Almost all of our methods for generating electricity are fundamentally the same. Coal-, nuclear-, and natural-gas-fired power plants boil water to produce high-pressure steam that spins a dynamo in a “steam turbine.” The wood-chip-fired combined-heat-and-power (CHP) plant that we want to build in Brattleboro works the same way. The heat sources used in these systems, of course, are very different—each with its own environmental and health concerns—but the way they actually produce the electricity is, fundamentally, the same. Hydro power plants use the gravitational force of flowing water to spin a similar turbine, and windmills use the power of wind to do the same thing.

只有一种根本不同的发电技术可以将大量的电力供电到公用电网：光伏或直接转化为电力。光伏（PV）细胞显着产生没有运动部件的电力。没有机械装置可以润滑或磨损，基本上没有任何维护。他们只是在阳光下坐在那里，并产生直流电能。

虽然其效果很简单，但PV细胞的工作原理非常复杂。这是对该过程的过度简化：PV细胞由诸如硅等“半导体”材料制成，而硅的材料是可以通过添加少量的杂质来改变电导率的材料。在典型的硅PV细胞中，有两层略有不同的硅，由“细胞连接”隔开。在细胞的一侧，硅希望摆脱电子。另一侧想要电子。

当光子的光子撞击细胞时，硅中的电子变得通电，其中一些会跳过细胞连接，但它们只能以一种方式交叉。结果，产生了电荷差异，在单元的一侧具有比另一侧的电子（电荷）更多。通过将电线连接到电池，可以通过允许电子流过电线来均衡该电荷差。这种电子流是直接电流（DC）的电力，我们可以利用该电流来执行工作 - 为电动机，产生光线等。

PV cells are wired together into “PV modules” so that the small amounts of current created by each cell are aggregated into useful quantities of electrical power. The modules, in turn, are often combined into “PV arrays”—sometimes huge arrays that generate many kilowatts or even megawatts of electricity when the sun is shining.

最后，逆变器将直流电转换为交替的电流（AC），该电流可以操作标准电气设备或使用特殊设备将其送入电网。

The practical photovoltaic cell was invented in the 1950s by scientists at Bell Laboratories in New York. Initially, the cost was extremely high—thousands of dollars per peak watt of output. The first applications for PV were on space satellites where that high cost was not a concern. In fact, local PV expert Richard Gottlieb ofSunnyside Solar在弗吉尼亚州吉尔福德（Guilford），在海军研究实验室工作，在1957年，在Vanguard卫星上安装PV细胞。

As the manufacturing of PV modules was streamlined and the cell efficiencies improved, costs came down. By the 1970s, PV power became practical for applications like remote radio repeater stations and Coast Guard signal buoys. In 1983, I organized a conference for the Northeast Sustainable Energy Association, “Photovoltaics: From Research to Reality,” which was held at MIT in Cambridge. By then, some pioneering designers were incorporating PV arrays into houses. Steven Strong, of Solar Design Associates in Harvard, MA, designed some of the first such homes, and he continues to be a leading designer of PV-powered homes.

Today, there are tens of thousands of homes throughout the country that derive a significant portion—sometimes all—of their electricity from PV arrays. Many of these are “off-grid” homes that are totally independent of utility power. They have battery banks so that lighting and other electrical devices can be used at night or during very cloudy periods. There are dozens of such homes in the Brattleboro area.

其他房屋与光伏发电“短时间”。类风湿性关节炎ther than batteries, these homes have “net-metering” systems that feed electricity into the grid when the sun is shining and the PV system is producing more electricity than the house needs to operate. In states with net-metering laws, including Vermont, New Hampshire, New York, and Massachusetts, when the PV system is producing excess power the electric meter spins in reverse, and the utility company effectively pays you for that electricity at the same retail price you pay for electricity.

Though PV costs have dropped a great deal since the 1980s, high first-cost remains an impediment to widespread use of PVs. For a grid-connected PV system (without batteries), installed costs today range from About $6,000 to $12,000 per peak kilowatt (kW) of capacity, before government incentives or tax credits. Thus, a 4.5 kW residential system, which would produce about as much electricity annually as a typical home uses, will cost in the range of $27,000 to over $50,000. Grid-connected PV systems are eligible for cash subsidies from the states of Vermont, New Hampshire, Massachusetts, Connecticut, and New York, as well as a 30% federal tax credit.

有关成本和补贴的更多信息，请联系安装人员，例如Richard Gottlieb或上周专栏中列出的一家公司之一。