This mechanism 'arrests' the electrical surge, keeping it from damaging the entire network of electrical systems.
METERS AND INSTRUMENTATION
Meters and other instruments can be employed to monitor a small renewable energy system's battery voltage, the amount of power being consumed, and the level at which batteries are charged. These monitoring systems could seem luxurious or extraneous, but they are in fact quite crucial and have the potential to considerably reduce expenses and headaches in the long run.
The Schneider Electric/Control Panel monitors the electrical currency in a solar electric system, demonstrating how much electricity is being generated and how much is being consumed. It displays system configuration and diagnostic information for all systems in the solar electric network, thereby eliminating the need for separate control panels for each device. Within the control panel, the battery monitor indicates the runtime for battery hours and the battery bank state of charge. Secondary Monitoring
The Schneider Electric/Automatic Generator Start tracks the flow and capacity of the solar electric system in order to autonomously and automatically recharge depleted batteries or assist with heavy electrical loads. The device can be configured to start in response to low battery voltage, thermostat operation, or load size on the inverter.
The Schneider Electric/Combox is an interface that manages the solar electric system remotely, using wifi. The system can be monitored remotely from personal computers, tablets, or phones. Additionally, the Combox can generate data and event logs for the solar electric system and a graphic display of the system harvest.
Any item that requires electricity has an 'electrical load', meaning that it is part of a circuit that consumes electricity actively. Such items include household appliances, computers, lights, etc. Supplying an electrical load to products requires an AC switch gear, which typically comes in the form of a breaker box; however, some manufacturers have switch gears specific to their electric systems.
Solar electric systems are designed for a certain capacity of autonomy, meaning that a system can continue running without exposure to solar radiation. Auxiliary power serves as a backup power source if a system is depleted due to lack of solar radiation. Examples of auxiliary power include other renewable energy systems (wind, hydro, thermoelectric), a generator, or a grid connection.
We are currently working on the design and configuration of a backup wind electric system. We will inform you as soon as this section is completed.
Coming soon. Stay tuned!
Coming soon. Stay tuned!
Coming soon. Stay tuned!