Small-scale embedded generation!

Small-scale embedded generation!

SSEG

Small-scale embedded generation!

What is solar PV (Photovoltaics) or solar electricity?

Photovoltaics (PV) could literally be translated as light-electricity. That’s just what photovoltaic materials and devices do, they convert light energy to electricity. PV generated electricity is a renewable energy source and can be used to save on normal grid electricity consumption OR to become totally electrically independent by replacing normal grid electricity consumption entirely.

What is a Small-Scale Embedded Generator (SSEG)?

Electrical generators that can connect and operate in parallel with the grid/network (by synchronizing with the grid), are seen as embedded generators (EG). EG smaller than 1MVA (1000kVa) are defined as small scale embedded generators. In south Africa solar PV is main technology type used as an SSEG, but wind, biogas, electricity, hydro power and diesel generators connected to the grid are also forms of EG.

Why solar PV? What about hydro power, gas, biogas and other forms of SSEG?

Since most small-scale embedded generators will be solar PV, best practice and processes for solar PV have been as a baseline for other technologies. The standard and specifications currently available, as well as those being developed do not exclude other types of generation, but do not address their specific detail requirements. More comprehensive standards and specifications which addresses specific issues of non-PV SSEG are planned, and some municipalities already have they own documentation in place.

What is the difference between a Solar PV and a Solar Geyser system?

A solar PV system is made up of different components. For an Embedded Generation Installation (EGI) systems these include

  • PV modules (made from a group of PV cells), which are also called PV panels.
  • Grid tied inverter(s)
  • Wiring,
  • And mounting hardware and/or a framework for the equipment.
Figure 1

Figure 1: A basic grid-tied solar PV system

For EGI with storage, additional components could include

  • one or more storage devices, like batteries.
  • a charge regulator or controller

and a battery inverter when alternating current (AC) rather than direct current (DC) is required

Figure 2

How does a grid-connected solar PV system work?

Here is a basic overview on how grid connected solar power systems work.

  1. During the day when the sun is shining onto your solar panels/modules is converted into DC electricity (the semi-conductor material in the PV module performs this conversion).
  2. This DC electricity is conducted to a solar inverter which converts this DC electricity into AC electricity.
  3. The solar inverter then feeds this AC electricity into your house’s switchboard.
  4. This AC electricity is then used to power your house.
  5. Any power being produced that is surplus to your house requirements/load will be fed back to the local grid (reverse feed-in).
  6. If there is a power outage from the utility, the EG will switch off and stop operating until the grid is switched back on (this is also a safety requirement discussed under Safety and standards for SSEG installations).
Figure 3

Why is there a recent increase in the number of solar PV installations in South Africa?

With the increase of utility electricity prices and the decrease in price for solar PV energy, it is becoming financially viable for citizens to invest in solar PV solutions that will save them money and serve as an investment in the long run (a PV system typically has a life of over 20 years).

There are also cases where some users install solar PV to meet all their electricity needs and go completely off the grid due to load shedding, bad power quality, or grid power price escalation trends, or just because they want to become grid independent.

What is the price trajectory for the SSEG components? What are the typical system costs and which trends are these costs following?

The cost of solar PV is on the decline and research has shown that the price is likely to decrease more over time; see figures below.

Figure 4

In the year 2018 the typical turnkey cost in South Africa (all equipment and labour costs included) per kWp installed is between R16/watt to R24/watt for residential. For commercial the cost can be R13/watt to R21/Watt.

For example, if you want to install a 2kWp solar system for your house at R20/watt, the total cost will be R40 000 to realize the installation.

The reason for the big price range has to do with the type and difficulty of installation, the quality of products used and economies of scale.

What is the voltage of a single panel?

A common PV panel voltage is high enough to charge a 12V battery, rated 18V DC. However, the voltage of one solar panel is based on the number of solar cells connected in series, hence the voltage output can be anything from 0.6 to 50V DC and in some cases can be even more.

How much space will a solar photovoltaic system require?

Depending on the module efficiency, the rule of thumb is 1.8m² for each standard 250Wp module installed flat on the roof’s surface, and 2.6m² for 250Wp modules installed at a tilted angle different to the roof surface and therefore requiring spacing in-between the rows to avoid one module shading another (Also known as inter module or mutual shading).

For example, if you want to install 2kWp of solar PV on your roof at a tilt angle, you need 8 Modules of 250W x 2.6m² = 20.8m² of space.

It is important not to install on roof areas with regular shade, as these areas will limit the overall performance of the solar system.

What is the difference between kW and kWh?

The solar PV system power output, kW, is an instantaneous measured value while kWh, or energy, is kW measured over time (generating 1 kW for 1 hour produces 1 kWh). A solar PV system performance is not determined by the maximum kW output measured, but rather how much electric energy, kWh, is produced over time. The financial savings in most cases is quantified by the amount of kWh produced from the solar system, also known as Levelized cost of Energy (LCOE).

How long should a solar PV system and its components last?

A PV system that is designed, installed, and maintained well will operate for more than 20-25 years. The PV modules has no moving parts and can last more than 30 years. Good quality and well installed inverters, on average can last 10 years or more. The best way to ensure and extend the life and effectiveness of your PV system, is by having it installed and maintained properly. Experience has shown that most problems occur because of poor system installation and/or using substandard/bad quality components.

What typical guarantees and/or warranties can one expect for solar PV components?

Modules/Panels

10-15 Year product warranty

25-30 Year performance warranty

Inverters

2-5 Year product warranty

Warranty can be extended up to 25 years at an additional cost

Mounting structure

5-10 Year warranty

Batteries

2–15-year warranty

NOTE: These are only estimates to give an idea. Number of years are different from each supplier and can increase/decrease over time.

When should batteries be installed?

Grid tied (EG) systems do not need batteries to operate and are seldom installed in such systems.  However, if the user wants to store excess energy and/or have alternative energy when the grid is down then batteries with a charger and battery inverter will be needed. If the user wants to be independent of the grid – i.e., to install a stand-alone PV system – then batteries are also required.

Why is PV rated in kWp and transformers/generators rated in kVA?

The kilowatt peak (kWp) is the maximum/peak power output you will obtain from a specific solar panel at Standard Test Conditions (STC). That means a 250Wp solar panel will give 250W output if the irradiance (sunlight) is 1000W/m², the cell temperature (the panel itself) is 25 degrees Celsius, and the angle of sunlight is almost 45 degrees (Air Mass (AM) of 1.5).

The ratio between the VA (i.e., rms volts times rms amps) and Watts is called the power factor (PF). In other words, VA x power factor = watts. Similarly, kVA x PF = kW, or kilovolt-amps times power factor equals kilowatts. Transformers have a kVA rating but will not supply the equivalent kW (active power) due to a power factor loss, hence the kW rating of the transformer is less than the kVA rating. The kVA and kW size of solar grid tied inverters are normally equal, because the power factor is Unity (one).

Can solar PV systems be dangerous?

Yes, electrical shock, arcing and fires are all possible with PV systems and can even be fatal.

What design errors can damage a solar PV system?

The following design errors should be avoided:

  • A solar PV system voltage output which is higher than the module/inverter/charger/other equipment voltage/current rating
  • Incorrect cable type or cable size too small
  • Improper connections or connection devices; especially on the DC side
  • Improper earthing of solar PV system
  • Having continuous shading on one part of a PV array
  • Improper weight loading calculations when installing on the roof

For systems with batteries

  • Having a too small solar PV generator to charge the batteries fully
  • Incorrect battery charging settings
  • Operating the batteries in a poorly ventilated area
  • Operating batteries in very hot conditions
  • Improper or bad connections

Written by,
Reinhard Groenewald