Solar Power Basics - Designing Your System

Many off-grid homes use photovoltaic panels to convert light into electric power. It's not that hard to set up a solar power system and, once you do, it is really great to have electricity without getting a bill for it every month. That said, it does cost money to get it set up. Some of the components are pretty expensive. Fortunately your system will deliver reliable power to your house for years without outages with proper design and maintenance.

Before we get into how to design your own off-grid solar power system here is some basic information about electricity, in case you don't already understand it.

ELECTRICITY BASICS

Electricity is a stream of electrons that moves in a circuit to produce work of some kind. It is called a circuit because the connections have to go full circle in order for the electrons to flow. The appliance or device that will use the power is called a “load.” In the drawing at left, "V" means the power source, "I" is the path of the current, and "R" represents the load. This is the bare minimum for a functional electrical circuit.

An open circuit means there is a break in this circle and, therefore, no electricity flows and nothing will work. A short circuit occurs when two parts of the circuit accidentally touch each other between the power source and the load. Short circuits are dangerous and can cause sparks, fires, and damage to you and/or your equipment. Fuses and/or circuit breakers are put into circuits to break the circuit in case of a short or a power draw that exceeds the capacity of the wiring or equipment.

The wires that carry electricity must be made of materials that are good conductors. Copper is the most commonly used in wiring but most metals can conduct electricity. Some non-metals can conduct, too, like water. This is why you do not want to be standing in water when using electrical equipment.

Learn this formula: Volts X Amps = Watts. Voltage is a measurement of the pressure of electron flow and Amperage is a measure of the volume of the flow of electrons.  A Watt is a measurement of power. That’s the bottom line – how much power you need your system to produce. If you play with this formula you will see that you can get the same amount of power with 100volts at 10amps as you will with 10volts at 100 amps. You can calculate Volts or Amps by re-working the formula – Amps = Watts /Volts and Volts = Watts /Amps. You will see how useful this formula is later.

Power use is measured in watt-hours. This is the amount of power used over time. For example, if you watch TV for three hours each night and your TV uses 100 watts, that adds up to 300 watt-hours per day of power needed. A 10-watt appliance that is used for 10 hours a day has the same total power use as a 100-watt appliance that is used for one hour per day.

Another important thing to know is the difference between direct current (DC) and alternating current (AC). With photovoltaic systems power is produced and stored as DC but is delivered to your house as AC. In the U.S. AC is approximately 117 volts at 60 cycles per second. The reason AC is used is that it travels farther than DC without significant power loss.

  

Higher voltages also reduce power loss over distances. This applies to both AC and DC power. Here is a link to charts showing AC and DC power losses for different distances and voltages. http://www.solar-electric.com/wire-loss-tables.html

Interesting Fact: Almost all electronic devices –radios, computers, etc. –run on direct current. Since normal house current is AC (because of the ease of AC power transmission). These devices all contain power supplies to convert house AC to the DC voltage that is required.  The little black plastic box that is part of your power cord on laptops, etc. is the power supply.

DC wiring has two leads – a positive and a negative. The standard is for positive DC wires to be red and the negative is black. If you stick to this it will make it easier and safer when connecting your equipment together. Don’t touch a positive and negative connection at the same time! You can be electrocuted. Just touching one connection will not do anything to you.

AC house wiring normally has three leads. – hot, common, and ground. The NEC color coding for this is hot=black, neutral or common=white, ground = either bare copper or green. Not all houses are wired using this color code, but it’s the standard for the U.S. and Canada. The ground wire is for safety. It will send the electricity into the ground if there is a short or malfunction. Once again, do not touch two different connections at the same time!

Series vs. parallel – There are two ways power sources are connected together to combine output. With series connections the voltage multiplies but the amperage stays the same. With parallel connections the voltage stays the same but the amperage multiplies. You can combine components like batteries in a combination of series and parallel (called series-parallel) to get the voltage and power output you need.

The diagram at left shows how to connect four batteries in series or parallel and also shows the result in volts and amps. As you can see, connecting the batteries in parallel gives you the same voltage as one battery but four times the amperage. Connecting them in series quadruples the voltage but the amperage stays the same. Solar panels can be combined in the same ways. You will want to configure your batteries and solar panels to output the voltage you need to match your other equipment.

Off-grid solar power systems consist of four main components: photovoltaic panels, charge controllers, batteries, and inverters.

PHOTOVOLTAIC PANELS (PV)

PV panels contain specially grown silicon crystals that give up electrons when light hits them. It is a very direct way to generate electricity although panels are currently only able to convert about 15% of the light into electricity. The panels harvest the sunlight and produce DC electricity.  Most panels produce electricity at somewhere between 12 and 35 volts DC, depending on the panel design. They can be ganged together in either series or parallel or combinations of the two to produce the voltage and power you need. It is best to match panels to each other by voltage and wattage as much as you can. Combining dissimilar panels works but is not optimal. Solar panel power output is rated in watts.

CHARGE CONTROLLERS

These electronic devices make sure that the power coming from the solar panels goes into the batteries at the correct voltage for the batteries without over-charging them. Charge controllers go between the solar panels and the battery array. They also monitor the state of charge in the batteries. If you want to see the status of your system, the charge controller has the indicator lights or a screen showing voltage and amperage. Smart charge controllers have several different charging modes depending on the batteries’ state of charge. They are designed to deliver the optimum voltage to keep your batteries charged and happy. You can have multiple charge controllers connected to the same battery array to handle the input from large panel arrays. Charge controller capacity is rated in amps.

BATTERIES

Batteries store electricity chemically so you can have power when the sun isn’t shining. Most commonly used are lead-acid deep-cycle batteries. Deep-cycle means they can handle fairly deep discharges unlike automotive batteries. Car batteries are designed to have a huge load on them for a very short period of time – like when your starter is spinning the engine to start the car. Deep-cycle batteries are designed to have small load on them for a long period of time. There are two main types used in PV systems – flooded batteries and AGM batteries. Flooded batteries are the least expensive but they require some maintenance. You will have to check the electrolyte levels regularly. If the electrolyte is low, you will have to add distilled water. AGM batteries are sealed units and do not require maintenance other than making sure the terminals are clean of corrosion.

Batteries can be connected in series, parallel, or a combination of the two to produce the voltage the inverter(s) require. When designing a battery array it is best to have as few parallel connections as you can. Series connected batteries tend to last longer. Battery power storage capacity is rated in amp-hours. A two-volt, 1000 amp-hour battery stores 2000 watt-hours of power. (volts x amps = watts.)

Batteries also have longer lives if they spend most of the time close to full charge. Deeply discharging the batteries will shorten their lives. I recommend making sure your batteries are more than sufficient to meet your load requirements and that you have more solar panels than you need to make sure the batteries charge quickly and fully.

Our inverter is old but works fine.

INVERTERS

Inverters are electronic devices that convert DC from the batteries into AC for your house. They must be purchased to match the output voltage of the battery array and to produce the amount of power you need for your home. There are two common types of inverters. The modified square-wave inverters are the cheaper ones. They produce a wave-form that is fine for running motors and lights but can cause problems when running sensitive electronic equipment. The better quality ones are pure sine-wave inverters. They produce the cleanest AC power to keep your electronics happy. Some inverters also have built in battery chargers for use when your system is running on back-up power, like a generator.  You can have multiple inverters connected in parallel to your battery array. Their power delivery capacity is rated in watts.

Six steps to design a photovoltaic system for your house.

For the purpose of this article we will design a sample system which happens to be pretty much the same system that we are running here at our house.

 

STEP 1. CALCULATE YOUR POWER LOAD

Make a chart listing all of you electrical appliances and their power usage. The first column should be the list of appliances. The second column contains the amount of power they use. The third column contains the number of hours each appliance is likely to be in use during one 24-hour period.  The fourth column is for watt/hours for each appliance. That is the number of hours of demand at the power level the appliance needs. To get watt/hours you multiply the amount of power the appliance uses by the number of hours the appliance is in use during one full day. Here is our chart:

All the appliances in your home have listed on them the amount of power they use at maximum. This is listed either in watts or in amps. If it is listed in watts, that’s easiest since we will be adding up the amount of watts your appliances use. If it is listed in amps, you will have to multiply that number by 120 to get the power use in watts.

The total estimated load for our house is 2000 watt-hours per day.

STEP 2.  CALCULATE THE AMOUNT OF SUN YOU GET IN WINTER


Insolation is the amount of sun you get. It is different in winter as compared to summer and there are differences in insolation based on the latitude and climate. We want to know the winter insolation because that is the time of year you get the least sun. If the system is designed to provide enough power in the winter then it surely will provide enough power in the summer when there is a lot more sunshine. The chart here shows winter insolation in the continental U.S. Values are in kilowatt-hours per square meter per day which basically translates to hours of full sun per day. It takes precipitation averages into account, too.

STEP 3. DETERMINE HOW MANY PV PANELS YOU NEED.

Each panel is rated for nominal output in watts. For example a 100-watt panel produces 100 watts with full sun. For our sample system you will need to divide the daily load in watt-hours (2000 watt-hours) by the number of hours of full sun in winter (4 hrs. according to the above chart) to get the wattage that needs to be generated by the solar panels = 500 watts. 

Our array - four 175-watt panels

We had four 175-watt, 24-volt panels donated to us which totals 700 watts of power output. The extra 200 watts will make up inefficiencies in the system. What are these inefficiencies? Each component uses a tiny bit of power, there are minor power losses in the wiring and connections, the solar panels could be a little dirty, stuff like that.

We connected the solar panels in series to produce 96-volts.This is to minimize power loss since our panel array is about 100 feet from the shed with the batteries, etc. in it. You will have to run DC current from your solar panels to your charge controller and batteries. Here is the link to the power loss tables again: http://www.solar-electric.com/wire-loss-tables.html

If you are scrounging panels, combining panels with dissimilar voltages and amperages will work but will be less efficient. If at all possible, find panels that have pretty close to the same output. There are often great deals on panels that have been replaced in commercial installations. I just got four additional 175-watt panels for $100 each. That's CHEAP!

STEP 4. DETERMINE BATTERY ARRAY SIZE

Batteries are rated in voltage and amp/hours. Amp/hours means the number of hours you can draw a set amount of current (amps).  Our sample system would require that the batteries supply 6000 watt-hours to power everything for three days without sun.

Our six Rolls-Surrette flooded deep cycle batteries.

Batteries can be connected together in series, parallel, or a combination of the two. For maximum battery life, series connections are better. We bought six 1050 amp-hour, 2-volt batteries. They are connected in series to produce 12 volts. This totals 6300 watt-hours - more than enough to meet our three-days-without-sun requirement We chose a 12-volt total output because we already had a 12-volt inverter and we needed to match the voltage to that. Inverters are pricy so we figured it was worth designing the system to use the inverter we have.

STEP 5. CHOOSE A CHARGE CONTROLLER

Charge controllers come in two types – PWM and MPPT. PWM stands for pulse-width modulation and is the cheaper of the two types. The downside to PWM controllers is that they simply chop off any extra voltage above the nominal input voltage from the solar panels. For example, it is not unusual for 12-volt nominal panels to produce 17 volts under full sun. The PWM controller takes 12-volts of this input and throws away the other 5 volts. Not efficient.

MPPT controllers are better. MPPT stands for Multi-Point Power Tracking. It constantly checks the output of the panels and adjusts its capacity to convert whatever voltage the panels are putting out into the voltage the batteries require without wasting power.

Our charge controller - 60amp MPPT

Charge controllers are rated in input voltage, output voltage, and output amperage. You can adjust the output voltage in most of them to meet the voltage of your battery array. With our little system we needed our controller to put out 12 volts to charge the 12-volt battery array. Since the output amperage (not the power) of the charge controller is a limiting factor, 700 watts (panel array output) divided by 12 volts (battery array voltage) will mean we need a charge controller that will handle up to 58 amps. Fortunately Morningstar makes a 60-amp MPPT charge controller. So that's what we bought.

If we had a 48-volt battery array and a 48-volt inverter a 60-amp charge controller could handle four times the solar panels. Why? Because 48-volts x 60 amps is 2880 watts. This is another reason to design your system to run higher voltages. If we wanted that much power to go into our 12-volt system we would need four of these charge controllers. Expensive!

Our charge controller can take any input voltage up to 150VDC and convert it to 12 volts to charge the batteries. Because higher voltage travels with less loss of power we have our four 24-volt solar panels connected in series.  The total output from our panels is 96 volts nominal. (They actually can put out quite a bit more voltage under certain conditions but not over 150 volts)

STEP 6. CHOOSE AN INVERTER

The inverter I wish we had! OutbackPower 3000W, 48V

The last piece of equipment you need for your system is an inverter to convert the DC current in the batteries to AC house current to run your appliances. We have determined our load as 2000 watt/hours. That is not meaningful for figuring out your inverter size. What you need to estimate is how many appliances are likely to be on at the same time and total the watts for that. Get an inverter that produces more power than is likely to be used at any one time and you will be good.

We have a 2000-watt inverter. That's equivalent to one 18-amp house circuit.  Make sure the inverter you buy can handle a continuous load at the wattage rating you need and that it produces a pure sine wave so your electronic equipment operates better.

Inverters are matched to battery voltage outputs. There are four standard voltages – 12v, 24v, 36v and 48v. Buy the inverter that matches your battery output. Higher voltage inverters don't cost any more than lower voltage ones. We already had a 12-volt, 2000 watt inverter so we had to match everything to that.

As you can see from our system, there is some flexibility in the design. If you have a line on some good equipment for cheap, re-designing the system to use that equipment can save you a lot of money. For our system we managed to get our solar panels donated to us. The inverter was already here so we didn't have to buy that. To create the optimal system I would have designed it differently but free panels and a free inverter meant we only had to buy batteries and a charge controller, which cut our potential system cost nearly in half. It was definitely worth figuring out how to make the free stuff work!

Our system has worked just great since we put it in three years ago. We have not run out of power once and we have all we need even though our system is small.

If you are starting from scratch and have similar power requirements as our system, I would recommend a 48-volt system. I would use the same 60-amp MPPT charge controller I am using now but, instead of the 12-volt battery array, I would get eight 6-volt batteries in series (48 volt total) and get an inverter that can take 48-volt input. With that system you could keep adding solar panels to your array up to a total of 2800 watts, four times the power that our system produces.

HOW MUCH WILL IT COST?

If I were to purchase everything today to build our sample system from scratch here's roughly how it lays out:

Solar panels - anywhere from 60 cents per watt for used commercial panels to over 2 dollars per watt for new ones. Prices have been coming down. Ten years ago new panels were close to 4 dollars per watt. Let's figure you are buying new panels at $2/watt. 700 watts would cost $1400.

Charge controller - I like the Morningstar Tri-Star 60-amp MPPT controller - $500

Batteries - We got Rolls-Surrette batteries because they have a 10-year warranty. Many cheaper batteries only have 2-year warranties. They were about $350 each. For eight of them (the optimal 48-volt system) that would be $2800.

Inverter - I would get an Outback Power 2800-watt, 48-volt pure sine wave inverter. $1900.

Hardware - This includes wire, switches, connectors, etc. - probably $200 to $300.

Panel Rack - You will need a rack to hold your panels. I built my own rack for about $200 in materials. I am a welder so it was no big deal to make it. I don't know what it would cost to have someone make you a rack.

Shed - The batteries, charge controller, and inverter need to be in a protected space like a shed. We already had a shed we could put this stuff in so there were no added costs. If you don't have a shed you will have to get or make one. I am leaving the cost of this out of the calculations.

As you can see, the batteries are the most expensive part of the system. They are also the part that wears out fastest. You will have to replace them every 7 to 15 years, depending on how well they were built and how well you maintain them. Badly maintained batteries in a poorly designed system may only give you 2 years of service so it pays to buy the best and take care of them.

If the rest of the equipment is good quality it should last a very long time. There are still solar panels in service that were made in the 1970's. They produce a little less power than when they were new but are still working. The good quality charge controllers and inverters also can last a long time. You will probably not have to replace them. Cheap stuff will likely die quickly and you will have wasted your money. Get the good stuff!

To save money, getting used solar panels and the other equipment is not a bad idea. Test them first to make sure they work. If they do, they are likely to continue to work.

SOLAR POWER CALCULATOR

I believe it is important to understand how your solar power system works. That's why I put all this info here. Once you know how it works and have done your first system design, you can simplify the process by using an online solar power calculator. Here is the best calculator I have found on the internet: http://www.sunsoglobal.com/calculator.html It calculates the number of solar panels you need, total charge controller amperage, and battery array size. Instead of using daily power use and production in watts to design the system, it uses monthly power use in kilowatts. You will have to multiply your daily load (watt-hours) by 30 and then divide by 1000 to get your monthly average load in kilowatt-hours.

Now that you have figured out the equipment you will need you will have to install the equipment. That will be the subject of my next article on the subject - Solar Power Basics - Installing Your System.

Water From the Air

The atmosphere contains about as much water as the terrestrial fresh water supply. That’s a lot of water! Wouldn’t it be nice if we could extract water directly from the air instead of having to depend on rainfall or groundwater?

98% of atmospheric water is invisible water vapor. The other 2% is condensed water that appears as clouds. The water vapor in the air is also known as humidity. For weather forecasting purposes they measure relative humidity.  Relative humidity is the comparison between the maximum amount of water vapor that the air can hold at the ambient temperature and the actual amount of water vapor present. For example, a relative humidity of 30% means that the air contains 30% percent of the maximum amount of vapor the air can hold without condensing out. Higher temperature air can contain more water vapor.

The temperature at which water condenses out of the atmosphere is called the dew point. The dew point temperature is higher as humidity increases.  Lowering the air temperature to below the dew point will cause the water to condense out. This is the basis for air wells and most atmospheric water generators.

Air wells are passive systems that need no external energy source and have no moving parts. There have been, historically, two kinds of air wells – the mass air well and the radiative air well.

The first ones were mass air wells and they did not perform very well. They consisted of massive stone buildings that, because of their mass, stayed cooler than the surrounding air thus accumulating condensed water on cool stone surfaces within the structure. The water would run down the stone surfaces into catch basins. A big one was built in the early 1900’s that was 45 feet high and made of stone. It only manages to capture five gallons of water per night. That’s really a lot of work to get only that small amount of water. The main problem with these, besides the fact that they are huge structures, is they also heated up during the day and could not cool down enough at night to produce more water.

Another ancient water collecting method was to create dew ponds. As it turns out, they produced most of their water from moisture in the soil or by collecting rainwater. Very little, if any, of the water collected in dew ponds actually came from dew.

The radiative air well uses night sky radiation to produce a cool surface for water to condense on. The thermal properties of night sky radiation are the reason frost sometimes occurs on roofs even if the air temperature is above freezing.  This phenomenon is now being explored for whole house cooling that doesn’t require air conditioners or evaporative coolers.

The material used to collect moisture has to be very low mass so it can’t hold heat. It also has to be insulated from the heat held by the ground. The material also has to be hydrophobic (water repellent) so that the water runs quickly off and into a storage container before it can evaporate. Typically, radiative air wells consist of a thin plastic film backed by insulation and placed far enough above the ground to mitigate heat coming from the ground. Most of the research and development on radiative air wells has occurred in the last 40 years. New thin-film materials are being designed that maximize the collection of the condensation. The best new materials have bumps on them to increase the surface area. 

You can make your own radiative air well using a sheet of polyethylene suspended at about a 30 degree angle with a trough at the lower end to collect the water and direct it into a storage container. I am thinking black polyethylene would work better than clear. That’s worth an experiment I think. You will want a filter between the collector and the storage container to keep out dust, etc.

How much water you can collect will depend on the humidity and how cold it gets at night. If you regularly get dew on your car windshield then a radiative air well could work for you.

There is a commercially available system that collects water from fog. The Warka Water system is being tested in Africa. http://www.vittori-lab.com/introductionwarka  Since the sides of the collecting fabric are vertical I don’t think it would work very well capturing water vapor.  They are interesting looking!

The radiative air wells are capable of producing more water than the mass air wells but not a lot of water. Larger surface areas produce more water. They can produce enough to provide drinking water under the right conditions - high humidity and a large temperature differential between day and night. Where I live, in central Arizona, humidity is very low most of the year so collecting water from the air would be difficult.

Atmospheric Water Generators (AWG) are active systems that use the same technology as air conditioners. They produce the most water of all and under a wider variety of atmospheric conditions. They contain cold surfaces and the water condenses on them. These are energy intensive appliances so are not practical in applications where cheap energy is not available. There are home systems that you can plug into house current. The use between 400 and 1000 watts which is a lot of you are off-grid. It may be worth it if clean water is not available and you have enough energy to run the equipment.

Some commercial AWG systems are being designed with their own renewable energy sources to provide the refrigeration necessary for atmospheric water generators to work. An example of this is the Eole Water Generator. http://www.eolewater.com/gb/our-products/range.html  This generator runs on wind power and also generates a considerable amount of extra electricity that can be used.

You could probably make your own AWG using an air conditioner. You would have to reconfigure the placement of the condenser so that water can be collected from it. I think you could use  a much slower fan, if you need a fan at all since the goal is to collect the condensation rather than deliver cool air into your house. To get a reasonable amount of water with AWG systems the humidity should be above 30% and the temperature should be above 65 degrees Fahrenheit during the day.

 

There is another method to get the water out of the air that involves hygroscopic liquid salts (desiccants) that absorb water vapor. These salts are then heated to boiling and the water evaporates and is collected and condensed.  It sounds like these might use less energy than the condensation systems although toxic salts don’t sound like something I want to fool with.

Even if collecting water from the air does not produce a lot of water via the above methods, the water produced is very clean. We really don’t use that much water for drinking so it might be enough for that purpose. It can also be a fun thing to experiment with. May someone will discover a new method to get more water from the air without using too much energy.