NEWS

Solar power backup for Critical Household Systems

Sun Volt Solar (Blog)
April 6, 2008
www.sun-volt.com

Almost everyone has experienced a black out. Thankfully, they are often for short periods and the line men work hard to get the power back on to everyone as quickly as possible. However, if major damage is inflicted upon the power grid, it can take quite a while to repair. Are you prepared to live without electricity for days, weeks or even months?

Many people have installed gas or diesel generators to power their houses. These units can run reliably for days or even weeks at a time, but they also require refueling. After Hurricanes Ivan in Pensacola and Katrina in New Orleans, it was discovered that motor fuel can often be in short supply after major disasters as the electric transfer pumps required to move the fuel from storage to use are inoperative. It can also be difficult to deliver fuel due to blocked or flooded road ways.

Additionally, generators require maintenance, the engine oil needs to be changed, they need to be  exercised periodically.  Solar panels need almost no maintenance, perhaps in a prolonged dry spell, the dust could be washed off once in a while.  Even that is optional.

As solar powered backup system requires no fuel and if properly sized can run your critical household electric load indefinitely. These systems can be individually sized according to the load calculations. Additionally, they can be configured as a normally solar powered system that is backed up by the electrical grid. If a location has a good wind resource, a small wind turbine and be used to augment the solar system during inclement weather.

Instead of using the PV panels to merely to keep a battery bank charged, the PV panels and battery bank can be the primary source of power. The electrical grid can be used as a standby in case the batteries get too low. A reserve charge can be built in to the battery bank to run the critical loads in absence of both sun and grid power.

This is best of both worlds.

Critical loads
Indentifying critical loads is the first step. At my house we have a well pump to supply water, an oil fired boiler for heat, an electric refrigerator, two sump pumps to keep the basement dry, and two circulator pumps on the solar hot water system. I would also like to power the computer network and one or two outlets for lighting. All of these appliances have name plate information which give the power consumed. If the loads are given in watts, they need to be converted to amps. To do this, use Ohm’s law, which states:

P=I x E

where P is the power in watts, I is the current in Amps and E is the Voltage.

Thus if something if something draws 300 watts, you would divide that by 120 volts and get 2.5 amps.

Load Name
 Manufacture/Model
 Load Current @ 120 VAC (Amps)
 Load Power (Watts or VA)
 Duty Cycle (percent/hr)
 Amp/day (Amps x duty cycle x 24)
 
Well Pump
 Goulds 10GS10422
 7.9*
 948
 2
 3.8
 
Boiler, heating
 Dunkirk/Grundfos
 4.3
 516
 10
 10.32
 
Refrigerator
 Kenmore
 2.1
 252
 10
 5.1
 
Solar Hot Water Pumps
 Taco 009B and 006B
 1.8
 216
 20
 8.64
 
Computer Network
 Various
 0.3
 36
 100
 7.2
 
Water Filters
 Kenmore
 0.3
 36
 100
 7.2
 
Misc loads, lights, etc
 
 3.0
 360
 40
 28.8
 

Total storage is 71.06 Ah per day at 120 VAC. Some of these loads are seasonal, e.g. the boiler will not run in the summer, but the refrigerator will likely run more. Over all, I think this represents a good picture of my critical household loads.

I plan to have a 24 VDC battery bank running two inverters tied together to derive 240 VAC for the well pump. I estimate a 355.3 Ah battery bank will give me 24 hours, but there are other considerations:

I would like to run for several days without recharging because sometimes stormy weather lasts for quite a while. Therefore 355.3 x 3 = 1065 Ah.

A battery bank should never be discharged by more than 40 percent. Doing so leads to shorter battery life and increased operating expense. 1065 Ah x 1.4 = 1493 Ah.

Finally, the inverter has some losses. A good inverter usually operates at about 95 percent efficiency. 1493 Ah x 1.05 = 1567 Ah.

That is a pretty big battery bank

The inverter chosen is a Xantrex SW2524 Plus, which as an internal battery charger and an optional controller for starting a generator. I plan to use the generator starting contact to close a power contactor from our house panel, which will be used to recharge the batteries when they reach 55 percent discharge and grid power is available. If the grid is not available, we have a 15 percent reserve, which means I will load shed until the batteries can be recharged.

Next consideration is the size of the solar array charging this battery bank. The total daily storage from above is 71.06 Ah. All solar panels are sized in Watts, therefore, using Ohm’s law, we have P=71.06Ah x 120 Volts = 8,527.2 watts or 8.5 kW rounded total charging per day.  We have about 5 sun hours per day average, so 8.5 kW / 5 = 1.7 kW of charging power.

I will be using a MMPT charge controller to charge the battery bank, as well as tracking mounts for the panels, both of which will increase the efficiency of the system.

Finally, I will be using a 400 Watt Southwest Windpower wind generator to keep the batteries topped off during story conditions. Without the wind turbine, I would have to double the size of the Battery bank.

Here is a list of parts:

Batteries; Surrette 4-KS-21PS 4V 1557 Ah, 6 each
Inverters; Xantrex SW 2524 plus, 2 each
Inverters, Xantrex SWI stacking kit, connects two inverters to generate electric for 230 VAC loads
Inverters, Xantrex GSM generator starting module
Charge controller, Blue Sky Energy Solar Boost 3024i MMPT controller
Photovoltaic panels, Sharp NE-170U1 170 watt panel, 10 each
Tracking Mounts, Zomeworks URTF90, 2 each
WInd Turbine, Southwest Windpower Air X 24 VDC
Wind Turbine Tower Kit, Southwest Windpower 45 ft
Total cost, retail $26,000.00