Off The Grid

Reducing Greenhouse Gas Emissions

I understand that, in Australia, households, schools and offices consume as much as 30% of generated electrical power. If we could convert one-half of this load to use off-the-grid solar power, then we would easily satisfy our international commitments for reduction of carbon dioxide emissions.

Our first step towards achieving this result is to carefully improve our energy efficiency. This is the subject of another paper. Here, I will concentrate on the method by which domestic consumers may use zero-carbon solar power to become independent of the electricity grid.

Is It Cost-Effective?

At current pricings, consumers can convert to off-the-grid solar power, at no nett cost. In my case, I spend $2,600 annually for electricity supply. For the same outlay, I could finance up to $10,000 worth of power generating equipment on a 5 year lease.  Importantly, I would be unaffected by inflation.

Since the operational life of the solar panel system is perhaps as much as 25 years, upon completion of the lease, the costs of routine maintenance and the re-cycling of storage batteries would be of the order of $1,000 per annum, giving me a nett saving of $1,600 p.a.

In 1967, I built a house in rural South Australia. It was about 300m from the highway, I was quoted a high price for the installation of power poles, and the standard rate for a rural electricity supply was about twice the town rate. So, I designed the house to be energy efficient and carried out a cost-benefit analysis for off-the-grid power. I used bottled gas for cooking, oil for heating, and a diesel-powered generator for electrical appliances and lighting. I saved money. Two years later, when the diesel generator was in need of overhaul, the Electricity Trust decided to re-zone the area as urban, with a very substantial reduction in installation charges and tariffs, so we changed to the public supply. If we had the present day availability and pricing of solar power equipment, connection to the grid would not have been a viable option.

What About The Night Hours?

As explained later in this paper, for the non-industrial consumer, there need be no loss of power during the hours of darkness. There would be no “power cuts” resulting from failure of the public power supply. There would be no need for emergency lighting systems. There would be no need for UPS equipment for computer or telephone systems. For new buildings, a part of the construction costs would be saved by the use of solar panels as roof cladding, and by removing the cost of connection to the public electricity supply.

Should I Feed Power Back To The Grid?

Most solar power systems, which are currently offered, cut costs by providing the minimum of equipment. During the daylight hours, the solar panels feed excess power back to the supply grid through a grid-tie inverter. At night-time, all power load is drawn from the grid.  Generally, water storage heaters are connected through a time switch, to benefit from a reduced night-time supply tariff.

A rebate for the excess power which is fed back to the grid during daylight hours is credited to the electricity supply bill. In practice, the saving is not substantial, because the rate for power credited is much less than that for power charged. Even using off-peak tariffs, such as for storage water heaters, the client loses.

Not generally understood, is that such a system does not function during a power cut. There are no lights, no house telephone, no TV, no computer!  I would also be concerned about possible failure of a grid-tied power inverter, as this can result in a house fire. The inverter is vulnerable to damage from mains supply voltage surges.

In a more complex system, storage batteries and a solar power regulator are added, so that excess daytime energy may be stored for night time use.  In order to minimise battery capacity requrements, the daily timetable should be organised to reduce power consumption after dark. Storage water heaters should be switched on during the day. If a mains power supply is available, it should not be connected to the solar power system, but of course could be used to charge batteries or to provide emergency power during periods of equipment unserviceability.     

Several brands of enclosed, silenced diesel mains power generators are available at a delivered price of under $1,500.  Such an emergency generator would provide reliable back-up power for many years, with occasional use, and so with little maintenance.

Community Centres, Supermarkets, Schools & Hospitals

Community Centres and other public buildings usually have large roof areas, and can mount large arrays of solar panels, producing large amounts of power.

For such large facilities, in addition to the use of storage batteries, other methods of 24 hour power generation may be considered, e.g. Rankine-cycle solar thermal, catalytic hydrogen gas production by electrolysis with storage gasholders and ceramic fuel cells, wind turbines, etc.

Excess energy may be used to produce potable distilled water from saline or waste water, using the low-footprint energy-efficient Passarell process.

Australian Manufacture

The increased demand for solar power equipment in Australia should support a viable manufacturing industry. At present, we are unnecessarily faced with the high freight cost of imported products.

We could grow our own silicon crystals and fabricate our own PV cells. The cost of labour is a small part of total cost.  

The mechanical parts and other assembly components for solar panels could be manufactured by the automotive components industry.

Inverters, regulators and control equipment could be manufactured by the telecommunications industry.

Deep-cycle sealed lead-acid batteries are the most cost-effective energy storage devices for fixed installations, and offer many advantages such as local availability of raw materials, existing manufacturers, and established recycling facilities.

System Design Notes

Battery capacity should be sufficient to supply energy demand for at least 24 hours, the longer the better!  I would suggest installing a 24V system, with at least four 12V 100AH batteries, providing a 24V 200AH capacity.  However, the capacity of the battery bank must be determined by the anticipated power consumption.

To get the best service from the batteries, deep discharge should be avoided whenever possible.

The most suitable batteries are of the SLA (sealed lead acid) type, which are specially designed for off-grid power systems, are fully sealed, and require no maintenance. 12V 100AH SLA batteries are available for around $250 each.

Lithium batteries are less readily available, are at least twice as expensive, and may need the addition of an equally costly ultra-capacitor bank to supply peak current demand. Furthermore, most of the available accessory devices, such as regulators, battery chargers and inverters, are designed to operate with lead acid batteries and are not directly suitable for lithium battery systems.

Choose your appliances carefully to minimise night time power demand, e.g. use LED lighting, low power desktop, laptop or tablet computers, LCD TV, evaporative coolers or fans. Make sure that any appliances, such as storage water heaters which are controlled by time switches, are set for daytime use.

Consider setting-up a separate lower power 12.6V DC solar system and wiring circuits to supply lighting, TV sets, computers and telephone equipment. In that case, the main power inverter need only be active when power appliances are switched on. Note that refrigerators and freezers should not be connected to light-up when the door is opened!

The batteries must be capable of supplying high current, e.g. 200A  for a 4.5kW load in a 24V system. The links between batteries and the inverter must be adequately sized to avoid power loss and heating (extruded aluminium flat strip makes excellent high-current battery links.)  If you opt to install batteries with inadequate peak current rating, then you may need to install a bank of ultra-capacitors to handle surge current.

Be sure to include an adequately rated DC Circuit Breaker and Isolator Switch at the battery terminals.  (These are available from Marine Suppliers). Connections between the individual solar panels and the battery bank are less affected by voltage drop and may be made with standard 10A building cable.

Solar panels are now available for around $250 each for a 24V 250W panel. The power rating is actually a peak figure at an optimum output voltage - greater than the actual operating voltage.   So, for example, in full sunlight a 250W 24V panel will charge the battery to 28V at perhaps 7.5A to 8A.  This charging rate is reduced as the azimuth and elevation angle of the sun changes. Unless space is limited, it is not economically practicable to provide a tracking mount for a solar panel array.  It is less expensive and safer to install more panels.

Solar water heaters are extraordinarily expensive. It makes more sense to dump excess power into a standard storage water heater, whenever the battery system becomes fully charged. You  need a controller which will close a Solid State Reay (SSR), feeding mains power to the water heater, when battery voltage reaches, say, 27.5V.  The SSR should open when the battery voltage falls to, say, 25.2V  If, of course, the water has reached full temperature and the thermostat has opened, then the water heater will draw no power, and in that case the battery voltage will rise to the float voltage setting of the Solar Panel Regulator (typically 28V) and the solar panels will be disabled to prevent overcharging.

Solar Panel Regulators work by short-circuiting the panels. They include reverse polarity protection diodes, which prevent battery discharge into the Regulator. Each Solar Panel Regulator may handle a number of panels, up to its total  current rating. The Regulator must be mounted with the cooling fins vertical and with unobstructed air circulation. It is probably a good idea to provide a Regulator for each panel connection, i.e. each single panel if using 24V panels, or each pair of series-conected 12V panels. If you need to connect panels in series to provide the correct charging voltage, e.g. 2 x 12V panels to connect to a 24V battery, then the panels must be identical - you must not mix brands or models! Also, use the appropriate voltage panels for the battery system - on a 24V battery system, 12V panels don't work and 48V panels only deliver half-power.

Modern inverters generate a pure sine waveform, which replicates the standard mains power supply. They are suitable for any application within their power rating. Choose a power rating which is adequate for your peak load - remember that motor-driven appliances such as refrigerators and airconditioners have a high starting current. For general applications, I would suggest a continuous power rating of 5kW. A 24V inverter with rated output of 9kW and peak rating of 18kW is available for around $750.  Other inverters with a lower power rating cost around $500.

Safety

Be very careful when connecting up an off-the-grid power system. You must include a high current DC circuit breaker and a high current DC switch in the battery circuit. Wear gloves and a faceshield to avoid injury. Cover the solar panels with a tarpaulin during installation, until you're ready for power-on. Panels may be damaged if they are exposed to sunlight without a load. Be aware that the 240V AC output of the inverter is just as lethal as the normal houshold mains supply, and requires the services of a qualified electrician.

C. M. Pearson B.Sc., C.Eng.
11/11/2013