Possible Koolbridge Solar Smart Load Center Installations
Solar Electric Components for an Off-Grid Home or Mixed Grid-Solar Home
- Solar Panels
- Means to meet Code rapid de-energisation requirements unless waiver can be obtained
- Charge regulator/Maximum Power Point Tracker
- Back-up generator
- Smart Load Center
Exemplary Solar Panels
- Consider 30-32, 36 volt nominal panels such as those supplied by Canadian Solar with the following specification:
- Price per Watt $0.52
- Rated Power Output 315 W
- Voltage (VOC) 40.1V
- Cell Type Monocrystalline
- Warranty 25-year linear warranty
- Dimensions LxWxH 65.9″ x 39.1″ x 1.38″
- Weight 40.8 lbs
- In a pure solar-powered home, solar energy is stored in the battery while the sun is up and taken out of the battery after sundown and before sun-up.
- The lifetime of lead-acid batteries is short if deeply discharged frequently, e.g. daily. Longer life lead acid batteries approach lithium batteries in cost.
- Lithium batteries tolerate many more charge discharge cycles and are smaller and lighter than lead-acid for the same capacity, but have been more expensive. They exist because their superior power-to-weight ratio is essential for electric transport. The Tesla 50kwhr or 100kwhr car battery is IDEAL for a solar home, but at a cost of maybe $15000 if you can get them. Lithium Iron Phosphate batteries from China are becoming less expensive and are quoted presently at about $9000 for the size needed for a totally off-grid home.
- The usual depth of discharge after sundown is not high, if the battery has been sized, as recommended in solar text books, to bridge at least three cloudy days. It takes a large battery, perhaps 50kwhrs, to bridge 3 days of no sun however and still won’t guarantee no interruption in power.
- The battery size, cost and life trade-off is greatly alleviated by two things:
- 1) use of the Koolbridge Smart Load center to manage energy consumption, and
- 2) Supplying a back-up generator that kicks in automatically when the battery gets low. The generator can operate from the same propane tank as a water heater and (possibly gas cooking appliances?)
Battery Voltage and Capacity
- With a back up generator and the Koolbridge Smart Load Center, lower-cost lead-acid batteries can in principle be used. If lead acid batteries are used, a suitable voltage is 48 volts, the NEC being easier on safety requirements for 48 volts than for higher voltages
- If lead acid batteries are used, any indoor battery compartment should be vented to the outside with e.g. a “ dryer vent” to dissipate odor and potential outgassing
- The choice of the inverter dictates the choice of battery voltage. Use of either the aforementioned SolarEdge optimizers or separate MPPT charge controllers decouples the choice of battery/inverter DC voltage from the solar array voltage. There are many inverters designed for 48 volt DC input.
- The battery AH capacity is related to the solar array size. The battery should logically be capable of absorbing at least one-days worth of sun, otherwise the array is needlessly large for the battery. An 8.85kw array will generate 45kwhrs on a good day; therefore a 48 volt battery for an 8.86 kw array should be around 1000 AH. This can provided by ten 100A-hr, 48 volt golf-cart batteries at ≈ $900 each for lithium
- It is possible to trade-off the cost of array and battery versus greater reliance on a back-up propane generator. As was explained on the previous page, if the battery is to be deeply discharged daily, it needs to be lithium, but a back-up generator can be controlled to kick in to avoid deep discharges, allowing lead-acid if the capital cost is critical
Solar Combiner/Charge Controller/MPPT Tracker
- Overleaf is shown one suggested wiring diagram for a 7 x 4 panel array.
- A 4-panel combiner combines each column of 4 panels, connecting pairs in series to get 72 volt strings, then combines two strings with (i) blocking diodes to prevent reverse flow, (ii) code-mandated per-string fuses, and (iii) two remotely-controlled relays for code-mandated rapid de-energization of each string (if installed on a dwelling)
- If required the combiner can be modular for different configurations. A combiner may not be needed at all if each 72 volt string is separately wired to a terminating box inside the dwelling but still within 1ft of the array. The required wiring would be 14 black and 14 red #14 THHN in suitable conduit. Junction boxes should be provided at the panels to convert the Solar module lead plus to THHN in conduit (e.g. EMT).
- The code requires all DC wiring prior to the first DC disconnect to be in metallic conduit. Since the combiner is the first disconnect, conduit is optional for the combined output of each combiner, but we recommend it as being superior to other wire anchoring methods, and for physical protection.
Seven of the 4-panel Combiners Below Could be Used (if required) for a 4 x 7 =28-Panel Array
Three, 60 amp MPPT Charge Controllers
The MMPT Charge Controllers
- The previous page illustrates three, low-cost ($178) 60-amp MPPT charge controllers. Actually there is huge variety of these on the market at widely disparate prices, and those of Chinese design have Code compliance problems. The market is changing rapidly as as these are relatively new and needs to be further addressed.
- The outputs of the seven solar combiners are combined 3,2,2 in a fused junction box. The fuses are code-mandated to protect the #10AWG wires from the total combined current of 54 amps that could back-flow in the case of a short at the combiner. The fuses can be user-replaceable, 30 amp fuses in finger-proof DIN rail fuseholders (They should never blow!)
- Alternatively Carling switch 30A 80 volt DC breakers can be used.
- The total current from 28 panels at 72 volts is 126 amps and no suitable single MPPT controller of that capacity was identified. However, it is an advantage to perform MPPT on the three groups of strings separately, as this gives better performance with partial shading. I
- The off-the-shelf MMPT change controllers are battery-technology agnostic (12,24,48 volt Lead Acid or Lithium)
- For a slightly smaller solar array, (6 x 4=24 panels) a single 100A MMPT charge controller could be used ($650 retail)
DC to AC Inverters
- Several DC to AC inverters are available operating from 48 volts, varying widely in cost. The cost is a function of
- (1) The power output
- (2) The AC waveform (Pure sinewave is a MUST).
- (3) Whether they provide only one or both hotlegs (120v versus 120-0-120 split-phase)
- (4) They must be listed to UL1741, as many cheap Chinese inverters are not.
- NOTE: Some cheap inverters do not tolerate even small reactive loads such as fluorescent light ballasts, and do not have the low standby power consumption needed so as not to discharge the battery overnight. Inverters sold at truck stops for use in trucks are not suitable.
- The best value for money seems to be the AIMS 8kw split phase inverter at under $2000 in small quantities. One 8kw unit for dual-power homes and two 8kw units for off-grid homes may be used.
- AIMS 8kw or 12Kw inverters
- Consider using two 8kW versions for the totally Off-Grid home. One would be used for all large (2-pole breaker) appliances and the other for everything else, giving a total of 16kw. A single one 8Kw unit can be used for the hybrid, dual-power home.
Deep-Cycle Marine Battery Possibility
- 40kwhrs of battery capacity can be provided by use of 40, 12v, 100A/H Deep Cycle marine batteries costing around $120 each, that is $4800 retail.
- However, they are good for only 300, 100% charge/discharge cycles. To obtain long life, they must not be discharged by more 25% of their capacity on a regular basis, that is 10kwhrs of discharge
- 10kwhrs will typically power a house overnight until sunup. With an 8KW solar array, the 10KWHrs will typically be replaced before 10AM on a good day. That requires energy management to ensure that the rest of the solar array output is utilized, by operating energy consuming appliances such as washer and dryer during peak sun (11am to 2pm).
- The Koolbridge Smart Load Center performs energy management by controlling the supply to appliances. Dish and clothes washer should be wired to dedicated breakers to allow individual control. Tumble dryer is already on its own circuit. HVAC can be set to over- or under-temperature when sun is available to delay making demands on the battery when the sun goes down.
- Of course it is possible to use a higher-life battery of only 10Kwhrs for an even lower price; the difference is, the 40KWhr capacity of the Deep Cycle marine batteries is available be used OCCASIONALLY to bridge successive periods of no sun. However, keep an eye out for deals on 48 volt lithium golf cart batteries
- In addition, a back-up propane generator of 3 to 5kw can be kicked in to prevent excessive battery discharge and to bridge .periods of no sun
Great Deals on Lithium Golf-Cart Batteries
- The above is a 48v, 100AH golf cart battery available from China at $900.
- The recommended capacity for an all-solar home of 40kWHrs would be met by using ten of these in parallel to obtain 48v at 1000AHrs.
- Coupling ten of these to a 48v high current busbar feeding the inverter(s) requires a 100A fuse or 100A Carling Switch breaker each for safety.
- The main choice is between an AC generator and a 48VDC generator.
- If an AC generator is used, and it is going to be used for battery charging, then a heavy duty charger is also needed, which is costly.
- Some inverters, like the AIMS inverter, are also chargers; however, either they are very fussy about the generator frequency being dead-on, or else they can’t be used as a charger and an inverter at the same time
- Moreover, with an AC generator, loads have to be transferred from inverter to generator using a transfer switch
- A 48v DC generator is therefore simpler to integrate with a 48-volt battery-backed-up system The generator can operate from propane, have autostart by contact closure either from the inverter or from the Koolbridge Smart Load Center to kick in to seamlessly maintain power when solar/battery is low, without transferring circuits from inverter to generator. The inverter monitors battery state and talks over a data cable to the Koolbridge SLC energy management unit.
- Residential standby generators traditionally had to be large enough to take the peak demand of many appliances operating simultaneously, and so were in the 15kW to 22kW region and costly.
- When added to an off-grid or solar installation having a battery, the back-up battery charging generator does not need to take the peak demand as the battery storage averages the demand over several hours. A 2kW generator can replace the entire 48kWHrs of battery storage capacity every 24 hours.
- Running a smaller generator at near capacity is much more fuel efficient than running a large generator that is underloaded most of the time
- A much smaller generator therefore suffices because it can run both before, during and after the high demand period to replenish the battery. A 48vDC propane generator in the 2 to 5 Kw region may therefore be used. A whole-house AC back-up generator would generally be in the 15-22KW regions
- The cost saving by being able to use a much smaller back up generator offsets the cost of battery and inverters to a certain extent.
This illustrates a 48A, 63A (3Kw) propane generator.
This one costs $2000 in small quantity and is made in Cyprus.