PV Powered CP Systems
Basics systems, and how to save money, time, and maintenance

System Design Factors

Reliability and efficiency

Solar powered cathodic protection systems have gotten somewhat of a bad reputation in the past. To some extent, this has been deserved, but the blame has often been applied in the wrong place.

It's not that solar is a bad choice, it's because many of the systems were designed without a complete understanding of the differences between PV power and AC powered rectifier systems. Another major factor is that up until recently, solar electric systems have been VERY expensive - that led to various methods of cutting costs, almost all of which reduced system reliability. Many systems were (and still are) built with CP controls and regulators that are very inefficient and waste a large portion of the power from the photovoltaic array. We have seen 400 watt arrays where less than 20% was actually available to the ground bed - the rest was wasted in system inefficiencies. Even the best designed current systems using conventional controls have overall losses of 40-60%.

Increasing System Efficiency

System losses come from three major areas:

1. Losses caused by mismatch between the solar array and the batteries
2. Losses and deterioration in the batteries themselves
3. Losses between the batteries and the ground bed (anode to structure) system.

1. Mismatch losses are caused because batteries are 12 volts and PV panels are typically 17 volts. What this means is that a 75 watt panel rated at 17 volts at 4.4 amps is actually only transferring about 53 watts into a 12 volt battery (12 volts x 4.4 amps = power in watts). This is a 30% (or more) loss before you even get started. (For a full explanation of this, see our "Why 120 watts does not equal 120 watts").

2. Battery losses are often overlooked, yet using the wrong type or cheap batteries can cause an additional loss of 20% to 40%. No battery is 100% efficient - but some are much worse than others. Sandia Labs did battery tests and found that some "budget" golf cart batteries had internal resistance losses of up to 50%. Even the best will have some losses (for example, our Concorde AGM will typically have about 5-10%), These losses occur (as heat) in both directions - when charging and when drawing a load. This can be reduced considerably by using batteries with heavy plates and very low internal resistance.

3. CP control losses: Usually the greatest losses comes between the battery system and the ground bed (anode to structure). This is because very few applications need a nice round number like 12 or 24 volts. So what happens is that the voltage is reduced between the battery bank and the ground bed to give the required current. This is often done with tapped resistors, variable rheostats, or electronic voltage regulators. The losses occur because all of these methods usually use equipment or controls adapted from AC powered rectifier systems.

If the ground bed requires 8 volts to obtain the correct current, then you are dropping 4 volts across the CP current control - another 33% loss. And it can be much worse in some cases - for example, a well head requiring 4 amp with a 1-ohm ground bed needs only 4 volts applied - so if you are dropping down from a 12 volt battery system, you are losing over 65% of your power just in CP controller losses.

Solutions

For a brief explanation of how we minimize these losses and reduce system cost, see the right side column on this page.

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Reliability and efficency, part 2

Reliability in PV power systems can be increased in 2 ways: better components and system design safety factors.

• Better components are a pretty obvious solution, but defining WHAT is better is not so easy. Better does not always mean more expensive - better is primarily matching the components to the application.
• Safety factors are much less obvious. Safety factors, or margins, or overdesign, or overkill - whatever it's called, it means designing the system for the "worst case" with an additional built in margin. We have seen some pretty bad examples of "low bid" systems with no margin at all - so after aging a year or two, the batteries no longer can sustain the load in the winter, leading to rapid failure of the system.
  • A system designed for the average or summer insolation (amount of sunlight) will be severely stressed and will fail early in the short-sun days and cold of winter.
  • Another factor not usually allowed for is system deterioration. Batteries lose capacity over time (especially if undercharged for long periods), and this must be allowed for. Solar panels also lose, but this is usually less than 1% a year.

Increasing System Efficiency, part 2

Reducing system losses

The major system losses can be reduced by:

1. Using what is called a power tracker, or "maximum power point tracker" (MPPT) type charge controller between the PV array and the batteries. This is cost effective on larger systems - that is around 250 watts and higher, but because of the cost of MPPT type charge controllers, for smaller (1 and 2 panel) systems, we recommend a standard charge controller such as the Morningstar ProStar 30 or Xantrex C40.
2. Using batteries with the minimum internal resistance possible to reduce IR losses. AGM deep cycle batteries have the lowest losses of all Lead-Acid batteries.
3. Use "smart" DC to DC conversion between the batteries and the load - in effect, use a MPPT to feed the ground bed. This is usually the single biggest money and power saver. Currently, only the AERL CP controller has this function.

1. Using one of our MPPT controllers can reduce the mismatch loss between the PV array and the batteries to less than 4%.A power point tracker acts like a voltage-current matching device between the solar panel and the battery. If the panel is putting out 5 amps at 17 volts and the battery needs 12.3 volts, it will change the panel output (with a DC to DC high frequency converter) to 12.3 volts at about 6.8 amps.

2. Cheap ("economical", "budget", whatever) batteries are cheap because they use less lead and cheaper construction methods. this can lead to high internal resistance, which is a direct loss of power. We use only Concorde AGM or Crown industrial (for large systems) batteries. Both of these have very heavy plates and heavy duty internal connections. A standard "golf-cart" type battery will have internal losses of about 20% when new - the Concorde about 4%, and the Crown average about 7% maximum (at C/10 charge or discharge rates - less at slower charge or discharge rates). Typical lifespan on our batteries using our system design varies depending on the batteries. With the Concorde AGM it is usually around 6-8 years, with the Crown industrial it is 20+ years. However, the industrial batteries are NOT suitable for smaller systems, as they are far too large and heavy.

3. CP controller losses: We have been pounding on folks about this one for years, but it's been a tough sell. It's been hard to convince people that a little extra spent on a good CP control can save you several times as much on the total system costs. Even the best of all other commercial units barely approach 80% efficiency, and most are in the 30-70% range. At about $3 per watt-hour system costs, that can get expensive very quickly. The worst of ours are 96% - and most are in the 98% range. This feature alone can save you at least 20% on most systems. For a 12 volt, 4 amp wellhead system with 2 ohm GB resistance, the savings would be in the 30-40% range.

We hope this has shed some light on the subject of solar electric systems for cathodic protection. Please see our other pages in this area for more detailed information, or give us a call or email.