SOLAR AND WIND POWER

FREQUENTLY ASKED QUESTIONS
Advice, Tips, Suggestions!

Here at Bright Green Energy we get asked lots of questions about all sorts of solar, wind and battery power systems. Whilst it is hard to keep on top of the variation of questions we get asked, we have captured some of the most commonly asked questions and our answers and listed them here for your benefit. NOTE: this information is provided as a guide and is not intended to be used to design or implement any system. Always seek expert advice before undertaking any project.

Battery Charger Settings and Batteries

If you plan on using a battery charger with GEL or AGM batteries then you do have to be careful to ensure that the battery charger is able to work with your chosen battery. If you connect a GEL battery to a charger that has the incorrect charge settings then your GEL battery will be damaged. A good charger normally has manually adjustable pots that can be set to the correct battery type to ensure correct charging. Of course, if you don’t know what you are doing when adjusting the pots then just get the battery charger manufacturer to preset the settings for you. It will save you a lot of money in the long run.

Maintenance of stand-alone power systems

Maintenance of remote power systems is crucial and it cannot be overlooked. The key issue is reliability and as such systems are, generally, located in remote and hard to access areas it follows that technical support can prove to be expensive. An often overlooked factor with remote power systems is the provision and quality of training that accompanies the system. Remote power system that combine wind turbines and solar PV are not just consumer items, at least not yet, that take “one size fits all” approach. Insufficient education and training does lead to lots of technical support calls and unnecessary time wasting. If you’re going to supply a customised system then ensure that the client is properly trained in the use and support of the system.

Grid Connected PV Systems

As interest in the use of solar PV increases we are constantly surprised by the lack of information available as to how pv can function within a building.

Of course there are many considerations to take into account, but these can be reduced to the following:

1. Architectural - solar pv can be used for generating electricity and can be fitted to walls, roofs, act as windows, skylights, facades and even shades. 
2. Load Management - solar pv can be used to offset peak loads and depending on time of year this can have a significant impact.
3. Energy Control Management - solar pv can be used to drive all sorts of devices, pumps, fans etc
4. Hybrid Energy Systems - providing emergency power supplies, lighting, pumps etc.

Whatever your function it makes sense to take a modular approach to the design and implementation of the array. Modular means simple to design and implement, allows for redundancy of the array where shading may occur, details array aspect and orientation and elevation and positioning.

Hybrid System Design Considerations

Batteries for use in remote systems must be of the deep cycle type. Most suppliers offer “solar” batteries and these should be used wherever possible.
Don’t forget that a function of the “days of storage” of batteries is the relationship between array size and load and this is known as loss-of-load probability. Battery storage gives availability of energy in relation to the array size. Of course this varies by the season of installation (summer or winter installations). This means that certain factors come into play on the solar array:

• the array size and tilt angle must be considered together before the installation takes place.
• increasing the tilt angle gives a more uniform output for all year round generation.
• the array generation should not be too far below that of the load. After all you don’t want the generator to be running all the time.
• system voltage is important. Opt for 24v or 48v system and choose inverters to suit.

Wind Turbine Control

All wind turbines have some means of controlling the speed of the rotor in high wind conditions. Most, if not all small wind turbines, except for a couple of rare exceptions, furl, or fold about a hinged joint that move the rotor towards the tail vane. Some turbines furl the rotor vertically whilst others furl the rotor horizontally towards the tail. Some designs furl by pitching the rotor blades and others furl by pitching the blades and furling the rotor. For small wind turbines to furl properly the axis of the rotor is offset from the furling axis. You can see this on many of the models available on the market today. If the wind turbine is going to be sited in windy location then a furling wind turbine would be advisable.

Micro Turbines

These superb range of turbines are suitable for almost every imaginable application including, but not limited to: off-grid battery charging systems; motor home and RV power; marine (boat) power; remote CCTV; telecom base relay stations; roadside lighting and a host of other applications.

So how do you choose a wind turbine suitable for your purposes?

You may think that the power output of a wind turbine is all you need to know, but you must not overlook the wind speed at which the turbine is rated. There is a huge difference between power at 9 m/s (20mph) and power at 12 m/s (27mph) wind speeds. It is easier to understand therefore that in wind speeds of less than 12 m/s, a small 150 watt turbine could produce as much power as one rated at 300 watts.

Wind Power Fundamentals

The amount of wind power at your site will determine the amount of power you can expect from a wind turbine.

How do you work you the power generated from a wind turbine? It is quite straightforward really!

The amount of power (p) in the wind is a function of air density (p), the area of the turbine blades intercepting the wind (A), and the instantaneous wind velocity (V), or wind speed. An increase in any of these factors will increase the power available from the wind.

Air density is affected by temperature and height. Warm air is less dense than cold air therefore a wind turbine will produce less power in the summer than winter, when it is colder, for winds of the same speed.

Depending on location relative to standard conditions at sea level, power can reduce by up to twenty (20) percent, and maybe more.

Power in the wind varies with the cube of the wind speed. Double the wind speed and you increase the power by eight (8) times.

The SWEPT AREA of a wind turbine directly affects power output. If you double the area you effectively double the power available.

Wind Turbine Height

Wind speeds do typically increase with height. This is very important. If the turbine is mounted in an area of rough terrain then height is important and the higher the better. Height equals stronger winds and less turbulence.

Estimating Performance

So, how much power does a wind turbine generate? A good question? Simple, but deceptive. If you’re into number crunching then it shouldn’t be a problem. On the other hand, if your working out the detail for your battery charging system then results can be difficult. Since there is no international standard for agreement on performance we must feel our way along.

Given that there are two different types of wind turbine uses: grid connected and off-grid (battery charging) systems we can work out what the norm should be for both types of wind turbine system. With a battery charging wind turbine system, and when the batteries become fully charged, the wind turbine must be able to dump, excess energy available. Some manufacturers offer dump loads with others offering special controls and circuits to get rid of the excess energy generated. In truth most small wind turbines simply disperse the left over energy.

Connecting a Grid-tie solar array - PV module configuration

As discussed in an earlier topic solar modules can be connected together in series, parallel or a combination of both. When the modules are connected in series the output current remains that of the individual modules and the output voltage is the sum of the voltages of the individual modules. When the solar modules are connected in parallel, the overall output voltage remains constant and the output current is the sum of the individual modules.

What connection configuration should I choose - Series or Parallel Connection?

In a grid-tied system solar modules are usually connected in series strings. Why?

• 1. the higher voltage allows the use of smaller cables (cross-sectional area , CSA) is reduced;
• 2. the installation is less complex thus making the install simple and quick;
• 3. there is little or no shading of the modules.

You should connect the solar modules in parallel if the modules are would be shaded or of different sizes (wattages) mixed together. These latter points are very important as you do not want the voltage levels to vary too much as damage to the inverter will result.

Battery Sizing - A Quick Check

Let’s assume you’ve done your homework and you have calculated that for your 24v remote power you will need 2400Ah of battery capacity to meet your load requirements of 10kwh.

Your preference is to choose 24 x 100Ah AGM, 12V batteries. Simple enough!

To meet the 24v system voltage requirement you will have to connect 2 x 100Ah batteries in series (2 x12v to give 24v). Each set of two batteries would then be connected in parallel so that the overall system voltage of 24v remains constant. You end up with a series/parallel battery configuration, perhaps 12 rows x 2 batteries.

Satisfied you do a check to ensure that your battery capacity is still 24 x 100Ah = 2400Ah.

But is that correct?

Remember, batteries connected in series voltages add and capacity is constant. Batteries connected in parallel capacity adds and voltage is constant. Thus, our series/parallel combination results in a total of 1200Ah of storage capacity, which is not what we originally envisaged.

Charging an AGM Battery

We’ve had quite a few questions recently about the correct settings for charging AGM batteries.

Q: When charging an AGM battery do I charge it at the setting for Lead Acid or GEL?
A: You need to set the charger to lead acid, assuming the charger does not have an automatic equalization setting, which will boil the battery.
This is not good for either Gel or AGM.

Solar Cells and Temperature

The efficiency of solar cells decrease with a corresponding increase in temperature. Crystalline cells are sensitive to heat and for every one degree Celsius increase in cell temperature the output decreases by approx 0.5%. Amorphous cells output decreases by approx 0.2% for every one degree Celsius increase. During the summer months, solar panel temperatures can go as high as 70 degrees C. If solar panels are to be used in in very hot climates then care must be taken to ensure that the correct panel is chosen.

Understanding Solar Cells

What is PEAK POWER?

The electrical output of a solar cell is directly proportional to the amount of solar radiation falling on it. On fine, clear days with lots of sunshine output of the solar cell is at its highest. In conditions of diffused solar radiation (cloudy, overcast weather) the output of the cells is much lower.

The maximum power that a solar cell produces is described as its peak power (Wp). This electrical output in watts is achieved under Standard Test Conditions (STC) - 1000W/m2 solar isolation at a cell temperature of 25 C, and an Air Mass of 1.5. Air Mass is a measure of the the thickness of the atmosphere that influences the composition of sunlight falling on the earth. Sorry about that!

Solar cell output is directly proportional to the cell surface area. The large the surface area the greater the output.

What is Short Circuit Current?

The electrical properties of a solar cell are voltage and current. The current produced by a cell depends on the quality and amount of light falling on the cell as well as the size (surface area) of the cell and the voltage at which it operates.

The relationship between voltage and current in a cell is described by it I-V curve. The cell produces its maximum current when it is short circuited, its short circuit current, denoted by Isc. When the cell is in open circuit - no sunlight falling on it - it produces what is called its open circuit voltage, denoted by Voc.

Under stable solar radiation, the current produced by the cell us determined by the operating voltage. The Maximum Power Point (MPP) of the cell is when the the current and voltage produce the maximum power. This can be expressed as a relationship with the formula Power = Volts x Amps or P=V x I

The current at MPP is denoted by Impp and the Voltage Vmpp.

Daily Load Requirements

With any PV System you must determine the Average Daily Load (ADL)

Identify all loads to be connected to the PV array

For each and every load, determine voltage (V), current (I), power (Watts) and operating Hours (h). Some loads will vary on a daily, weekly or monthly basis and this must be taken into account when calculating the daily averages.

Separate the AC loads from DC loads
Determine the average daily load in Ah for each load from the Watts (W) and Operating Hours (h). If the operating hours vary from day to day during the course a week, then daily average operating hours over the week should be calculated. The same applies if the operating hours vary from week to week or month to month.

Add up the Wh for all AC Loads - they should all operate at the same voltage. For AC loads, the DC input current to the inverter must be determined.

Add up the Wh for DC loads - they should all operate at the same voltage

Work out the Ah for the AC and DC loads.

Add the Ah for the AC loads to the Ah for the DC loads and divide by the cable efficiency factor to obtain corrected daily average Ah for the total loads. In a battery storage system you will need to include battery efficiency factor.

The total AC power will determine the size of the inverter.

Cable sizes are determined from individual loads and the total load current will need to be compared to the total array current to ensure correct wire is used.

PV System Design Suggestions

• Keep it simple!
• Be realistic as no system will be 100% efficient.
• Be careful and realistic when estimating loads. Note: the bigger the safety factor the more money it will cost you.
• Know your weather - getting solar estimation wrong will cost you in performance.
• Understand your hardware and costing.  Talk to people and ask the questions you don’t understand. Never buy anything without knowing what it does.
• Install the system properly.  Never use poor quality goods.  Solar arrays are designed to last 25-30 years so do the job properly.
• Plan for maintenance - a bit of TLC goes a long way.
• Stay safe - always comply with local and national building codes electrical codes.  Never take short cuts with electricity.

Simple advice, but you’ll be surprised how often it is forgotten.

Grid Tie Inverters - Part 1

Depending on the size of the pv array there are different inverter configurations.  They are broadly separated into inverters for small single string installations; central inverters that serve a single installation - more about this later on and inverters for single and multiple strings of modules. Small arrays are usually connected to a single inverter. If the array if affected by shading,  is very large, orientation varies - solar panels are mounted in portrait and landscape mode, or the installation varies in height then several inverters may be used. See our website www.brightgreenenergy.co.uk for more information

A grid connected inverter should function in a number of ways:

• it converts DC electricity into AC and feeds it into the electricity grid network;
• it maximises the output of the solar array. This is know as Maximum Power Point Tracking (MPPT) - more on this in
• it automatically disconnects itself from the grid in the event of a grid failure, or voltage and frequency variation.

What is Inverter Islanding?

This is better understood as a function of the electricity grid.  If the grid fails or is turned off the inverter will automatically disconnect itself from the grid.  If the grid fails and the inverter remains active attempting to feed electricity into the grid - it can be dangerous - This is known as  Naturally, this must be prevented from happening for obvious reasons.  Where there are multiple arrays, spread across buildings within the same area, for example, this could be a problem.  Inverter electronics are very sophisticated and operate within very fine tolerances.  Exceeding this tolerance ensures that the inverter will cease to operate.

Grid Tie Inverters

Grid connect inverters convert the DC produced by solar array into AC required by the electricity grid.  The inverter can convert into single-phase or three-phase AC at the frequency and voltage required by the grid. Inverters for use in the UK are referred to as G83 compliant.

There are few variations of grid connect inverters and connections required for arrays.

Studer Inverters

Q: Does the Studer AJ inverter range support positive earth?

A: Yes.

Components of Grid Connected Solar PV System

If you are planning to install a solar photovoltaic (PV) system to a home that is already connected to the electricity grid comprises at least seven essential components. 

These are:

• Solar PV array
• Solar panel mounting frames (various options available)
• Array combiner or junction box
• Grid connect inverter
• Import / export meter - provided by your energy supplier at a nominal cost
• Grid connection point - incoming distribution board or similar
• Loads - appliances that consume energy

The DC energy produced by the solar pv array if fed via cables into the array conjunction/combiner box where the array is connected together in the appropriate way before being fed by cables into the grid tie inverter. The inverter converts the DC produced by the solar panels into AC which is either consumed by the building appliances or fed directly into the electricity grid.  If the solar array is not able to supply enough electricity to power the appliances then the electricity is supplied by the grid.  The role of the export meter is is purely to monitor the amount of electricity that the system supplies to the grid.

Off Grid Energy Systems

We are constantly being asked questions about the design and implementation of an off-grid, stand alone or remote power system. There are basically three types of system:

• solar photovoltaic (PV);
• wind power;
• PV hybrid system - which also have another source of power (diesel, bio-fuel ,wind or hydro-generator).

What they all have in common is that they are autonomous systems that are not connected to the electricity grid.  Depending on energy requirements the systems will vary in size and output.

In a typical stand-alone PV system the DC current produced by the solar panel is used to charge batteries via a charge controller.  If DC lights are used they are usually connected directly to the charge controller. If AC mains voltages are to be used, this is done via an inverter connected directly to the batteries.  Inverters used in off-grid systems and inverters used in grid-connected systems, although they perform the same function converting DC electricity to AC electricity, are very different and are not interchangeable.

Of course, there are some situations where the DC output of the solar panel can be used directly without the need for batteries. A good example of such an application is water pumping.  Whatever the application it is absolutely vital that the energy produced by the off-grid system is sufficient to cover the energy requirements of the application's load.  System voltages are usually 12v or 24v and occasionally 48v. Stand alone PV systems are typically used as follows:

• security - CCTV, alarms, lighting, street lighting and communications 
• water pumping - livestock watering
• residential - lighting, street lighting, radio, TV computers, small refrigerators 

All stand-alone systems need to be managed as there are limitations of such systems.  Energy consumption and state of charge (SOC) of the battery being two important considerations.  A well designed, installed and managed system should be able to meet the demands of the application for which it is designed.