Designing a Stand-Alone PV System: Page 6 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

An inverter with a continuous rating of 2,500 W and a minimum surge rating of 6,436 W will meet the household’s instantaneous power and surge requirements. The inverter model chosen must have an input voltage of 24 VDC to match the nominal voltage of the battery bank, and have an AC output voltage of 120 VAC to meet the needs of household loads. There are no 240 VAC loads in the Ackerman-Leist home, but if there were, the following options would be available: specify an inverter with 120/240 VAC output; stack two 120 V inverters in series; or use a step-up transformer for the loads that require 240 VAC. Inverter features are also important to consider, such as an inverter-integrated AC-DC battery charger. This feature is convenient for use with a backup generator when the batteries need supplemental charging. A digital interface can also be a helpful feature.

System Recap

This system was sized appropriately given the design parameters and, along with the backup generator, should provide the family with a reliable and long-lasting PV system. The daily and annual energy production of any PV system is largely dependent on how much available sunlight there is and weather patterns, which vary from year to year.

It is interesting to examine how the system design would change if a backup generator was not incorporated. Using the month with the lowest peak sun-hours (December, 2.8 daily sun-hours) and increasing the days of autonomy from three to five would require 20 batteries and 20 modules—a 66% increase! Of course, higher-capacity batteries and larger modules could be used, but the increase in cost would still be substantial.

Since it was installed in May 2004, the Ackerman-Leist system has performed well and has provided the family with almost all of their electrical needs—minus about 30 hours per year of generator run time to equalize the batteries and make up for occasional shortages during the winter months. Although the system was sized for 12 modules, they started out with 10 for budgetary reasons. But with the addition of two children to the family (making them a family of five) and a few new loads, they will be adding the other two PV modules soon. In addition to the use of efficient appliances, the family is also in-tune with the weather and their energy usage patterns; they only do laundry on sunny days and only use a clothesline to dry their clothes. The system powered the entire construction of their three-level home and has since served as an educational model for them, their community, and students at Green Mountain College, where Philip Ackerman-Leist teaches.

It’s inspiring to see a family of five use so little energy and yet live so comfortably—a system of this scale would be vastly undersized for almost any other full-time residence, at least here in the United States. A testament to energy conservation, efficiency, and awareness, the Ackerman-Leist family lives with their system, paying close attention to the ebb and flow of energy.

Access

Khanti Munro is a Green Mountain College alum, an ISPQ-certified PV instructor, and SEI’s PV online coordinator and instructor trainer. Tied to the grid since childhood, Khanti lives vicariously through his off-grid friends and clients, with ambitions to someday unplug.

The sizing method presented is the sole intellectual property of Solar Energy International (www.solarenergy.org), which acknowledges that there are many sizing methodologies available today, and assumes no liability for systems sized using this method. Omitted from this sizing exercise were some technically complex aspects including nonoptimal tilt and orientation derate factors, conductor and conduit sizing, overcurrent protection sizing, grounding, and PV mount selection.

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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