Power Backup, Power Hybrid And Grid Tie Systems

Introduction

The concept of a (1) normal power back-up system differs somewhat from a (2) grid interfacing power back-up and we refer to the latter as a Hybrid power backup system, or a Hybrid solar grid system.

Furthermore, there is a third option for Photovoltaic systems, referred to as a (3) Grid Tie system, in which there is NO Backup power available. It is purely a system which generates and supplies power when the Sun is up and photovoltaic energy harvesting is possible, and is only really suited for reducing electricity bills.

Very briefly, the differences are as follows:

1. Power Backup – This system usually employs a normal inverter or UPS type inverter. Whenever the main grid power (Eskom) is available, it will feed the load via Eskom power. At this time the battery (energy storage) is being charged by either Eskom and/or via Solar panels. This system does not have the intelligence to give preference to whatever alternative energy is available at that time. Thus it will only feed-in a solar supply, when Eskom falls away.
   Please note that it is also possible to build up a small system on which Eskom, or the panels only, charge the batteries, and the load is actually always connected through the inverter, and also works from the inverter- however, this is not the system discussed here. These are the most common systems employed, as they usually utilize a "lower cost" Inverter/Home Ups/UPS, or inverter with an automatic transfer switch.
   An Inverter with no-load power, common on most of these inverters, is usually a problem and not advisable for use with the systems mentioned below
   
   
2. Hybrid power backup system – these systems are more intelligent. Here a more expensive type inverter system is generally used - such as those supplied by Victron or Outback, etc. The concept here is as follows:
   
   
   1. System is a Hybrid, where input energy sources are usually via a Solar Panel array and Eskom, Solar Panels and a Generator, or even Solar Panel, Wind Generator and Eskom, as examples.
   2. The System is designed to offer a backup facility for when Eskom would not be available
   3. The backup period and autonomy is therefore determined by the size of the battery pack
   4. When the "safe"(full state) storage level of the battery is reached, all additional energy from the alternative energy source (Solar Panels) is then used as the main energy source to the load. However, when the load's requirements exceed what is available from the alternative energy source, only then is the additional power required, drawn from Eskom
   5. This kind of systems can successfully be used for a system from as small as 750 Watt, up to practically around 18KW. (bigger systems are possible, but for a household this is typically a maximum)
   6. The ratio of Solar Panel array versus Inverter size, is determined by optimal appliance draws and, for battery bank sizing, the storage time required. For instance, it could be logical to have a 9KW inverter, with only 1kW (6kW.H) of solar panels and 12 KW worth of batteries.
   7. The Inverter size is determined by the optimal load employed
   8. Single, and/or 3 phase systems, are possible
   9. Battery backup autonomy can vary from 3 hours backup-up, right up to 72 hours.
      
      
3. Grid Tie system – these systems are very intelligent, usually bigger systems, and a Battery backup does not form part of the greater system.
   
   
   1. These systems are usually not practical for homes/dwelling but more for offices - which have only day-time occupancy.
   2. These systems are Grid-tied in such a way that whenever photovoltaic power is available, it will use that power as first priority, but when no photovoltaic power is available, it is then drawn from Eskom.
   3. The Solar Array on these systems is usually linked to the rating of the inverter itself. E.G. a 3KW inverter will have a 3kW solar array. Power from the alternative source is thus only available when the sun is shining, or with certain wind turbine systems, when the wind is blowing.
   4. Inverters used here are typically SMA, Victron and for Big systems of 100KW upwards - Hitachi/ Siemens.
   5. It would not make sense to have such a system with a 3kW panel array when the average power usage for instance is only 1KW per hour, as the additional energy is then simply not used and thus wasted (unless it can be fed back, or sold back, to the Grid). On bigger system however, although not legislated in SA yet (but in progress) , that additional power can be sold back to the grid. (In contrast, the Hybrid power backup system will store the extra energy in the battery bank for use at a later stage).

The Understanding of these differences between the various systems is very important in order to understand the respective cost implication associated with each system.

Suggested KITS for systems

Stand Alone Photovoltaic systems (No Grid Power)

These systems kits are for dwellings from small Rural to Urban homes, depends on system requirements – 40 Square Meter to 600 Square meter houses with no Grid supply

Power Backup (With Grid Power)

These systems, are for dwellings and offices in which the main purpose is backup power, Alternative energy is an optional input. Eskom grid is available and backup is given to critical components or all

Hybrid power backup system

These are the first of the so-called Grid-Tie systems, but with a backup component present. It is not necessary to use all the alternative power that might be available, as the energy gets stored and usually a level of autonomy is already built into these system. Such a system could supply back up from 3 hours, to 72 Hours. Such options are for small rural dwelling of 40 M^2, up to 600M^2 such as for use in Rural houses, Urban houses, Offices etc. The purpose here is to allow for expansion at a later stage and even to a full hybrid operation (when possible). More advanced systems can "sell" back to Grid although - it is not legal in SA (yet?!).

Grid Tie solar grid system

These KITS do not offer backup power options. The concept here is to install a certain rated Grid Tie inverter, with matching rated Photovoltaic energy; i.e. 3KW inverter and 3KV Panel array's. The load should typically be greater than the Panel array, so that whatever is supplied by the Photovoltaic array is directly consumed into the load (any excess could be sold back to the Grid) . Systems such as these are ideal for Office complexes or dwellings, Mines, Hospitals, or wherever occupancy only occurs during the day. The savings represented below are directly calculated on a Municipality rate of 75c per kWH and exclude any other rebates and Carbon Credits which can help to amortise the cost of such systems.

© Dr Gawie van der Merwe - Plan My Power (PTY) LTD