Solar Power System Design – Part II

I’ve been contemplating about what to buy for the last month. Its worthwhile to view installed renewable systems and talk to the owners about pros and cons. There is a lot of bullshit published on the Internet, the vast majority being copy and paste articles which are neither detailed nor discuss alternative solutions.

As mentioned in the Part I, the design of my Power System depends vastly on my budget. Ideally, the main focus will be the power requirements, but like anything else I’m doing on BENIsLAND I cannot afford what I want nor what I need. This project will also be a compromise, and I explain my rationale for each electrical component in the following.

This, following articles, including downloadable material to which I link to, shall NOT be treated as advice, instruction or a form of guideline for any sort of power system.

The aim of this series of articles is to provide a significantly higher standard than the usual, free online publications about renewable power and to write about all aspects of this project. The scope does not allow to write in exhaustive detail, however, I intend to publish an e-book in the future that can be used as a reference to designing, purchasing and installing a photo voltaic power system.

Finding a Supplier

In general, suppliers that provide detailed product information (including specifications of components) AND provide a regularly updated price-list are the ones I would contact. Too many suppliers ask you to contact them for pricing, which is usually a strong indication for relatively steep prices. As mentioned in Part I, you can expect a better service when you purchase all goods from one dealer, rather than shopping around.

Batteries (Deep Cycle Lead Acid)

Storing energy is just as or even more important than generating it. The batteries are the heart of your power system and making the wrong decisions will lead to significant future costs. Batteries also weigh relatively much and transport costs should be taken into account.

Let me start with a bad example:

A person takes the advice of a trades-person and purchases 8 flooded, 6V deep cycle batteries, each weighing about 40 kg and links two strings of four (24V operating) together. Flooded batteries require maintenance, they have to be stored outside, they need to be equalized regularly and their self-discharge rate is much higher than maintenance-free batteries.

During equalization, these batteries require a higher voltage than during normal (bulk) charging and require a high current input as well. Equalization is of utmost importance for flooded batteries. During this process, solid residues of lead(II)sulphate which adsorb on the electrodes get ‘shaken off’ and return to a dissolved state. During this process, the level of electrolyte (the liquid phase inside the battery) will decrease, explosive gasses will be released and pure water needs to be added to maintain optimum electrolyte level.

If equalization is not done regularly, the solid residues decrease the conductivity of the electrodes drastically, reduce the storage capacity and they can even short a cell, and cause permanent battery damage.

Furthermore, the required input current to charge to a specific state will increase accordingly. Since the requirements stay constant, the user will have to increase the depth of discharge (DOD) to meet the demands, and, as a result, the lifetime of those expensive batteries will decrease exponentially. To equalize flooded batteries, a generator is usually necessary; over-sizing PV panels is another option (works in summer, but not that well in winter).

8 of such batteries with a C100 storage capacity of about 400 Ah can cost anything from 3200 – 6000 NZD excluding expensive, high ampacity cables and transport. A decent generator will set you back 2-4 grand and on top of that you’ll have to pay for fuel and maintain the generator.

Other than the noise, pollution and fuel costs aspects, the generator needs to be stored safely (they do catch fire). The (stupid) risk that one may take by storing it under the house, with a full container of fuel close-by, becomes apparent when your and your neighbour’s house burns down, and your insurance company informing you that your fire insurance policy was void due to your decision-making.

I’ve seen plenty generators stored under a timber-built home, and the toxic fumes that you’re breathing while watching TV and eating supper is yet another reason to store generators appropriately.

Maintenance-free batteries, such as gel or amalgamated glass matt (AGM) are superior to flooded types. They don’t require any maintenance (and for most users, proper maintenance of flooded batteries is outside the scope of their self-motivated understanding and competence), as a result of this they last much longer, they don’t require equalization, they can be stored inside in any orientation as they do not release gasses,  generally have a higher DOD rate, the self-discharge rate is much smaller, the charging efficiency much higher and they can even be cheaper than flooded ones.

I have seen online prices of up to 800 NZD for a single 6V 400 Ah flooded battery. The AGM batteries I’ve got my eyes on have the same capacity and cost 500 NZD a piece.

In other words, save the money for a generator and invest it into your renewable power system.

What I Plan To Buy

a) 4 x 6V AGM batteries, 395 Ah C100, 300 Ah C10 (298x178x345 in mm, 48 kg, L16 size) or
b) 8 x 6V AGM batteries, 286 Ah C100, 220 Ah C10 (260x180x243 in mm, 31 kg R220 size)

a) 499 NZD per unit and b) 269 NZD per unit.

PV Panels

My advice is to reduce the total number of panels. 6 x 200 W panels produce in ideal conditions the same as 4 x 300 W panels. The former requires however more transport and installation costs (circuit breakers, cable and aluminium stands). Further, higher power panels are more efficient and therefore require less (roof) space. The specifications of the charge controller determine how many panels you can use and how many you can connect in series.

Example: a 50 A solar charge controller with input specifications of 16-112 Vdc (max. 40 A) and output of 24 Vdc (max. 50 A) with four 295 W panels (two pairs of two panels in series).

Solar panel specifications:
Voltage at maximum power (Vmp)  = 36.7 V
Current at maximum power (Imp) = 8.65 A

In this scenario, the total maximum input power equals to 2x (8.65 A x 73.4 V) = 1270 W and total max. input current is 17.3 A. No problem here.

To work out the maximum output current, divide max. input power by battery operating voltage. Here, 1270 W/24 V = 53 A. This is a rule of thumb. For instance the battery operating voltage will be between 24.4 – 28.8 V. However, this example shows that a 50 A solar charge controller could be at times inadequate for this setup. Moreover, it shows clearly that it cannot be upgraded with further panels.

This controller would, however, be suitable in this example if a 48 V battery bank was being used.

Orientation of Panels

The amount of peak sunshine hours on your site depends on the geographic location, time of the year, time of the day and obstacles such as clouds, trees or hills. Search for peak sunshine hours, solar insolation, solar irradiance to get a better idea. These are usually given in kWh per (square meter multiplied by time), MJ per (square meter multiplied by time) or when averaged over time W/m2. The time being a day for a specific month or a year.

The first two units can be converted into W/m2 (Watt per square meter). It is paramount to consider that this unit is averaged over time. The sun doesn’t shine all day though and the amount of sunlight to your site depends not only on the weather conditions but also of the time of year.

For Auckland, New Zealand I worked out an average solar insolation of about 150-170 W/m2. Conservatively speaking, my site receives 4 hours of peak sun shine on average per day during summer (+3 hours) and about 2 hours on average during winter (+3 hours). The amount in brackets is non-ideal sun shine on the panels.

From empirical knowledge, I’ve lived on my site for more than three years and have a good idea of the ‘path’ of the sun, I worked out that I can install the panels length-wise almost flat onto the roof for maximum exposure during summer. I’m referring to when the sun is ‘highest’, i.e. in zenith position.

My roof faces NE (40 deg) and has a 12 deg pitch. For the winter, maximum exposure to light will be utilized by facing them NW (300 deg) with an angle of about 30-40 degrees as opposed to being installed flat on the roof. New Zealand is on the southern-hemisphere (if you are confused why they are not facing south…).

The ideal solution for me is therefore, to allow for two sets of lengths of aluminium feet. One side of the panels being mounted to the roof flat, the other side being either flat (during summer) or tilted (during winter).

It is a misconception to think that panels are most efficient when being perpendicular to the sun. Light refraction and reflection are important. Further, what might be a perfect orientation during cloudless days will not be perfect for cloudy ones.

Stationary orientation will of course not be as efficient as one that can be altered, and it is worthwhile to look at a protractor diagram (to quantify potential efficiency losses) which takes into account both angles of the panels (one being the orientation of the roof, the other a sum of roof pitch and elevation angle).

Perhaps this would be clearer if I used azimuth and elevation (altitude) angles and perhaps I should discuss the orientation using these terms after installation.

I couldn’t find any trustworthy data for the protractor diagram for New Zealand (even NIWA publishes quite abstract data).

The protractor diagram I did find was from a thesis on PV systems and the gist I got from it is that losses are quite small, averaged over the year when the panels face NW.

If you have to chose between a winter-optimized or summer-optimized orientation, in general the former should have priority but essentially this will depend on your requirements. For instance, if it is a holiday or permanent home.

What I Plan To Buy

4 x 295 W panels
Voc = 45.5 V
Vmp = 36.7 V
Imp = 8.65 A
Dimensions in mm = 1958H x 992W x 50D
Weight in kg = 24
Price = 366 NZD per panel

Charge Controller

What I Plan To Buy

Powermaster MPPT 80 A
Dimensions in mm = 414.8 x 225 x 147
Weight in kg = 7.1
Max input current = 70 A
Max output current = 80 A
Input voltage range in Vdc = 16 – 112
Max open circuit voltage in Vdc = 140
Battery Monitor included, requires optional shunt
Price = 1099 NZD

In order to reduce ampacity and therefore save on expensive, thick conducting cables, it is sensible to increase the input voltage. This controller handles 16 – 112 Vdc under normal operation. In other words, high voltage, low current (DC) goes into controller and the output voltage is reduced to matched battery bank voltage, while the current is increased.

When connecting two or more panels in series one needs to know that if one panel is even partially shaded, input energy is drastically reduced. Shade could come from a tree or a chimney. On a yacht, there are a few things that can create partial shade. Therefore, in shade-prone areas, it might be more efficient to connect the panels in parallel and invest in thicker cables.

MPPT (Maximum Power Point Tracking) controllers are much more efficient than other types. In a nutshell, other forms of chargers require a higher input voltage than battery voltage to charge. For instance, if your battery bank is semi-charged and operating at 12.8 V and the input voltage from the panels is 12.7 V, the controller cannot continue charging the batteries.

A MPPT controller, on the other hand, modifies voltage and current continuously. In this scenario, it will increase the voltage to above 12.8 V (therefore reducing the current) and continue charging the batteries. MPPT type controllers are much more efficient especially on cloudy days and during the months with less peak sunshine hours.

I suggest purchasing a MPPT controller that can be upgraded in the future with more panels, wind or hydro power.

I downloaded the manual for the device I like to purchase but could not find any information about operating noise level. Ideally, one would want to install this unit inside but this might not be a peaceful option when living in a small cabin.

Battery Monitor

Most charge controllers have a data logger for input and output current, power, energy and voltage but to check the state of the battery bank, that is how much storable energy the batteries actually receive, how much power, energy is discharged from the batteries etc., a battery monitor with an in-line shunt needs to be installed. The Powermaster MPPT 80 A is equipped with a battery monitor but requires an optional shunt.

Price of shunt (500 A/50 mV) is about 50 NZD.

Temperature Sensor

A temperature sensor (mounted onto the battery) is a sensible option. Performance and storage capacity depends on the temperature and the controller needs to adjust for variations. Depending on where you live, the range of temperature could be large, minus zero to 40 deg Celsius.

Again, the batteries are the heart of the power system and when you pay a grand and more for a controller and a few on batteries, don’t skimp on 50 bucks for the sensor and potentially 200-400 bucks for a monitor.

Price of sensor is about 80 NZD.

Inverter

Will likely purchase a Powermaster 1200 W -24V with a surge capacity of 2400 W. If you are planning to use a generator, be it for backup or something else, you would want to buy an inverter-charger. The price of this unit is about 850 NZD.

The inverter is the component that transforms direct current (DC) to alternating current (AC). During this transformation energy ‘loss’ occurs in form of heat. In general, inverters have efficiencies of 80% and above. They reach maximum efficiency under full load. Therefore, while a 300 W inverter might be 95% efficient powering a fridge and a laptop, a 2000 W inverter powering the same appliances will be less efficient.

It is good practice to minimize AC load requirements for any renewable power system. As an example, my laptop uses 70 W power peak. Its battery requires DC (AC cannot be stored in a battery), the big thing in the middle of the cord is an AC->DC transformer (I believe the correct term is rectifier). The opposite of an inverter.

Hence, the inverter inverts the DC from my batteries into AC and there is energy loss involved. Further, the rectifier for my laptop will transform it back into DC and energy ‘loss’ is attributed to that process as well. In a nutshell, wherever possible, use DC for your appliances to save on two losses. 

You might ask why the electricity coming from your socket is of AC type while most of your appliances need DC. Maybe, Ben is confused and what he claims is not right… Well, look up the ‘War Of Currents‘ between Edison and Tesla.

The short answer being that if electricity companies provided your house with direct current, most of your cables would have the thickness of jumper cables.

Inverters come in different forms. Obviously how much power they can generate will be one factor, but also the form of AC electricity they generate. In NZL, we use 240 Vac with a frequency of 50/60Hz. This means basically that the poles of the current are alternated (switched) 50/60 times per second, respectively. Therefore, the current will have alternating maxima and minima.

Ideally, the form of the current is smooth like the shape of a sine wave. Modified sine inverters create a current form where the maxima and minima are cut off either by a square shape or a sharp angle. Some appliances cannot deal with such modified sine waves.

Worst case scenario the rectifier makes a loud noise, heats up and damages itself and/or the appliance. Good case scenario, it makes heaps of noise and creates heat, that reduces efficiency, but works otherwise without issues.

Pure sine wave inverters are not that expensive any more in comparison and I believe that it pays off to buy one of these. Especially if you are using only one inverter.

Power and Surge Power

Grid-tied electricity is so cheap (it is astonishing how cheap), although people constantly complain about continuous price increases, that most consumers would not even look at power consumption specifications of appliances they buy.

For instance, you’ll find all types of information online when buying a TV. How many HDMI inputs, resolution, screen size, viewing angle, image refreshing frequency and the list goes on, but the thing that is almost always missing is peak and average power consumption.

Strangely, it is challenging to find the peak power usage of appliances like washing machines, surge power and averaged energy for a year. I would have thought there is a page that compares price, specification and power consumption of mass produced appliances. 

It also appears that primary school mathematics is not common knowledge any more, and instead of specifying a power in its appropriate unit Watt (energy per second [J/s]), the government and appliance manufacturers use a non-standardised star or energy efficient label.

Ergo, grid-tied electricity is so cheap that detailed information of peak and average power consumption is not a priority when purchasing appliances.

It is, however, of utmost importance when using an off-grid power system!

I looked up average power consumption of appliances like washing machine, chest freezer and TV and decided that a 1200 W will be adequate for my needs. However, one needs to account for surge power as well. A washing machine may use only 600 W peak during a cycle, but may require for a brief time a much higher surge power.

It is paramount to leave your intuition behind and just read the power specification labels of the appliance you want to purchase or use. Such labels are on any electrical appliance in accordance with legal requirements.

A 2500 W vacuum can of course not be powered by a 1200 W inverter and even if you think that boiling two cups of water in a kettle can’t require much power, I can tell you that you couldn’t do that with a 1200 W inverter either using a normal kettle.

Water has a relatively very high heat capacity and the life-span of many inverters and batteries have been shortened abruptly due to using electricity to heat water.

Education is, as usual, the key to an alternative power system. Treating each electrical appliance as if they all use the same amounts of power and/or relying on your intuition is an indifferent approach to put it diplomatically.

Last but not least, inverters will also make some noise, again I couldn’t find information about that in the manual.

What I Haven’t Mentioned

Price of circuit breakers and box, fuses, cable, aluminium feet and other equipment necessary for appropriate installation. I’ll discuss this after consulting the experts and drawing a wiring diagram. Further, lead acid batteries are not necessarily the best option for an off-grid power system. They are, however, arguably the most practical and affordable solution.

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