2.2 Basic Solar Jargon for Energy and Power

Solar Energy Journal was established for the International Solar Energy Society (ISES) and has been around for some time now. Solar Energy Journal stands as an important forum for peer to peer sharing of solar research for energy conversion and human applications of solar energy. What I want to establish here is that there is precedent for the complex system of notation used in the solar energy world that has been in use for decades. The original authors have established the following observations:

"Many disciplines are contributing to the literature on solar energy with the result that variations in definitions, symbols and units are appearing for the same terms. These conflicts cause difficulties in understanding which may be reduced by a systematic approach such as is attempted in this paper.

It is recognized that any list of preferred symbols and units will not be permanent nor can it be made mandatory, as new terms will emerge and old ones become less used with the development of the subject. But in the meantime, a list would be appreciated by the many workers who are entering this multi-disciplined field...

...Energy: The S.I. (Systèm International d'Unités) unit is the joule ($J=kg{m}^2{s}^{-2}$ ). The calorie and derivatives, such as the langley (cal cm^-2), are not acceptable.

No distinction between the different forms of energy is made in the S.I. system so that mechanical, electrical and heat energy are all measured in joules. However, the watt-hour (Wh) will be used in many countries for commercial metering of electrical energy...

Power: The S.I. unit is the watt ($W=kg{m}^2{s}^{-3}=J/s$ ). The watt will be used to measure power or energy-rate for all forms of energy and should be used wherever instantaneous values of energy flow are involved. Thus, energy flux density will be expressed as W/m² or specific thermal conductance as $W{m}^{-2}{K}^{-1}$ . Energy-rate should not be expressed as $J/h$.

When energy-rate is integrated for a time period, the result is energy which should be expressed in joules, e.g. an energy-rate of 1.2kW would if maintained for one hour produce 4.3 MJ."

W. A. Beckman, et al.

In summary: received energy flux density (or power density, called irradiance) can be expressed in units of W/m². We also note that the received radiative energy density (called irradiation) can be expressed in units of J/m², or in units of Wh/m². Notice that we did not use radiation, which is an expression of light glowing outward (emitted light, different direction than what we want).

In today's maps of the solar resource, you will often see the units expressed in kWh/m². You should be aware that these are still only representations of solar light energy density, and not the hourly/daily/annual quantity of potential electricity that could be produced. To find that value, we need a simulation tool like SAM (System Advisor Model, which you should have downloaded at the end of Lesson 1), which takes irradiation data and converts it into power data.

I would like you to now take a short self-quiz to see if you recall the common uses of the notation and descriptions for solar energy (used in particular in this class.).