The phone alarm startles me awake at 6:30. Heavy curtains prevent the dawn light from passing through the studio’s window, so I switch on the bedside lamp. As a graduate student, I like starting my day with caffeine, with stove-warmed black tea. As the water heats, I open my laptop to check email. As usual, the computer takes too long to initialize so I grab milk from the refrigerator for my oatmeal. As soon as I sit, the teapot whistles. It is only 6:45 a.m. and I have already used more than a handful of appliances, all of which require electricity.
Most Americans wake up and start their day the same way. Millions of lightbulbs, coffee makers, and toasters come to life, all at roughly the same time, time zone after time zone. By noon the demand for electricity declines, but by sunset people are back at home and the high energy demand repeats. In Arizona, where I live, energy usually peaks from 3:00 p.m. to 6:00 p.m. This is bad news for solar energy technology: solar power systems collect more energy at noon, off-peak.
What we need is a way to store solar energy so there is enough electricity during peak energy use periods. Energy storage is one of the major challenges that renewable energy industries face—and it is something that scientists and engineers must take into consideration when designing solar systems.
At the University of Arizona’s College of Optical Sciences, my research group investigates a particular type of solar energy collector called the hybrid solar energy conversion system. Our project, which is funded by the Department of Energy through the ARPA-E program, combines the advantages of two of today’s most common technologies: concentrated solar power and concentrated photovoltaics.
Concentrated solar power systems use large, curved mirrors to capture solar radiation (sunlight and heat) and focus it onto a tube that carries dense oil. As the liquid heats, solar energy is stored as heat because of the large mass of the liquid. Electricity is then created with steam turbines driven by the stored heat. One of the major challenges with this system, however, is that it is not very efficient. It requires multiple steps to convert energy: solar energy transformed to heat transformed to electricity (from steam power).
On the other hand, concentrated photovoltaic systems use large mirrors to concentrate light onto small, efficient solar cells. The solar cells are connected to an electrical transformer that sends the collected energy to the electrical grid. Since sunlight is transformed directly into electricity, these systems tend to be relatively efficient. But solar cells cannot use the heat from the sun and, more importantly, are unable to store energy.
The hybrid solar system we are designing likewise uses large mirrors, but it also has an additional mirror with a special filter that allows us to separate sun radiation into heat and visible light. This way, the heat can be used to heat oil and the light can be transformed directly into electricity using solar cells. We combine the efficiency of concentrated photovoltaics and the storage capability of concentrated solar power at a reasonable cost.
Hybrid solar systems also support large-scale production, making harvesting sunlight a viable source of energy. With this research, it is possible to match our energy storage needs while also reducing the consumption of fossil fuels.
Now if we could only create another kind of “hybrid system” that helps society rethink our energy consumption habits altogether. Perhaps we don’t really need all those lightbulbs and coffee makers and toasters to start the day….
The Carson Scholars program at the University of Arizona is dedicated to training the next generation of environmental researchers in the art of public communication, from writing to speaking. Partnering with Terrain.org, the program will present essays and other writing from students and alumni of the Carson Scholars Program — A Life of Science — with hopes of inspiring readers to understand not only research findings but the textures of the lives of scientists and others engaged in the crucial work of helping the planet along in an age of unprecedented change.
Liliana Ruiz Diaz was born in Matamoros, Mexico and studied physics at the University of Texas at Brownsville. She is currently working on solar energy concentrators while completing a Ph.D. in optical sciences at the University of Arizona.
Header photo of the hybrid solar energy conversion system in storage mode by Liliana Ruiz Diaz.