Unlike solar and wind systems, fuel cells run on fossil fuel, even though they reduce fossil fuel usage in total. However, that is changing — as biomass-based fuel cells are being developed. Another way they differ from solar and wind systems is they don’t rely on the weather. This reliability factor is extremely attractive. While one goal with solar and wind is to make them as reliable as the existing U.S. power grid, an advantage claimed by fuel cells is they can readily be made more reliable than the grid.
Wind energy is increasingly popular — from the large-scale wind farms that utilities run to the small-scale units that reduce the electric bill of small buildings such as residences. Due to the unreliability of wind, small-scale wind systems are typically not the sole or primary source of electricity for a given facility. They usually supplement power provided by the utility grid, but are increasingly used with other systems. For example, when it’s storming outside, the sun isn’t shining to power that solar array. But the wind is most likely blowing.
Fuel cell systems
A fuel cell is defined by the NEC as “an electrochemical system that consumes fuel to produce an electric current.” Fuel cell power systems can be designed as stand-alone or interactive units. These systems typically consist of a reformer, stack, power inverter, and miscellaneous auxiliary equipment items. They can also be designed for AC or DC current output.
The 2011 NEC didn’t change the requirements for fuel cells very much. There were only five changes. The most significant revision is that now only qualified persons can install these systems, including the associated wiring and connections [692.4]. The other changes are in Part VII and address fuel systems that are interactive — that is, they connect to other circuits.
Fuel systems must now not only be listed, but also marked as interactive to be permitted in interactive systems [692.60]. The other three changes reference Art. 705, Interconnected Electric Power Production Sources:
- Output characteristics must be in accordance with 705.14 [692.61].
- Unbalanced interconnections must be in accordance with 705.100 [692.64].
- The utility-interactive point of connection must be in accordance with 705.12 [692.65].
These three changes don’t actually present new requirements. Section 692.3 already listed these references in the 2008 NEC, and that text is unchanged with the 2011 NEC. However, adding the references in Part VII makes the NEC a bit easier to use.
Small wind electric systems
The 2011 NEC added Art. 694, which applies to small wind (turbine) electric systems that consist of one or more wind electric generators having a rated power up to and including 100kW (see Figure below). Typical system components are a generator (or alternator), inverter, and controller [694.1] — see SIDEBAR: Where the Wind Blows.
One way that wind differs from solar and fuel cell systems is physical access. The typical roof-mounting of residential and commercial solar means requires minimal climbing for the installers, but access to a wind system nacelle (where the generator or alternator is located) may require climbing a 75-ft tower.
On larger towers, the ladders are inside the tower body. The towers of the smallest residential wind turbines (under 10kW) are really pole-mounted systems where you reach the nacelle from the outside of the structure.
You need climbing safety training and climbing equipment to work on any but the very smallest of wind systems. You must be adept at completing elevated work as well. This means carefully planning each task and following procedures that prevent you from dropping your parts bucket onto the customer’s new pickup truck 75 ft below.
Height is also a factor in the NEC requirements. Obviously, a disconnect in a nacelle 75 ft off the ground isn’t “readily accessible” as required by 694.22(C)(1).The language of 694.22(C)(1) specifically permits locating the disconnect at a readily accessible location either on or adjacent to the turbine tower, on the outside of a building or structure or
inside, at the point of entrance of the wind system conductors.
Circuit requirements — Part II of Art. 694 applies to systems up to (and including) 600V. That’s the voltage limit for systems connected to one- and two-family residences. Systems over 600V must comply with Part IX, instead of [694.10(A)].
Two of the circuit sizing requirements are familiar from solar installations:
- Inverter output circuit current. It’s equal to the continuous output current marked on the inverter nameplate [694.12(A)(2)].
- Stand-alone inverter input circuit current. It’s the stand-alone continuous inverter rating when the inverter is producing power at the lowest rated input voltage [694.12(A)(3)].
The NEC also requires determining the turbine output circuit currents. It says to base these on the circuit current of the wind turbine operating at maximum power 694.12(A)(1)], but what does that mean?
If you need to calculate the number yourself, then it’s a matter of converting the turbine kW rating to amps using the turbine output voltage — not the inverter voltage. For a residential system, the inverter output voltage is typically 120V or 240V, but the turbine output voltage may be considerably higher. Typically, the manufacturer has already done this, because these systems are usually sold as packages with a wiring kit.
Overcurrent protection — Size the overcurrent protection devices per Art. 240. In addition, consider the loads to be continuous [694.12(B)(1)]. That means the circuit conductors and overcurrent devices must be sized to carry not less than 125% of the maximum current calculated in 694.12(A) [694.12(B)(2)].
Stand-alone systems — You won’t find “stand-alone” defined in 694.2, but it is defined in 690.2 for solar PV systems — and in terms of fuel cell systems in 692.2. Since this term is now being used in two or more Articles, its definition should find its way into Art. 100.
Stand-alone systems must meet the installation requirements of the Code for a similar installation connected to a service. The inverter output can supply power to the structure disconnecting means, but only at current levels less than the calculated load connected to that disconnect [694.18(A)].
The inverter must be rated at least large enough to supply the largest single utilization equipment connected to the system. Don’t consider calculated general lighting loads as a single load [694.18(A)].
Grounding — Bond all non-current-carrying metal parts of towers and equipment to the equipment grounding (bonding) conductor [694.40(A)] — that is, follow the bonding requirements of Art. 250, Part V. But make sure you ground the tower itself by connecting it to one or more auxiliary electrodes for lightning protection purposes [694.40(C)].
A bright future
Advances in solar technology are making this option ever more practical and affordable. Wind systems are now available in predesigned kits that result in a better installation at lower cost. This same advantage also exists for fuel cell systems. For example, a refrigerator-sized unit can meet all the electricity needs of a typical suburban home. Of course, it doesn’t just plug in the way a refrigerator does, so 692.4(C) is a welcome addition to the NEC. As the demand for alternative energy systems increases, you now have a good foundation for correctly applying the NEC to that work.
Holt is the owner of Mike Holt Enterprises, Inc., Leesburg, Fla. He can be reached at www.mikeholt.com.
SIDEBAR: Where the Wind Blows
Conversion of wind power to electricity is not new. This concept was used in rural farming areas prior to the Rural Electrification Act of 1936, which brought rural electrical cooperatives into existence. Installing and maintaining a battery system remains one of the barriers to wide-spread use of wind power. As a result, the stand-alone small wind electric system is not seen as commonly as interconnected systems to supplement the electric utility power supply.
Most of the wind generation installation work is for large-scale wind farms. Wind farms consist of wind turbines too large to be covered by Art. 694. The types of installations that fall under Art. 694 are typically for a specific building or facility. They can serve as the sole supply for a structure, but more commonly serve as a supply in addition to other sources [694.7(A)]. The scope of Art. 694 limits it to one or more wind electric generators with individual generators having a rated power up to and including 100kW [694.1].