Terms used on the live data feed and the instructional video

Total Energy

At the bottom of the Live Data Display field there is a "Total Energy" ticker. This ticker shows the amount of energy - electricity - that the chosen PV array has generated since it was connected to the live data feed. You can find the connection date for a particular array by going to the UQ PV Sites page, navigating to the site and array of interest and then looking for the entry next to "Date Connected to Display".

If you are viewing "All Sites Combined" then the energy ticker shows the total amount of electricity recorded by all of the University of Queensland live PV sites since the first site was connected to the live data feed (June 2011).  During 2012 the University is planning to connect all of its sites to the display.

CO2 Savings

CO2 savings achieved by the chosen array since it was connected to the live data feed is shown on the LHS of the display.  The savings achieved in the relatively short time since a website visitor has opened up the live data feed ("this session") are also displayed.

Only a small amount of fossil fuel is involved in the maintenance and operation of a PV array. The electricity generated by PV is usually considered emissions free. The CO2 savings are based on the amount of grid electricity displaced by the PV.  

Electricity supplied from the SE Queensland electricity grid is generated mainly by coal fired power stations. Electricity from gas fired plant and from renewable sources also forms part of the mix. The Commonwealth Government publishes CO2 emission factors covering the generation of grid electricity consumed within each State. The CO2 Savings for the chosen PV array are obtained by simply multiplying the "Total Energy" by the emission factor applicable to Queensland.

All plant and equipment has embodied energy and hence greenhouse gas emissions associated with its manufacture and installation. Information is still being gathered and analysed on the embodied energy in different types of power generation plant. Meanwhile an assumption is made that the embodied energy associated with a PV installation, expressed as kg CO2 per kWh of output, is not a lot different to that for other forms of power generation. Consequently the CO2 savings calculator does not, at this stage, considered embodied energy and related emissions.

$ Savings

The $ Savings (year to date) created by the chosen PV array, since the start of the calendar year, are shown on the bottom LHS of the display.  The figure is derived by multiplying the amount of energy generated by the PV array since the start of the year by the average cost of grid sourced electricity in Queensland.  If you want to know how much energy has been generated since the start of the year then choose "Pick a Year"  and then pick the current year. You will get a bar chart of energy generated for each month of the year. By placing the cursor over each month you can use the tool tip function to read off the output for each month.  Further development of the live data feed will see the total year to date energy production shown when you select "Pick a Year"

Daily Energy - Culmulative

If you have chosen "Energy-Cumulative" as the data to display, then you will see a curve showing the total amount of energy that has been generated by the chosen PV array at different times of the day, up to the time shown in the display's title block.  The unit of measurement is kWh. A technical definition of "Energy" is given in a section below.  At the start of each day, the cumulative energy value is set to zero and the energy generated is accumulated and displayed as the day progresses. The data is updated approximately every minute.

Flat panel PV arrays typically create a flattened "S" shape cumulative energy curve. Energy generation early and late in the day is limited by the low sun angle and this is reflected in the gently sloping cumulative energy curve at those times of the day. In the middle of the day, when the sun is more at right angles to the panels, energy is generated at a fast rate and the cumulative energy curve is steep (assuming no clouds). The Concentrating Array, however, points at the sun all day. When the sky is clear, this means the array generates energy all day at a fairly constant rate; this means the cumulative energy "curve" is in fact very straight for most of the day.

Power - Instantaneous

The power output from PV arrays usually changes from moment to moment, albeit by only small amounts in relatively steady sunlight. Every minute a snapshot of the chosen PV array's instantaneous power output is taken and shown when you chose "Power-Instantaneous" to display . The unit used for power is kW. A technical definition of "Power" is given in a section below. On sunny days the power curve for flat PV panels will adopt a symmetrical bell shape.

If clouds move to block the sun the power output will drop immediately and this is shown by a sharp dip in the power curve. Having clouds blocking the sun does not mean complete loss of output though. Flat PV panels can make use of scattered or diffuse light and consequently useful power can still be produced in haze, uniform light cloud or broken moderate cloud cover. On days with heavy cloud the power output can, however, remain very low and close to zero.  The concentrating array relies on direct sunlight. If the concentrating array cannot see the sun then power output will drop to zero. 

Energy - Delta Time

The Energy –Delta Time display shows the amount of energy generated by the chosen array over a series of 15 consecutive minute increments during the day. Each bar represents a quarter hour interval beginning and ending on the hour: 09.00 to 09.15; 09.15 to 09.30; etc. When viewing today's live data the last bar shown on the chart represents the current 15 minute interval. The bar will gradually grow in height (data is updated every minute more or less) until the end of the 15 minute period. Then, the next 15 minute bar will start to develop. The Energy - Delta Time bar chart takes on a similar shape to the Daily Power Instantaneous curve. It could be thought of as smoothing the power output curve which can vary markedly minute to minute when clouds are present.

Energy - Scientific Overview

The PV Data Display shows electrical energy in units of kWh (kilowatt hour). This is the everyday unit for electrical energy. A definition of a kWh is covered under "Electrical Power and Energy Terms". A quasi scientific overview of "energy" is given here.

Energy is a characteristic of a state that is capable of doing work. Work is the product of force acting through a distance. Pushing a box across a floor is work. The greater distance you push the box the more work you do. A heavy box requires more force (to overcome friction) to push it than a light box. Consequently, pushing the heavy box a given distance involves more work than pushing the light box. This matches with our everyday experience. At an atomic level, the processes involved in producing electricity from PV panels - getting an electron to break free of its bonds, move through a semi conductor and then, power equipment via a circuit – all requires work.

Energy can take many forms; potential or stored energy, the kinetic energy of a moving mass and electro-magnetic energy, to name a few. Fossil fuels have chemical potential energy that is released when they are burnt and this can be transformed into electricity via heat engines – steam boilers and turbines. The kinetic energy of wind can be converted to electrical energy via wind turbines. Sunlight is electro-magnetic energy. PV systems convert sunlight into electrical energy. Electrical energy can then be used to do what we more readily perceive as work; such as to power motors that push and pull things around.

Energy can be transformed from one type to another. Sunlight can be converted to electricity using PV technology, for example, and this electricity can be used in a lamp to produce light. During this transformation some energy is, in common parlance, "lost". Energy is, however, always conserved. What is really happening is that some of the energy is not captured by the process at all (e.g. light may be reflected off PV panels) or some is transformed into forms that are less useful in terms of human utility.  During the conversion of light to electricity and vice versa some of the energy involved in the conversion processes is converted to heat, or infrared energy, which is not useful if the objective is to produce visible light..

Increasing the efficiency of converting sunlight to electricity (by improving capture and reducing losses) is a major focus of PV research.

The scientific community uses the "joule" or "newton-meter" (N x m) as the units for energy or work. The "newton" is the scientific unit for force and was named in honor of Sir Isaac Newton who described the three laws of motion that form the basis of mechanics. He also identified the specific relationship between force, mass and acceleration (F=MA). The alternative energy unit, "joule", is named after the English physicist James Prescott Joule who pioneered the field of thermodynamics. He discovered the relationship between heat and mechanical energy, a relationship that underpins the conversion of fossil fuels into kinetic energy (a spinning turbine) and then electrical energy today.

Power - Scientific Definition

Power is the rate (energy amount per time period) at which work is done or energy converted. The scientific unit of power is the watt (W), which is equal to one joule (energy amount) per second (time period). The PV Data Display shows electrical power units as kW, which is equal to one thousand watts.

Energy and power are often confused and used interchangeably. Quite often people will refer to the "power" generated by their PV panels today when they really mean the electrical energy produced during this time period.

The difference between energy and power is that the former is an amount and the second is a rate. Consider two full 1000L tanks of water on stands of equal height. Both tanks contain the same amount of potential energy. One has a large diameter discharge pipe connected to a water turbine. This tank will be emptied more quickly than its neighbor with a small discharge pipe. The turbine connected to the large diameter pipe will generate energy at a faster rate and will be more "powerful" than the other turbine. It will of course generate electricity for a shorter time than the turbine connected to the small diameter pipe. Ignoring the slight differences in efficiencies, both systems will generate a similar amount of energy by the time each tank is empty.  

The term watt is named after James Watt, a Scottish engineer. Watt greatly improved the performance of the steam engines that existed at the time. He also measured the power output of draft horses and ponies and refined the unit "horsepower". This unit was originally used to compare the output of steam engines with the power of draft horses and is still in use today, especially to describe the power output of vehicles, boat engines, etc.

Electrical Power and Energy

The electricity sector generally uses the unit "kilowatt" (1,000 watts) or multiples thereof, such as megawatt (MW, i.e. 1,000,000 watts) as its standard measure of power.

The sector also uses the unit kWh for energy, which is different from the scientific units of newton–meters or joules. The kWh unit is the product of the rate at which energy is consumed or produced (power) and the duration or time period of consumption. If an appliance draws power at a rate of 1 kW (kilowatt) for one hour then the energy consumption over this period is 1 kWh (kilowatt-hour). A kilowatt hour can be confirmed as an energy measure by breaking it down into it basic units:

1 kWh = 1 kW x 1 hour
  = (1000 joules/second) x 60 minutes x 60 seconds/minute
  = (1,000 joules/second) x 3,600 seconds
  = 3,600,000 joules or 3.6 MJ

Industry specific units of energy are not unusual. Coal was once used extensively as a local heat source to create steam which was used drive steam engines, presses and other devices. The emphasis was on using measures related to heating water. The energy unit in common use was the BTU or British Thermal Unit. A BTU is approximately the amount of energy needed to heat 1 pound (0.454 kg) of water from 39 to 40 ° F (3.8 to 4.4° C). This unit of heat is still in common use in the United States and a number of other countries. In Australia, you may still see a gas appliance with a BTU rating on its nameplate.