Hybrid Power Systems
Communications
Residential
Microwave Repeater
A hybrid power system has more than one type of generator - usually a fueled-engine generator and a renewable energy source such as PV, wind, or hydropower. A PV-engine hybrid is the only type considered here. A hybrid system is most often used for larger applications such as village power; residential systems where generators already exist; and in applications like telecommunications where availability requirements are near 100 percent. Almost all PV-generator hybrid systems include batteries for storage.
Hybrid power systems are used in the applications described below. For a discussion of the issues regarding Hybrid Power Systems go here.
This hybrid system was located in the mountains in Idaho. System availability was critical. Two charge controllers were used in parallel to provide reliability. Both were housed in a NEMA 13 enclosure with analog voltage and current meters mounted to the door. The controllers had the temperature compensation feature with temperature sensors attached to the batteries. An adjustable low-voltage sensor was used to control operation of the generator. When the batteries reached 2.0 volts/cell, (approximately 50 percent state-of-charge) the generator was started and provided load power and charged the batteries to 80 percent SOC (2.3 volts/cell). A battery charger was connected to the generator through the load center located inside the repeater building. When the generator starts, the battery charger turns on and remains on until its cycle is complete. The shutdown of the generator terminates the battery charger cycle. The array was mounted on an aluminum support structure that was attached to a wooden platform elevated 7 feet above the ground. Two inch PVC conduit was used for all exposed wiring. A fused, two-pole, 30-ampere dc rated switch in the communications building was installed to disconnect the array. A dc to dc converter was installed, because this particular repeater had both 12-volt and 24-volt dc loads. The converter obtains its power from the 24-volt battery bank through conductors running in conduit from the control box to the converter and its loads.
Site
Iron Mountain, Idaho
Location/Elevation
44°N - 115° 3' W - 2540 meters
Environment
Mountaintop
Temperature Range (°C)
-30 to 24
Maximum Wind Speed (m/s)
40
Availability Required
near 100 percent
Days of Storage
4
Load Profile
Variable
Installation
The array support structure consists of pressure treated wooden utility poles that provide an elevated platform for the array. This mounting prevents heavy winter snow accumulation from obstructing the array. The array mounting system consists of aluminum supports designed to meet the wind load requirements of the site. All module interconnections were made using type USE sunlight resistant cable, secured at the module junction boxes with strain relief connectors. The parallel module connections were made inside weatherproof junction boxes mounted on the back of the array frame and interconnected with PVC conduit. The array conductors were run in PVC conduit from the parallel junction boxes to the control box inside the building. The array and its mounting structure are grounded to a grounding electrode at the base of the support structure. The negative conductors in the photovoltaic system were grounded. Positive conductors were fused in a double pole safety switch. All metal enclosures in the repeater station were bonded to the existing grounding system. As a precaution against transient voltages, metal-oxide varistors were installed between each ungrounded conductor and the grounding system. A current limiting fuse was placed in the positive lead to the battery to prevent load and array fuses from blowing in the event of a serious fault.
Worksheet 1-Calculate the Loads
This system has both 12 V and 24 V dc loads, both approximately equal in magnitude. The designer decided to use a 24 V system and a converter to provide the 12 V loads. The system must be able to handle the maximum load of 300 W. that occurs when both transmitters are operating simultaneously.
Worksheet 2-Design Current and Array Tilt
The array tilt of 60 degrees maximizes wintertime energy production and helps any snow to slide quickly off the PV array.
Worksheet 3-Calculate System Battery Size
Heavy-duty industrial batteries are selected for this application.
Woksheet 4-Calculate System Array Size
Worksheet 5-Hybrid Design Determination
Starting the design of a hybrid system requires new decisions about the size of the battery and PV array. In this case the load was designed to be split 75:25 for PV:Generator respectively.
Worksheet 1HY-Calculate the Battery Capacity
Four days of battery storage (vs. 16 days) are used for the hybrid system because of the availability of the generator. The generator was derated 37.5% because of the 7500 feet altitude of the site.
Worksheet 2HY-Calculate the Number of Modules
Although the same type of battery is used, the model is changed to better match the required capacity of the hybrid power system.
Controller Specification
Power Conditioning Units Specification
Switches and Protection Components
DC Wire Sizing Specification
Hybrid Power System for a Residential Application
This hybrid system is located on a house on a private island off the coast of South Carolina. The homeowner lives on the island year-round. The electrical demand is high because of a large air conditioning and space heating load from a ground-source heat pump. A hybrid (photovoltaic-generator) system was determined to be the most cost-effective design to accommodate the average daily load of about 5 kilowatt-hours. Since the homeowner already had the generator, the PV array was designed to supply about 45 percent of the total loads. This fully automatic system includes a sophisticated control system that starts the generator at specific battery states-of-charge and controls all aspects of system operation.
South Carolina
29°N - 80° W - 20 meters
Coastal
-5 to 37
90 percent
The array was mounted on the south facing roof of the residence with 3 inches of space between roof and array. Spacers were installed in the attic between the joists and the array was attached to these boards to prevent system damage during the hurricanes that may occur along the coast of South Carolina. The mechanical support structure was placed in pitch pans to reduce the possibilities of moisture penetration and to facilitate replacement of the roofing material. The array conductors were secured to the module junction boxes with strain relief connectors. Interconnecting wires were tied to the back of the modules to prevent chafing against the roof. Anodized aluminum was used for all metal supports to prevent corrosion in the humid climate. The array conductors were run in conduit to the battery room and inverter area in the attic space. The enclosures for the flooded-cell batteries were vented to the outside. The array and all equipment were grounded to a copper rod beneath the house. The negative conductor of the dc circuits and the neutral conductor of the ac circuits were connected to this same ground. All ungrounded conductors in both ac and dc circuits were fused. Lightning arresters were installed on all ungrounded conductors. A set of schematic drawings and an owners manual was provided as well as a battery maintenance kit including maintenance procedures, electrolyte replenishment container, hydrometer, and battery terminal corrosion inhibitor.
Worksheet 4-Calculate System Array Size
Worksheet 1HY-Calculate Battery Capacity
AC Wire Sizing Specification
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