Solar Power Systems

Is there a business case for Solar Power (PV) ? Solar Power (Photo Voltaic) systems for own consumption has a favourable payback period with the added advantage that the investment continues to produce positive return for 25 years. With the right finance from banks, your cash flow can be positive from day one. Unfortunately banks are dead in the water, when it comes to financing PV systems. They only finance utility scale systems, which we all know is not the solution, because the Eskom distribution network cannot handle it. But, its a government guaranteed loan, and that is why they do it. Don’t let them convince you for one moment they are interested in financing (or have financed any) green investments because they care. Fortunately, an industry which will provide loans for PV systems are developing outside the commercial banking system. The ongoing Eskom increases are a constant reminder of additional future benefits. Before buying a PV system, you have to understand why you want to do it, and being fed-up with Eskom is not a reason. The questions are: Are you trying to save on your electricity bill and protect yourself from energy cost increases? Do you want to ensure continuity during load shedding and electricity blackouts? Both the above? Are you prepared to compromise during power outages and to what extend? Do you want to build a system in stages, or once off? Once you have answered these questions, you have to consult a professional who specialises in energy for expert advice. Especially for commercial or industrial applications.

How much does it cost ?

This may seem the wrong place to start, but it is apparently where everyone wants to start. You have to understand three figures. The amount of energy produced, i.e.: the units of energy as charged by Eskom and Municipalities - kWh), The installed system peak power (kWp) and The capital cost per installed power unit (Capital cost in SA Rand / kWp). Every day the sun comes up, gets to midday and starts going down. You can take the sun’s height curve as the pattern along which a PV system produces power. i.e.: It starts from zero, increases to midday and then starts going down. It is difficult to calculate the total energy using this curve, so industry uses the concept of full-time equavalent sun hours. So let’s say instead of rising slowly and going down slowly, the sun switches on, shines at full power, and then switches off. How many hours per day does the sun have to stay on to produce the same energy as it currently does? In South Africa it’s mostly 4.8 hours per day. Many contractors will stretch the truth to more than that, they are like fisherman, its how big the fish would have been if it he did not catch it. So the 4.8 hours means, if you want the amount of energy produced per day, take the installed base (total panel power in kWp) and multiply that by 4.8 hours, this is the amount of energy saved (kWh). The capital cost of the installed base is about R 14,000 per kWp. That is a budgetary rough order of magnitude figure, and gets cheaper with economy of scale. So, for R 1 Million you get a over 70 kWp system, which will produce 336 kWh per day (or 10,800 kWh pm). At a rate of (for example) R 1.70 per kWh, that will save you R 17,136.00 per month. To understand the business case for a commercial installation, is much more complex, you can for example depreciate the whole system in year one.

What is a Solar Power (PV) System?

When saying PV system, we actually talk about two things, with possibly a 3rd component: Generation part - A PV System consisting of panels with its associated electronics, cabling and switchgear. This part generates power. Storage part - An inverter-battery system consisting of batteries with its associated electronics, cabling and switchgear. This part stores power for when the grid is off. Generator - A standby generator as backup during long power failures and heavy cloud periods. Depending on your goal, you may need the one or the other or both. Adding a generator is a low capital addition, with high operating costs. If you want to run a system with all three components, you have to incorporate additional control systems to manage the generator runtime and loading, the battery discharge cycling and the solar power delivery as one integrated system. The following diagram shows a complete System with a Generation part and a Storage part.

Let's look at the PV Generation part first:

Sunny South Africa is an ideal location for PV power generation. We can get about 65% more power from the same PV system than in Europe. The main components of a PV system are the solar panels and the inverter.

The panels.

You are buying panels for 25 years so make sure the supplier is credible. And yes, many Chinese panels are very good, BUT, those good manufacturers are represented in the country or they have long term relationships with local suppliers. Bloomberg new energy finance corporation rank solar panel manufacturers in terms of their bank-ability or financial stability. It is worth considering their ratings. Don’t be too hung-up on the panel power rating. Higher power panels are simply bigger, and the designer may need to use different panels to suit the inverter better.

The Controller (or Inverter)

Inverters vary greatly in price, features, quality and performance. It is probably the most important component of the system and the features must match the requirement (resulting from your answers to the questions above). This is where expert advice is needed and it is not the component to save money on!

Grid Tied Systems

A grid tied PV system is one that connects to the grid and generates additional power. Two important facts are: A grid tied system need a grid establisher to produce power. A grid establisher is either 1) the grid, 2) a generator or 3) an inverter-battery backup system. It does mean that the PV system will switch off if the grid goes off. It is a legal requirement in every single country on this earth, so don’t let anyone tell you his system simply continues to operate if the grid is off. And yes, city electrical engineers, you are not 1st ones in the world to think of this problem, more like number 7,000,000, and it was solved after number 10 thought about it. A grid inverter will always force its power into the grid. It is therefore always the 1st priority power being used. This means it will push power into the grid if it is not being used for self consumption. It also means it will reduce the generator’s power load. It will even force power into the inverter-battery system and damage the batteries if the system is not controlled, so don’t buy your system from an idiot. Generators need to deliver more than 40% of their capacity if they run. If they don’t the piston sleeves glass to the extent that the generator will seize if this happens over a prolonged period So the fact that the PV system forces it’s power into the system is good, but may lead to problems in an integrated system. This is why a large system has to be designed properly.

Installing a PV System

The first consideration is where to place the panels, and where do they point to? The sun is about 150 Million Km from the earth and putting your panels on the roof only gets you 5 meters closer, so don't automatically assume they have to be on the roof. Ideally they have to point north in South Africa (for any Americans reading this, our country is South of the equator) and be tilted about the same angle as the latitude. So in Johannesburg that is 26 Degrees, Durban about 30 degrees and Cape Town 34 degrees. The tilt angle is an average between the summer and winter angle. In winter you ideally want the angle higher and in summer lower. So if you make the angle higher, you will get better performance in winter and less performance in summer. That may be what you want, so circumstances can dictate that you place panels very different to the rule-of-thumb angle. Panels are normally placed in rows. The length and width of each row depends on the wiring, space available, etc. The distance between rows of panels should be more than two and a half times the height of the panels to prevent excessive shading. Panels perform badly with even a little bit of shading (for example: a cable shadow will show a measurable degradation). Some final comments on where to point the panels: If your roof is more or less pointing north and the angle is not too far out, then put them flat against the roof. If it is a large system, have someone simulate the system and calculate the exact yield. Placing panels flat on the roof may have less impact than you think, and you can fit a much bigger system. Do however only consider that if you do a proper simulation and intent to keep the panels clean.

The Battery storage system

A typical house has an energy consumption curve over 24 hours which looks like the Red and Blue curve below. A PV system typically produces power along the Green curve. So generally the PV system produces too much power by midday and too little power from about 16:00 in the afternoon up to 09:30 in the next morning. So the battery system stores energy while the PV system produces more than needed and supplies power while there is a shortage of PV power.

The Batteries

Car batteries and Solar batteries are designed for different load conditions. A car battery can supply lots of instantaneous current but cannot be discharged much below 90% of full charge before the battery is damaged. The industry refers to Depth of Discharge (DoD), which is the percentage of the rated capacity which is utilised per cycle. So, for example, a car battery rated at 100 Ah can normally only be used in a design which uses 10% DoD, so it’s useful capacity is 10 Ah. We generally refer to nominal capacity (the 100Ah) and cycle capacity (the 10 Ah). Solar batteries are designed to supply less maximum current but the very best, can be discharged many times to about 50% DoD. Essentially when you buy batteries, you buy cycles. Good manufacturers will give you a graph of the Service life in Cycles versus the Depth of discharge. The deeper the depth of discharge, the less the service life in cycles. If you need a battery you calculate the capacity you need in Amp-hours. Then you determine the number of cycles you need the battery to last. Then you choose a battery which will give you that number of cycles for the given capacity at a given DoD. Not all manufacturers will publish the number of cycles for a given depth of discharge. Ask them why not, the stories are normally a good laugh. A good battery manufacturer should provide the following information: Battery Nominal Voltage. Capacity in Amp Hours. Note that the capacity varies with the amount of current you draw from the battery. The higher the current, the less capacity. The manufacturer will give you a set of values for different discharge rates (in hours). A typical curve is shown below. Note that the 5 hour (C5) capacity is only 65% as much as the 100 hour (C100) capacity. Life span in cycles. The lifespan varies depending on the average Depth of Discharge (DoD). The deeper you discharge a battery, the shorter it's life. A typical curve is shown below. These curves are for extremely good batteries, and they are floated tubular cells, meaning they have to be filled, like our fathers had to do with the car battery on a Saturday morning. Many batteries offered as Solar batteries will only produce 300 cycles at 50% DoD, versus the 2,500 cycles shown in the curve. Good 12v maintenance free batteries are around 1,000 cycles. Get the curves from the battery supplier. If he cannot supply them, don’t buy, its probably rubbish. If he does supply them, make sure he guarantees it. So you can see a battery is not just a battery and selecting one is not trivial.

Days Autonomy

This is how long the battery bank can provide the design power. The size of the battery bank is directly proportional to the number of days autonomy. For domestic systems the batteries should store at least on day’s power produced by the PV system. Take the nominal battery capacity and divide that by two (remember 50% DoD?). Compare that to the PV system capacity (kWh per day), which is about 4.8 times the nominal power (kWp power) of the panels. If the battery power is less, there is not enough batteries.

PV Water Pumping Systems

Using PV systems to pump water requires special mention. Except for systems which has to deliver water pressure on demand, most pump systems can be sized to pump for only a portion of the day. This means we can limit our pumping to when energy is being generated by the PV panels, and thereby eliminate the need for a battery storage sub-system. Without batteries the PV system is such cheaper. The PV pump system electronics control the pump speed to match the amount of energy available form the panels. This is done by using a variable speed drive on a 3 phase induction motor or by using a DC motor driven pump. There are a number of manufacturers who build such controllers, either sold separate, or as part of an integrated solution. The fact that you don’t have to include the battery storage sub-system means that it will always be more cost effective to separate your pump PV system from the rest of the PV loads. If you buy a good quality system, you can have a system that will run for 25 - 30 years.

Preparing for PV Power

There are a number of factors to consider when preparing your business or home for alternative power. Reduce the energy consumption by using more energy efficient devices. Use solar thermal as far as possible for heating. Note that solar water heating panels are four times more efficient than PV panels in terms of panel area and cost. Consider alternative energy sources. Using gas for cooking, boiling water etc.

Generators vs. Inverter-Battery System Cost

From a cost point of view, Generators and PV systems are fundamentally different investments but, if you analyse the cost properly, the cost of the electricity you get from them is the same. Some generator facts: The initial cost per watt of a generator is low. A good small generator use about 400 ml diesel per KWh, so the generation cost is approaching R 6.00 to R 7.00 per KWh. They are noisy and engines don't last long. At least buy one that runs at 1500 RPM then they last a lot longer. You have to service them regularly. They can be used any time (except when the neighbours complain). Generators are good for providing lots of power for short periods.

PV plant facts:

The capital cost is higher than a generator. They last for 25+ years if you buy quality. If you use the power generated every day, the power costs below Eskom rates per kWh. If you have to store the power in batteries, the systems costs 2.5 times more for one day autonomy systems. The business case is then based on business continuity rather than the cost of electricity. They have no maintenance or running costs, except for the occasional cleaning. They generate power only when the sun shines, but batteries provide power in between. They are as green as you will get. The trick is to combine the best of both in a solution. This can be done by powering the low power, high duty cycle devices from PV power and powering the high power, low duty cycle devices from a generator, or even better, do the washing only when the grid power is on.

Safety and Electrical Standards

I did not touch on safety and electrical standards. The reason is that, if you know so little that you have to read about it here, you should not touch the system. Many industry players prefer to ignore the regulations but, at the end of the day, your premises are your responsibility. Having studied the regulations endlessly, I can assure you they are all justified and ensures safety. Not adhering to them will catch up one day and most likely that will coincide with your day in court.
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