Designing a Stand-Alone System: Page 2 of 6

Intermediate

Inside this Article

Ackerman-Leist Pole-Mounted Array
The Ackerman-Leist pole-mounted array stands at the garden’s edge, an integrated part of the family’s homestead.
A Watt-Hour Meter
A watt-hour meter gives precise figures on consumption for appliances already owned. Without that information, the values in the “Loads” table must be estimated.
Battery Bank
Contained in this simple battery box, three parallel strings of four 225 Ah Trojan T-105 batteries make a 24 V, 675 Ah battery bank.
Back of Pole-Mounted Array
This pole-mounted array offers unimpeded solar access at the site from 8 a.m. until 4 p.m.
Outback Power Controller
Step-down MPPT controllers can help decrease wiring costs by allowing PV array voltage to be higher than the battery bank voltage.
Xantrex Inverter
Select an inverter to handle the maximum loads that will be on at once in the home. Choosing the next larger size will help ensure your system can meet the demands of future loads.
Ackerman-Leist Pole-Mounted Array
A Watt-Hour Meter
Battery Bank
Back of Pole-Mounted Array
Outback Power Controller
Xantrex Inverter

Size it Up: A Case Study

Let’s explore an example sizing scenario, component by component, with a method Solar Energy International (SEI, see Access) uses in its classes to size stand-alone systems using an maximum power point tracking (MPPT) controller:

Who: The Ackerman-Leist family

Where: Pawlet, Vermont, approximately 3/4 mile from utility service

Solar window: 8 a.m. to 4 p.m.

Average daily solar resource: 4.6 peak sun-hours*

System backup: 4 kW backup engine generator

System voltage: 24 VDC

Projected energy use (AC and DC): 2.2 kWh per day

Expected avg. ambient temperature for batteries: 60°F

Record low temperature: -35°F

Desired days of autonomy: 3

Desired battery depth of discharge: 50%

Battery: 6 V nominal, 225 Ah, deep-cycle flooded lead-acid

PV modules: 12 V nominal, 80 W STC , array tilt equal to the latitude (43°)

Charge controller: MPPT, 60 A

Array mounting: Pole-mount

*Peak sun-hours are based on Concord, New Hampshire, values, which more accurately reflect the site’s latitude and weather patterns.

Step 1: Estimate Electric Load

Determine the amount of energy (kWh or Wh) that will be consumed on a daily basis. If it is for a home not yet built, this can be a very involved and time-consuming step. A designer will need to work closely with the homeowner/builder to realistically estimate the daily and seasonal energy requirements.

The power (W) of individual loads and their estimated energy consumption (Wh) can be tallied to calculate the household’s average daily load. This step will help identify opportunities for efficiency improvements and pave the way for sizing the system components. The table below lists the electrical loads found in the Ackerman-Leist household. The family heats their home with wood, cooks with wood and propane, uses a propane refrigerator, and heats their water with a solar thermal system and a backup propane boiler, so those are not factors in the load analysis.

According to the table, daily household loads average 1.8 AC kWh and 0.36 DC kWh (from the chest freezer), totaling almost 2.2 kWh a day.

Comments (5)

Marty Rosenzweig's picture

Very straight forward article but am I missing something here? Where is the "array sizing" adjusted for the C/10 optimum charge rate for those batteries? 3 X 225 Ahr /10 = 67.5A. necessary for the battery charging (plus accommodation for the load amps).
Even at C/15 that's 45A. I speak from experience since I designed an identical system for my house in Mexico. After about a year, the battery inertia (don't know what else to call the increased internal battery resistance) made bringing up to full charge those three strings more and more difficult and I experienced somewhat premature battery failure in less than 4 years. Even the addition of 300 W.more panels were not satisfactory. I've settled on 2 strings (450 Ahr.) with the 1200 watt array and two days of autonomy. We'll see how long the new batteries last under these conditions.
It would be beneficial to hear how the Vermont design is currently holding up and to track the battery life in the future.

Justine Sanchez's picture

Hi Marty,
Thanks for your comments! Joe has already covered the key aspects to battery longevity. I would also just like to add a few additional thoughts...while we can dial in the charge rate of an battery charger utilizing a generator (or the grid if avail) to charge batteries, from my experience an array is usually sized to simply replace lost energy from the battery bank on a daily basis. If we oversized the array to meet a specific charge rate (including during the winter months), we likely would have a significant portion of our array producing excess energy the rest of the year...and in an off-grid setting no place to utilize that excess other than a dump load. Historically it is the job of a backup generator to get those batteries topped off when the array (sized for estimated daily energy consumption) cannot.

A few other notes...Not sure what the rest of your system is comprised of but an MPPT charge controller will help wring some extra amps out of those modules, especially during cool sunny days. Also keeping the parallel battery strings to a minimum (one is ideal) will help keep battery bank charge/discharge imbalances from reducing battery longevity. And of course making sure the batteries are regularly maintained (watered, tops cleaned, connections checked for corrosion or loose hardware, etc.) will also increase battery life.

Best,
Justine
Home Power Magazine

Joe Schwartz's picture

Hey, Marty. We'll check in with Khanti and see if we can get some information on the performance of his system/batteries to date. Few thoughts related to battery charging:

First, historically, most off-grid PV systems were not designed to meet a battery manufacturer's optimal charge rate. The cost of modules was simple too high for most people to afford/achieve a c/10 charge rate for example. With the falling cost of modules higher charge rates in the range of c/10 are becoming more common.

Designing a system to meet a battery manufacturer's optimal charge rate is significantly less important to battery longevity than the following:

1. System's should be designed to replace all of the energy used on a (sunny) daily basis including system efficiency losses.

2. The battery bank should be fully recharged as often as possible, at least once a week and more frequently is always better. It should be completely recharged every sunny day.

3. Systems should be sized to limit the average daily depth of battery discharge to around 20% and again, less is always better. For example, my off-grid system is designed for a daily depth of discharge of 10%.

4. Off-grid system owners need to avoid the common scenario of falling into a pattern of cycling their batteries between say 80-60% capacity on a daily basis and rarely recharging them fully. Run the engine generator when necessary to avoid this scenario.

5. Flooded batteries should be equalized regularly per the manufacturer's recommendations.

-Joe

Geoffrey Kaila_3's picture

Hi Khanti,

I refer to your article designing a stand alone PV system in HP 136. I have two observations; 1. Cound't you have used a higher voltage than 48V on the PV side to maximize the MPPT benefit. 2. When I use the PWM sizing method I get the same # of modules. I thought the MPPT method would reduce the # of modules. Please clarify. Thank you and best regards,

Geoffrey kaila

Art Drayton's picture

Nice article - particularly like the emphasis at the start of energy efficiency first! Cheers - Aussie Art

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