- J.R. Brownson, Solar Energy Conversion Systems (SECS), Chapter 8: Measure & Estimation of the Solar Resource (section on Human Vision).
As modern society, we each seem to lack a cultural awareness for measures of light per radiometry is our biological link to vision. Along with your reading, think about why your vision has quite different criteria for performance than a solar hot water panel, or a PV module. I would like you to shift your metrics from clarity and information, linked with photometry, to that of irradiance on a given surface. We present this page to "illuminate" our collective bias that vision brings in to our solar resource estimations.
Read the following description of your eyes, and think about the type of equipment that we will need to assess the solar resource for economic decision-making. There are numerous physical challenges incorporated into vision that strongly bias perception of a solar resource. But solar resource assessment is about metrics (physical measurements), not perception.
Consider: I have personally heard people tell me that solar technology is not viable in Pennsylvania, North Dakota, and Minnesota. The news media have erroneously stated solar is too diffuse for all of the United States (old Fox News report: because Germany was supposed to be brighter?). Additionally, my colleagues have been told stories that solar is not viable in places such as Santa Barbara and San Francisco, as well.
None of these armchair philosophy assessments is correct. First, speculation on the solar resource using visual cues is not appropriate; our eyes do not actually measure the solar resource in a meaningful way for SECSs. Second, speculative resource arguments (even measured data) must ultimately be tied to economic arguments. Here, we lack the financial argument (the economics) associated with the avoided cost of fuels from incorporating solar energy technologies in a given locale for a client of interest. We will cover the financial and economic discussion in the next lessons to come, but let's go back to vision for a moment.
Sight Perception is a funny thing
Sight perception works to our advantage as individuals when we wish to minimize risk, such as avoiding that lion prowling through the forest in the evening. It also adapts with weak or intense signals, trying to feed the brain a useful stream of information. As such, sight also has limitations, in that our sensory systems are combined with a cognitive system to extrapolate small signals into big information or really intense signals into reasonable information. The goal of sight is information about the world around us, not the amount of light delivering that information to us.
How does the eye work?
The eye has two main conversion molecules: rods and cones, located in the retina. The two systems have adapted for dim lighting (rods) and full daylight. Rods absorb only certain wavelengths of light that are longer and lower energy, while the system of cones (actually multiple kinds of cones) absorbs the wavelengths of light that we interpret as color. In both systems of absorption, the maximum range of wavelengths is limited and does not include the ultraviolet and infrared regions that comprise about 50 percent of the shortwave band of solar irradiance. So, our eyes do not detect a large range of solar wavelengths, and the response factor of those receptors is not linear either. This is a detraction to using the eye as a quantitative solar detector.
Notice in the figure that the rods and cones are distributed across the back of the eye, but the two systems are not distributed in the same fashion. Rods are distributed broadly across the retina, with the exception of the fovea centralis. In complement, cones are distributed tightly within the fovea centralis. The two distributions are linked to the lens system of the eye.
- Cones: found mainly at the focal point of our lenses, the fovea centralis
- Rods: found distributed everywhere except at the fovea centralis
The lens system in the eye conveniently allows us to think about concentrating solar systems ahead of schedule (CSP, concentrating solar power for thermal steam production; and CPV, concentrating photovoltaics for electricity production).
Any lens will focus light onto a focal point (the fovea centralis here), but can only collect light from the direction that the lens is pointing. Meaning, a contracting system has to track the bright light sources for better performance.
The implications of this property of concentrating optics is that cones will only detect light in the direction that the eyes are pointing. So, our color detection system is relatively poor at sensing diffuse or scattered light from areas of the sky or ground at which the eye is not pointing. Mark one more detraction for the eye as a solar detector.
Going back to the rods...they are distributed everywhere except the focal point, and so will detect diffuse light entering the eye from all angles. Recall that rods are not color sensitive; they just detect long wavelength photons, such as those at night or during the twilight. If you want to see more at night while riding a bike or running, you are encouraged to defocus your vision to allow peripheral light to be detected. The optical implications are that your rods are not part of a concentrating system and will detect diffuse light better at the expense of color discrimination, but only at low levels of light. Mark yet another detraction for the eye as a quantitative solar detector.
And now, your additional macroscopic feedback system to control light acceptance, the iris. As the object of your eye is to provide your brain with the best information, not power, the iris is a feedback system meant to open wide when the light is dim, and squeeze up small when the light is intense. No matter what your rods and cones are doing, your iris is constantly adapting to maximize the signal of visual information to the brain. In a power detection system, we do not want an adaptive iris system, because it again detracts from our goal of linear detection of irradiance changes across the day.
We can also add the eyelids and eyebrows to your optical system, as they block or shade much of the bright light to your eyes. We can add behavior to our eye system, in that very few people actually look up to sense the light in the sky, and tend to look to the horizon instead, meaning our lenses are not trained vertically upward, but more along a horizontal plane (we mount solar detectors flat, to point up to receive the entire sky dome of light). All in all, we can come to the conclusion that the eye is not the ideal constant solar detector to inform us quantitatively of the amount and changes in irradiance during the day.
Power vs. Information
When a device takes one form of energy as an input and transforms it into different new forms of energy as outputs, the process is called energy conversion. The source of that input energy is called a resource. Now, if we were to draw upon the resource of the Sun from the point of a dude on Earth's surface, the energy form is electromagnetic radiation. We will call this light, bearing in mind that light from the Sun can be visible or invisible (ultraviolet and infrared) to the eye.
If one wished to transform the light into an electronic, or electrochemical signal, the resulting device (an energy conversion system) could be tailored to provide lots of power (more energy, less information) or to provide lots of information (less energy, more information). Let's use two examples: a photovoltaic cell (solar-electronic transducer), and the human eye (solar-electrochemical transducer).
- The photovoltaic cell is designed to respond in a linear fashion to the intensity of solar irradiance, thus delivering the maximum amount of power, but not functioning well at all in the night or during twilight. The device will provide changing levels of voltage and current (voltage times current is power) in a direct linear response to the intensity of irradiance.
- The human eye is designed with an iris, rods and cones, and lenses to respond in an adaptive, logarithmic fashion to the intensity of solar irradiance. The eye will detect the tiniest of solar signals on a starry night, and will adapt to the brightest of desert suns during the day, while always providing very similar levels of electrochemical voltage and current to the brain, but with a high degree of information content that can separate the light reflecting from a "lion" vs. a "tiger" vs. a "bear." A device that can sense a signal over many orders of magnitude is a logarithmic detector.
The eye is designed to provide you with the information sufficient to avoid bumping into bad things in extreme lighting conditions. This is called information, but it is not the useful information for assessing the solar resource quantitatively. A linear detector like a photovoltaic device is required to accurately measure the power of the solar irradiance.