PV (photovoltaic): light to DC electrical energy
If solar thermal captures solar radiation as heat, PV systems convert this radiation into electrical flow in one direction (direct current, or DC, like batteries). This is convenient for battery storage, but to be used in most homes and businesses, AC (alternating current, 60 Hz) is needed. Inverters are electronic devices that turn DC from PV and/or batteries into AC for use.
Since HPA is on one meter with HELCO, we are essentially a “micro-grid” meaning any electrical energy harvested from PV (or released from batteries) goes to slow down or reverse the HELCO meter. Since we do not presently get any credit for energy out, we want to make certain we can store any excess energy on campus for our night time use.
Since the sun is brightest at noon, PV engineers use an estimation of a PV array output called “solar hours”, meaning the equivalent amount of energy harvested if noon lasted that many hours. This is like making a camel hump curve into a rectangle, adding the edges to the top.
For example, our PPA (purchase power agreement) array behind the elab produces about 100 kW maximum. This is true at noon, but less so either side of noon, so we use “solar hours” to estimate energy harvest each day. For us, this is about 5.5 solar hours, depending on season:
100 kW x 5.5 solar hours = 550 kWh or about $200 saved each day.
|Click for full-size image|
PPA arrangements usually charge us a fraction (about $0.20 per kWh) of the HELCO cost, but we have to pay for what it produces, not what it uses. If we are pushing energy out the door to HELCO using the PPA array, we are in effect paying to give this energy away, which happens during vacations (summer, winter, spring).
Net zero energy is when we have effectively stopped the HELCO meter, meaning we are producing exactly how much we are using.
We hope to harvest enough to reach net zero around 10AM each day until about 2 PM each afternoon. The extra energy during that time we hope to capture using battery and other storage systems (pumped storage hydro, hot water activation, etc.)