PHOTOVOLTAIC CELLS IN YOUR FUTURE
SAFETY
Increased energy costs are continuing to make alternative sources attractive to the public. One such source is the photovoltaic cell, which makes electricity from light. Although photovoltaic systems are relatively expensive at present, there is great promise that the cost will rapidly be reduced to the point where the systems will be fairly common.
What is believed to be the first photovoltaic-powered residence in the United States built using only private funding was rec ently constructed in the Eldorado Fire District of Santa Fe County, N.M. This particular house is a passive solar design, to which has been added a photovoltaic system to supplement the electricity supplied by the electric company.
Because of the nature of photovoltaic cells this house has presented new challenges to the Eldorado Volunteer Fire Department. The house has a massive array of photovoltaic cells on the roof. The panels and related hardware impose a static load on the roof of about 1500 pounds In addition to the problem of having this static load over your head while conducting interior fire fighting operations, the panels also make it difficult to open up the roof for ventilation. This is especially important in this house because there is a 12-inch insulating barrier between the ceiling and the built-up roof.
For this location — with a 7000-foot altitude and a high number of sunny days — there is only one such photovoltaic array on the roof. But at lower altitudes and in cloudier areas, there would have to be two arrays to generate the same number of kilowatts. The weight would also double.
The roof-mounted cells generate DC current, which is fed into a system inside the house for conversion to AC. This system feeds the house electrical system. When the solar array is generating more power than is being consumed by the house, the excess power is fed back to the public utility for credit against the electric bill.
While there are shutoffs for both the positive and negative sides of the photovoltaic array, the cells will continue to generate power while the sun shines on them. Meaning this: a fire fighter could receive a bad shock while working on the roof if he came into contact with the cable feeding through the roof. The DC current
is converted to AC by a synchronous inverter. If the utility loses power while the sun is shining, the inverter will cease to function, and no power will be fed into the house. However, DC would continue to be generated. The current will feed into the utility line as far as the house transformer (transformers do not “see” direct current, so no power will feed back into the transmission lines under this condition). Because of this, fire fighters still face problems even though the outside power is off, and so does the utility. Public Service Company of New Mexico has adopted a special code in their computer which alerts them, when receiving a service call, that there will be DC power on their lines between the transformer and the house.
As the price of photovoltaic cells continues to fall, more and more photovoltaic systems will go up. The fire service had best be prepared to handle this new challenge.
