Somewhere right around the 39th parallel on the east coast you will find a site that receives, on some reasonably conservative average, about 3 kWh per square meter per day of raw solar energy. If you put a building there, and you find a way to harness and utilize all of that energy, (in which case you are very clever, indeed), you could on an annual basis heat, cool, and electrify maybe as many as seven stories of finished space (assuming current construction methods and building technologies). That is the good news.
The barriers to such wanton success include: the time value of energy, the laws of physics and our current ability to apply them economically, public policy, and the taller building in the lot just to the south of you that hogs all the renewable resources.
Time value of energy means that there is too much solar cash flow in the summer, not enough in the winter, and no bank to store it in so you can even it all out. Keeping energy around for just when you need it is expensive and difficult. Life does this brazenly well, (polar bears are quite happy to eat nothing at all for months at a stint), but we haven’t quite figured out how to copy it yet. Life performs amazing chemico-physical feats on a whim, while creating and wasting very little energy as heat in the process. We, on the other hand, struggle daily with these concepts and do not enjoy such heat-avoiding chicanery as placing sunlight bit-by-bit into the shape of a tree: we are most comfortable working with things that burn as they transform.
It is certainly visually appealing.
Speaking in terms of universal speed constants, there is no heat engine known to humans that could convert sunlight into forms of energy we would like at 100% efficiency. Plants, which are most definitely NOT heat engines, are about 2% – 4% efficient at converting sunlight to stored chemical energy (as biomass). Photovoltaic panels do 3-4 times better. And solar thermal systems can even beat 80% capture rates, but there’s only so much heat we can stuff into a building before we decide there has to be a better way. Converting the captured heat into a more valuable form of energy, such as electricity, is fairly ineffecient. And storing it for six months to use in the winter with current technology is work for Sisyphus.
So, realistically, the unabashed deployment of present solar/renewable technologies on, in, near, and cleverly juxtaposed with all the surfaces of a modern building can probably sustain the realtime energy needs of 1-2 floors of finished space. Anything larger than that will almost definitely require energy imports. That is sounding like bad news.
Our job until such time as energy is truly cheap and abundant is to use it and transform it as wisely as possible. It is just about this spot where we aspire to enter the conversation.
By Michael Mark, PE
