OFF-GRID Design Dr. Gay E. Canough, Master trainer 1
Understand the Customer s load 2
Load sizing Gay E. Canough AC Appliance watts amps how many of these? number of hours it is used per day equals watt-hr/ day number of days per week equals watt hours per week EXAMPLE: lights 15 0.5 4 4 240 7 1680 kitchen lights 0 7 0 livingroom lights 0 7 0 other lights 0 7 0 well pump (voltage= 120 or 240?, max amps =, running amps=?) refrigerator (kwh per year/365 from yellow sticker) 0 7 0 0 7 0 freezer 0 7 0 cooking stove if electric 0 7 0 microwave 0 7 0 computer 0 7 0 TV 0 7 0 pellet stove 0 7 0 bolier (pumps for heating) 0 7 0 3
Beware of adding just one thing. If the one thing is a refrigerator, it will need about 1.5 kwh per day. If the load without the fridge is 2 kwh per day, adding this one thing increases the load by 75%! And you have to increase the solar array by this much to keep up with the new load. Beware of the phantom load Unplug charging bricks and other transformers. Put TV on power strip and turn off strip with its switch. Anything that keeps the inverter on uses power 24/7 and the inverter itself uses about 20 W or 0.48 kwh in a 24 hour period. 4
Implement Energy Efficiency Measures First. Each dollar you spend on reducing the load saves $20 on PV panels needed to power it. Use compact fluorescent lights or LED lights Use Energy Star appliances 5
Anatomy of an Off-grid PV System Photovoltaics generate DC electricity Charge Controller regulates current and voltage to battery Battery stores energy Inverter converts DC to AC Fuses protect electronics from overcurrent faults Disconnects isolate system components for maintenance 6
Size up the solar array depending upon the type of use. For a summer cabin, use the average summer sunhours. For a year-round weekend camp or remote home use the average year-round amount of sun-hours For a remote home (full time use), recommend a PV/ generator hybrid. More PV will lead to less generator use. 7
Find out the solar resource 8
PV array sizing Procedure Off-grid people tend to plan for kwh per day use, rather than yearly. Take the daily average amount of sun (4.1 sunhours for Binghamton) Use an efficiency typical of battery charging (65%) Take daily load in kwh from load-sizing sheet PV array size = Daily load (daily sun-hours x efficiency) Example: 5 kwh per day (4.1 x 0.65) = 1.876 kw of PV 9
For summer cabin: The solar array needs to generate at least the amount Of energy needed each day. For year round: The solar array needs to generate more than the energy needed each day to keep batteries as close to full as possible, ready for cloudy weather. 10
Choose a Charge Controller The charge controller should be sized to handle: PV array short circuit current x 1.5625 This is because the current is continuous (heating) AND the irradiance can be greater than 1000 W/square meter. Example: module/string short circuit current = 7.99 7.99 x 1.5625 = 12.48 60 A charge controller can handle 60/12.48 = 4.8 Strings. 11
Where: Choose wire size to minimize voltage drop (0.2 x d x I /V) x (Ω/kf) = %Voltage Drop I is the circuit current, which for source circuits is usually taken as the maximum power current, Imp, d = one way distance in feet V is the voltage at which you want to find VD, and Gay E. Canough Ω/kft is the wire s resistivity in Ohms per 1000 feet and is found from NEC Chapter 9, Table 8, Conductor Properties. Math note: 2d is the round trip distance. To convert to %, one multiplies by 100, but to convert feet to kilo-feet, one divides by 1000. 2 x 100/1000 = 0.2 12
Example For a source circuit with 4.4 A, 50 wire run using size 10 wire What is the percent voltage drop (%VD) for a 24 V system? Vdrop in % = 0.2 x 50 x (4.4/24) x 1.24 ohms/kft = 2.27 % This is OK, but lower than 2% is better. We would probably use larger size 8 wire instead of size 10. Alternatively, we could go to a 48 V system. 13
Outback true sine wave XW true sine wave TR modified sine wave 14
Grid-tie with Backup Gay E. Canough These inverters can be used for offgrid or for grid-tie with backup. They stop feeding the grid during power outage and feed the critical loads only. Many stand-alone inverters have an input to charge the battery from an AC source such as a generator. they are then called an Inverter/ Charger. 15
Inverters prefer cool, dry, clean places. Batteries will perform better in a heated space. vent Power panel Battery box 16
How big should be off-grid battery storage be? Battery Sizing Example Let s say you need enough storage for a 3 day weekend. Loads: a) Lights: 0.5 kwh/day b) Stereo: 0.5 kwh/day c) Well pump: 1 kwh/day d) Microwave: 0.25 kwh/day Total = 2.25 kwh/day Or 6.75 kwh for 3 days 17
Load: 6.75 kwh Multiply by 2 because you want to discharge batteries at most 50%. So now you have 13.5 kwh Choose a battery For regular cycling, use a wet cell battery such as the Rolls S-460, 460 Ah at 100 rate of discharge (that is slow discharge) Inverters for off grid applications can have 12 V, 24 V or 48 V inputs Solar modules are made for either 12 V or 24 V outputs. 18
With 12 V modules such as Sharp 123s And A 24 V input inverter such as the Outback 2524 We must have a 24 V battery bank wire 2 modules in series and multiple strings in parallel With Rolls S-460s, we need 4 in series for 24 V, because each battery is 6 V 19
So, how many Amp-hours do I need? 13.5 kwh at 24 V (6 V x 4 in series) means I need (13.5 kwh x 1000 W/kWh)/24 V = 562.5 Ah. [This is our old friend Watts = Volts x Amps.] So one string of S-460s would be too few Ah. Get 2 strings or 8 batteries. Unless the system is really small, use 24 or 48 V. This leads to smaller wires and switchgear, which is less expensive. 20
Size battery cables to carry maximum DC current Example: A 4000 W inverter on a 24 V battery bank can draw up to 4000 W/22 V = 181.8 A because the battery voltage can be as low as 22 V before inverter cuts off due to low battery. As required by NEC, add a safety factor: 181.8 x 1.25 = 227 A Now check NEC ampacity table for wire size: USE-2 size 4/0 would be OK. Make sure cable is rated for batteries. 21
Strong PV installer holds S-460 battery! Use wet-cell batteries for offgrid homes and cabins. Use sealed batteries for grid-ties with back-up. Discourage batteries for backup power when power outage is rare. 22
A Few Notes on Batteries 1. Always use deep cycle batteries that can be discharged up to 80% without damage. 2. Batteries that need watering should be filled with distilled water and equalized once per month. 3. Lead-acid batteries like to be warm. Always put non-sealed batteries in an enclosure and vent to the outdoors. 4. Battery capacity is usually stated in Amp-hours. It is best to convert this to Kilowatt-hours. Do this by multiplying by the battery bank voltage. 5. Larger batteries generally are longer-lived. Surrette CS batteries have 10 year warranty. KWh = (Amp-Hours x Battery Bank Voltage) 1000 Battery ratings are usually given in Amps-hours at a 20-hour rate. This means it will give the rated Amp-hours if discharged over the course of 20 hours. Slower discharge rates usually have a slightly higher Amp-hour rating. 23
Take care of your batteries! 1) Keep them fully charged 2) Put in Distilled water when needed 3) Equalize once per month Gay E. Canough Charge State Table Supply customers with: The Battery Book A large box of baking soda A fire extinguisher (to be mounted next to battery box) 24
There must be: Don t forget disconnects and fuses 1. A fuse and disconnect between the battery and inverter. (DC ratings) 2. A fuse and disconnect between the PV and the charge controller. (DC Ratings) 3. An AC disconnect between the generator and the inverter. 4. A circuit box to feed from inverter. 5. A fuse on each series string if there is more than 2 strings. If you are feeding both sides of load center with 120 V from a single inverter, then all loads must have their own neutral (no sharing). See NEC article 690. 25
About Generators Make sure your generator is capable of remote start. Generator size: under 10 kw is fine for most off-grid homes. 26
Photovoltaic Array on Pole-top Rack Gay E. Canough 8 x 11 9 x 8 1.65 kw, working since 1994 10.0 Kilowatt-hours per day 7.5 kwh 5.0 2.5 0 1 2 3 4 5 6 7 8 9 10 11 12 month 27