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Introduction System types On-grid or off-grid? Hybrid energy systems Analyzing Your Loads

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Hybrid Power Systems

According to many renewable energy experts, a stand-alone “hybrid” system that combines generation sources, such as wind and PV, offers several advantages over a single generation system. In much of the United States, wind speeds are low in the summer when the sun shines brightest and longest. The wind is strong in the winter when there is less sunlight available. Because the peak operating times for wind and PV occur at different times of the day and year, hybrid systems are more likely to produce power when you need it.

For the times when neither the wind generator nor the PV modules are producing electricity (for example, at night when the wind is not blowing), most stand-alone systems provide power through batteries and/or an engine-generator powered by fossil fuels.

If the batteries run low, the engine-generator can be run at full power until the batteries are charged. Adding a fossil-fuelpowered generator makes the system more complex, but modern electronic controllers can operate these complex systems automatically.

Adding an engine-generator can also reduce the number of PV modules and batteries in the system. Keep in mind that the storage capability must be large enough to supply electrical needs during noncharging periods. Battery banks are typically sized for one to three days of operation.

Balance-of-System (BOS) Equipment

In addition to wind turbines, PV modules, or a microhydropower generator,you must purchase BOS equipment. This may include battery charge controllers,batteries, inverters, wires, conduit, a grounding circuit, fuses, safety disconnects,outlets, metal structures for supporting the PV modules, and any additional components that are part of the system.

In very small systems, DC appliances operate directly off the batteries. If youwant to use standard appliances that require conventional household alternating current (AC), however, you mustinstall an inverter to convert DC electricity to AC. Although the inverter slightly lowers the overall efficiency of the system,it allows the home to be wired for AC, a definite plus with lenders, electrical code officials, and future home buyers.We’ll discuss BOS configurations first for loads requiring direct current, then for loads needing alternating current.In grid-connected systems, the only additional equipment needed is an inverter that makes the turbine outputelectrically compatible with the utility grid. No batteries are needed. Work with the manufacturer and your local utilityon this process.

When examining the costs of wind turbines, PV modules, or microhydropower generators, remember that thesecosts do not include the cost of BOS equipment.

Direct-Current System Equipment

Battery. In off-grid systems, the battery stores electricity for use at night orfor meeting loads during the day when the generation source (wind turbines, PV, or microhydropower) is not generatingsufficient power to meet load requirements. To provide electricity over long periods, renewable systems requiredeep-cycle batteries. These batteries, usually lead-acid, are designed to gradually discharge and recharge 80% of theircapacity hundreds of times. Automotive batteries are shallow-cycle batteries and should not be used in renewable systemsbecause they are designed to discharge only about 20% of their capacity. If drawn much below 20% capacity morethan a few dozen times, the battery will be damaged and will no longer be able to take a charge.

The cost of deep-cycle batteries depends on the type, capacity (amperehours), the climatic conditions in whichit will operate, how frequently it will receive maintenance, and the types of chemicals it uses to store and releaseelectricity. An off-grid PV or wind system may have to be sized to store a sufficient amount of power in the batteries to meetpower demand during several days of cloudy weather or low winds. This is known as “days of autonomy.” Consultyour dealer before selecting batteries for your system.

Most types of batteries contain toxic materials that may pose serious healthand safety problems. The National Electric Code, battery companies, and renewable system designers recommend that leadacid and wet-cell batteries, which give off explosive hydrogen gas when recharging, be located in awell-ventilated space isolated from the other electrical components of the system and away from living spaces. Also, allow enough space for easy access during maintenance, repair, and replacement. Most importantly, maintain the battery according to the manufacturer’s instructions, and recycle the batteries properly when they wear out.

Charge Controller. The charge controller regulates the flow of electricity from the generation source to the battery and the load. The controller keeps the battery fully charged without overcharging it. When the load is drawingpower, the controller allows the charge to flow from the generation source into the battery, the load, or both. When thecontroller senses that the battery is fully charged, it stops the flow of the charge from the generation source. Many controllerswill also sense when loads have taken too much electricity from batteries and will stop the flow until sufficientcharge is restored to the batteries. This last feature can greatly extend the battery’s lifetime.

The cost of controllers generally depends on the ampere capacity at which your renewable system will operate andthe monitoring features you want.

Alternating-Current System

Equipment

Inverter. Alternating-current (AC) systems also require an inverter, whichchanges the DC electricity produced by renewable systems and stored in batteriesinto AC electricity. Different types of inverters produce a different “quality” of electricity. For example, lights, televisions,and power tools can operate on lower-quality electricity, but computersand other sophisticated electronic equipment require the highest-quality electricity. So, you must match thepower quality required by your loads with the power quality produced by the inverter.

Inverters for most stand-alone applications (i.e., those systems not connectedto the utility grid) cost less than $1 per rated output watt. The cost is affected by several factors, including thequality of the electricity it needs to produce; whether the incoming DC voltage is 12, 24, 36, or 48 volts; the number ofAC watts your loads require when they are operating normally; the amount of extra surge power your AC loads needfor short periods; and whether the inverter has any additional features such as meters and indicator lights.

Tell your dealer if you plan to add additional AC loads in the future. If you are considering building another roomonto your house or adding electrical loads, consider purchasing an inverter with a larger input and output ratingthan you currently need. This may be less costly than replacing it with a larger one later.

For more about sizing an inverter, see http://www.nrel.gov/buildings/pv/.

Selecting a Dealer

Your dealer can help you analyze your loads and size your system correctly.The dealer will install your equipment, making sure that it runs properly. Dealers will also have contacts at the localutility if you are connecting your system to the utility grid.With any major purchase, you must have confidence in the dealer’s products and services. Becoming an informed consumerwill help you feel more confident in your choices. With the growth of the renewable energy industry, the numberof regional dealers, mail-order and ecommerce businesses, and local distributors has rapidly expanded. Manytelephone directories contain listings for dealers under the “solar” heading. After you identify dealers, you will want to dosome research to learn more about them.

Professional credentials are one indication of a dealer’s knowledge and qualifications. Ask dealers about theirtraining, certifications, and licenses. A second consideration is the dealer’s experience in the field. How long has the company been in business? Your local Better Business Bureau can advise you whether any customers have registeredcomplaints about the dealer. You should also ask the dealer how many systems like yours he or she has designed andinstalled. Ask for references, and to speak with owners of systems similar to the one you want to purchase.A third consideration in selecting a system installer is the variety and quality of products offered for each componentof the system. Because PV systems are often designed for a specific site, one company’s products may not be appropriatefor all applications. Competent dealers will stock components manufactured by several companies. A variety ofproduct options will help ensure that the most appropriate components are available for your system. When a dealer recommends a product, ask what the recommendation is based on, whether there are consumer or independenttesting facility reports you can read, and whether the products are listed with Underwriters Laboratories.Also, consider the service agreements and performance guarantees the dealer provides and the warranties given by theproduct manufacturers. No system is maintenance-free, nor will all components function flawlessly forever. Whenproblems emerge with your system, what services will the dealer provide? What warranties do the manufacturers provide?What costs should you expect to pay, and which costs will be assumed by the dealer and/or the manufacturer?Finally, you should compare prices from different dealers. Because distribution channels and dealer networks haveexpanded dramatically, the opportunity to “shop around” is much greater today.

If possible, approach more than one dealer about a draft design and cost estimate for your system.

С целью увеличения продолжительности работы системы в автономном режиме, система бесперебойного электропитания помимо АБ и преобразовательного блока содержит еще один или несколько разновидностей ВИЭ. В качестве ВИЭ используются вырабатывающие электричество: солнечные батареи (СБ), ветроэлектрические установки (ВЭУ) и термоэлектрические генераторы (ТЭГ). ВИЭ подключаются к АБ через контроллер заряда, защищающий АБ от перезаряда.

ВЭУ состоит из 3-х лопастного ветроколеса, повышающего редуктора, электрического генератора, мачты с растяжками высотой от 6 до 18 м. Мощность ВЭУ пропорциональна квадрату диаметра ветроколеса и определяется мощностью электрического генератора. Номинальную мощность ВЭУ достигает при ветре около 10 м/с. По ветровым условиям в Средней России, за лето ВЭУ вырабатывает менее 20% количества электроэнергии от своего годового потенциала. Зато в остальное время года ВЭУ работает эффективнее СБ. В Московской области, где среднегодовая скорость ветра 3м/с, ВЭУ вырабатывает 10-15% от указанного производителем номинального количества годовой электроэнергии. Например, ВЭУ мощность 1 кВт за год выработает не 8760 кВт.ч, а лишь 876-1314 кВт.ч. Стоимость ВЭУ мощностью 1 кВт составляет около 1200 у.е. и зависит от состава комплекта.

Термоэлектрический генератор (ТЭГ) состоит из термоэлектрической батареи, рассчитанной на напряжение 12 В, системы нагрева горячих спаев и охлаждения холодных спаев, обеспечивающей на батареи перепад температуры 2500С. При этом батарея площадью 75 х 75 мм2 вырабатывает 12-15 Вт. Пропорционально выбранной площади растет мощность ТЭГ. Нагрев может происходить за счет сгорания газообразного (биогаз), жидкого (рапсовое масло) или твердого топлива (древесина). В домашних условиях целесообразно использовать печь с тлеющим, медленным режимом горения имеющую КПД > 80%, в которую вмонтирован ТЭГ с системой водяного охлаждения. Нагретая вода в этой системе используется для бытовых целей.
В тлеющем режиме работы печи ТЭГ мощностью 15 Вт вырабатывает до 120 Вт.ч/сутки. В режиме длительного горения (6-8 ч) печь помимо обогрева помещения позволяет греть воду, вырабатывать электричество и готовить пищу. Стоимость ТЭГ без печи составляет 140 у.е. Стоимость печи зависит от мощности и составляет около 100 у.е. при тепловой мощности 3 кВт.

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