Photovoltaic Grid-Interactive System

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Related Pages:

GI system Diary
System Performance Data

Household Electrical Wiring Diagram & Notes
Household Power Summary

 

Introduction

Photovoltaic Grid-Interactive systems use the light available from the sun to generate electricity and feed this into the main electricity grid. These systems differ from a 'Stand Alone' or 'Remote Area Power Supply (RAPS) system in that there are no batteries to store the energy produced. The houses are connected to the 240V electricity mains and use the same appliances as found in other houses. If at a particular moment in time more power is being produced than is required in the house, the extra power is sent back onto the grid to be used by neighbouring households. At night or when there is insufficient power being produced to supply the households needs, electricity is drawn from the grid in the same manner other households do.

Carport Roof mounted panels

 

How they work

The main components in a system are the Photovoltaic (PV) panels, Grid Interactive inverter(s), and the meter that records the amount of power produced.

Simplified System Block Diagram
The blue arrows indicate the possible paths for power that the system produces, while the red arrows indicate power that is consumed from the main electricity grid.

 

PV Panels: The PV panels convert the light reaching them into DC power. The amount of power they produce is roughly proportional to the intensity and the angle of the light reaching them. They are therefore positioned to take maximum advantage of available sunlight within siting constraints. Maximum power is obtained when the panels are able to 'track' the sun's movements during the day and the various seasons. However, these tracking mechanisms tend to add a fair bit to the cost of the system, so a lot of people either have fixed panels or compromise by incorporating some limited manual adjustments, which take into account the different 'elevations' of the sun at various times of the year. The best elevations vary with the latitude of your location. In the Southern Hemisphere, these panels should face as close to True North as possible.

Grid-Interactive Inverter: The Inverter takes the DC power produced by the panels, converts it to AC, and feeds it back into the main electricity grid. Some systems may have a number of smaller Inverters connected in parallel.

Meter: The Meter records the amount of power produced by the system. Note that in some installations, a single meter is used - It goes backwards when power is being generated, and forwards when power is being consumed. There are several different metering configurations available, each with their pros and cons. Ultimately, it's up to the local electricity authority as to which configuration they'll approve. The above diagram represents my configuration of separate generating and consuming meters.

 

Overview of my System

PV Panels

I have eighteen 75 Watt BP Photovoltaic panels which produce a nominal peak generating capacity of 1350 Watts.

House Roof

Nine of these panels are fixed to the house roof, while the other nine are mounted on adjustable frames on the carport roof. Each panel within these groups of 9 are connected in series. The two groups are then connected in parallel.

Carport Roof

On the carport roof there are 3 frames each holding 3 panels. These panels are facing true north, and their elevations can be set at 3 different levels to optimise the amount of power produced at different times of the year.

 

House roof mounted PV Panels surrounded by Solar Hot Water System
The nine series-connected PV panels on the house roof surround the Solahart Hot Water System and face 35 west of true north with a fixed elevation of 30. This photo was taken before the SunLizard Solar Air Heater/Cooler was installed. It now occupies the space in the lower right hand corner of the photo.

The table below shows the approximate dates that the panels need to be at the stated elevations to produce the maximum amount of power:

Approx. Date Range Elevation
26 Feb - 14 Apr 34
14 Apr - 31 Aug 55
31 Aug - 17 Oct 34
17 Oct - 26 Feb 20

In practice, there is a window of at least 10 days either side of the dates specified where you'd be hard-pressed to notice any discernible difference in output power.It takes me around 30 minutes to change the elevations on all of the frames.

 
Carport Roof mounted PV panels
The panels on the carport roof are facing true north and are mounted on 3 frames with adjustable elevations. Pictured here they are set for their winter elevation of 55.
 

Grid Interactive Inverter

The inverter used is an SEAG-150-1k5 Grid interactive inverter made by Solar Energy Australia (This is a discontinued model). It's rated at 1500 W max power input and 1350 Watts max power output. Using Maximum Power Point (MPP) tracking, it takes the (approximate) 150 Volts DC power produced by the PV panels and converts this to normal 240V AC power which is then fed through the SunPower meter and either back into the house or onto the main electricity grid.

The inverter senses the incoming power from the street, and if this should disappear for any reason, it shuts down and won't start up again until a short time after power has been restored. This is a safety feature to protect anyone that may be working on the lines further up the street. Therefore, if the mains fails during the day, this system can't supply my house with any power.

 

Grid Interactive System Electrical Board
Grid-Interactive Inverter (Large box on left), with the supporting components.

The Liquid Crystal Display (LCD) on the inverter's front panel can display a number of parameters such as the instantaneous power being produced (in kW) and a running total of power produced for the day so far (in kWh). The unit also has a serial interface which feeds out the above stats every second. A PC can be used to log this data.
     

Metering

Note: There are actually three 'meters' used to measure household consumption, although two of them are inside the same case. In the block diagram above I've combined these meters and shown them as one for simplicity.

There is an additional meter on the main electricity switchboard to measure how much power the system is producing. This is called the SunPower meter. By adding up the totals on the three normal usage meters and subtracting the reading on the SunPower meter, it's possible to tell whether more power is being produced than consumed or vice versa. On average, production is slightly higher than consumption, although on a day by day basis these figures can vary between an excess of about 4 to a deficit of around 7 kWh. Over the last year, the system has produced nearly 1500 kWh, which averages out to around 4.1 kWh per day. See the Household Power Summary page for a more detailed breakdown on these figues.

 

Main electrical baord for the houseMain Electrical Switchboard in my house.

The only additions to the switchboard for this system are an extra meter and a switch. The upper right hand meter (SunPower Meter) records the amount of power produced, and the switch at the top right corner of the board is to remove power from the system when maintenance is required. The meters at the bottom of the board measure the household electricity consumption. The one on the left is actually two meters in the one case.

At the end of the billing period if more electricity has been produced than consumed, the electricity retailer credits my bill with the difference. The amount they pay me for electricity is the same as I buy it from them. Despite normally having higher production than consumption figures, I still have to pay a small electricity bill. This is because as well as electricity usage costs, the retailer also charges each customer a small daily charge for access to the electricity network. The excess power that is generated is not quite enough to compensate for this additional charge. I'm normally left with a bill of several dollars.

Economics

The initial full price of the system was $17,000, but this was partially offset by a rebate from the Commonwealth Government's Photovoltaic (PV) Rebate Program which operates through the Australian Greenhouse Office (AGO) and is administered in NSW by the Sustainable Energy Development Authority (SEDA). In my case, the rebate was for $7,425, leaving me to find the remaining $9,600. (At the time, the rebate was $5.50 per watt, up to a maximum of $8,250 for 1500 Watts. These rebates have since been reduced. See the website above for the current levels).
In June 2001 a further $200 was spent on parts for an additional carport mounted frame to try and increase the amount of power produced. See System Diary for more info.
As mentioned above, last year I generated on average just over 4 kWh of electricity per day. At the current 11.7 cents per kWh, this means that I'm saving or being credited with around 48 cents a day on my electricity bill - Not a lot, but...

Renewable Energy Certificates - RECs

In April 2001, the Federal Government through the Office of the Renewable Energy Regulator (ORER) mandated that all electricity retailers had to source a certain percentage of the power they sell from renewable energy sources. This is done by allowing accredited renewable energy producing power stations to create Renewable Energy Certificates (RECs) for each megawatt-hour of electricity they produce (or are deemed to produce), and surrendering them to electricity retailers in exchange for a small payment.
Although only a very small system in comparison with many others, my system became an accredited power station on the 20th of April 2001, with an accrediatation number of SRPVNS04. I was able to generate my first REC on the 21st of December that year. I have subsequently generated 1 REC per year since. See www.rec-registry.com for a list of accredited power stations and how many RECs they've registered.

Enough talk of money. I'd like to give you advance notice of a little event I'm busily planning:

Payback

P a r t y

This is a tentative invitation to anyone who may be able to make it to a party at my place to celebrate the 'break even' or payback date of the solar grid-interactive system. Based on the amount credited to my house by the electricity supplier from the date of installation (15th of August 2000) until the 4th of June 2001, this date is currently projected to be:

17th of March, 2059*

Please mark this date in your diary.
More details posted as the date draws nearer.

* This date was arrived at purely by working out how much money I've been credited with on an average daily basis so far, and using this data, calculating the date at which my total credits will add up to the same amount as I paid for the system (including the government rebate). Of course, this is way too simplistic. This assumes that I will produce the same amount of power and be paid the same amount by our supplier over the entire period. It doesn't take into account many other significant factors such as inflation, future maintenance or repair costs to my system, added household insurance costs and interest I could have earned if that money was invested elsewhere. As the system is working more reliably now and generating slightly more average power, the payback period should decrease a little over the next year or so.

I guess what I'm trying to say is that at present you wouldn't consider installing a similar system on economic grounds alone. I see this project as an investment in a more sustainable future both for myself and for the generations following.

Related Pages on this site:

Floris Wouterlood from The Netherlands is compiling a list of people with similar systems around the world. Click here to see his list.


Last Updated: 18/06/05
Email:
robkemp@tpg.com.au