Renewable Power and Energy, Volume I

Photovoltaic Systems

Gary D. Price

• Editorial: Momentum Press
• Año: 2018
• ISBN:
  9781947083875
• Categoría: Tecnologia e ingegneria
• Categoría: Risorse energetiche rinnovabili

Photovoltaic power systems are becoming a significant source of energy in our energy resource mix today. It is essential these systems are reliable, safe and secure.

Precise engineering design is required to insure these new power systems meet these requirements. In particular, interconnected systems with existing utility power systems must operate in synchronism and improve overall quality of the electrical power grid.

This book is intended to identify and explain engineering procedures for the design and operation of photovoltaic systems. It includes a review of conventional electrical power systems as implemented in the United States and common to all electrical systems throughout the world and introduces other types of renewable energy systems. The heart of the book is focused on the design of interconnected and stand-alone PV systems–battery storage is becoming an integral part of PV systems, and a significant portion of the text is dedicated to energy storage for stand-alone and back-up power systems. The author also highlights how economics and structural considerations are an essential part of the engineering design process.

CHAPTER 1

CONVENTIONAL ELECTRIC POWER SYSTEMS

Power and energy are important concepts that must be understood before beginning to explore the application of renewable energy systems. For example, a common misunderstanding is the confusion between watt and watt-hour. Watt is a power term, and watt-hour is an energy term. Adding to the confusion are shortcuts used by the power industry professionals who frequently use the power term megawatt when they really mean megawatt-hour (energy). A utility energy trader will almost universally use the term megawatt when buying or selling a megawatt-hour of energy. We begin this book with definitions and the relationship between energy and power, as used by electrical power engineers.

1.1 POWER ENGINEERING CONCEPTS AND TERMINOLOGY

Power is defined as the time rate at which work is done or energy emitted or transferred. Energy is the capacity for doing work. Watt is a unit of power equal to 1 joule per second—or in electric terms, 1 ampere under the pressure of 1 volt ( joule is the unit of mechanical power equal to a meter-kilogram-second or 0.7375 foot pounds). The important fact to remember from these definitions is that power is a rate, and energy is a quantity. Therefore a watt (w) is the rate at which energy is being produced or transferred. A watt-hour (wh) is the quantity of energy transferred or produced.

Electrical power equipment is typically rated in terms of watts. A 100-watt light bulb will use 100 watt-hours of energy when operated for 1 hour at 1 amp and 100 volts. A photovoltaic (PV) panel rated at 100 watts will generate 100 watt-hours in 1 hour when operated at a specified solar intensity and connected to an appropriate load. If these devices operate for 6 minutes, the resulting energy will be 10 watt-hours. A kilowatt (kW) is 1,000 watts, a megawatt (MW) is 10⁶ watts, and a gigawatt (GW) is 10⁹ watts.

Power plant generators usually are operated at a constant power near the nameplate rating. This makes it simpler for dispatch operators to communicate by referring to 100 MW and assuming the generator will operate at 100 MW for 1 hour to produce a 100-MWh block of energy. We cannot make that assumption with PV systems or wind generators because the output power is variable. Wind and solar power must be integrated over time to determine energy.

The British thermal unit (BTU) is a common measure of energy. One BTU is equal to the energy required to heat 1 pound (lb) of water 1 degree Fahrenheit (1°F). A BTU is also equal to 0.293 watt-hours (wh), and 3,412 BTUs equal 1 kWh. The BTU provides a useful conversion between electrical and solar thermal systems. The gas industry commonly uses the acronym MBTU to represent 1,000 BTUs and MMBTU to represent 1,000,000 BTUs. (The M is derived from the Roman numeral value.) This textbook will avoid using MBTU, as it causes frequent calculation errors and confusion. K represents 1,000 and M equals 1,000,000, as used in kW and MW.

A common energy term used in the natural gas industry is the therm. One therm equals 100,000 BTUs and represents the heat or energy content of natural gas. A cubic foot (cu ft) of natural gas contains approximately 0.9 therms of energy. The energy content of natural gas is not consistent, depending on the chemical make-up of natural gas. Natural gas consists of several components: primarily methane, heptane, and propane. Small amounts of nitrogen, carbon dioxide, oxygen, and an odorant are also present in natural gas. The energy content varies from source to source. The energy content also depends on the delivery pressure. The price of natural gas that shows up in our utility bill is usually expressed in dollars per therm.

As aforementioned, a joule is equal to 0.7375 foot-pounds (ft-lbs), and a joule per second is equal to 1 watt. From these relationships, we can determine that 1 kWh of energy is equal to 2,655,000 ft-lbs. If a 150-pound person climbs a 17,700-foot mountain, the amount of energy expended is about 1 kWh! Most of us use that amount of energy for home lighting every day! Another reference to the value of 1 kWh is the content of energy in petroleum: approximately 3 ounces of oil contains 1 kWh of energy if we assume 1 gallon of oil contains 143,300 BTUs. By comparison, if we install 2 square meters (m²) of PV on our roof, the array will generate about 1 kWh of electricity per day.¹

Demand is defined as the amount of work to perform a desired function. In the electrical world, the term demand is used to measure the peak power required to operate all connected loads for a particular circuit. If a circuit consists of five 100-watt light bulbs, the peak demand is 500 watts. Demand is determined when the load is at maximum, or when all five lights are on. As utilities are concerned with the maximum load on their system, they measure peak energy in a specified time interval for demand. An instantaneous value is not as significant as the average value during this time interval, usually 15 minutes. For example, if the power on a circuit ranges from 5 kW to 15 kW during the 15-minute interval, the demand will be about 10 kW. The average value of 10 kW is more significant than the 15 kW peak value, because it more accurately represents the load that the generators must meet.

Capacity is the instantaneous ability to provide energy required to do work. Again, the electrical use of the term capacity usually refers to the size of the generator(s) required to maintain a circuit load. A 100-MW generator will provide the energy required to operate 100 MW of peak load. Capacity is also used to define the load capability of other electrical equipment on the system. A 115-kilovolt (kV) transmission line may safely transfer 100 MW of power without overheating and sagging. A 100-megavolt-ampere (MVA) transformer may safely transform 90 MW of power before it overheats. The safe operating rating of the equipment defines its capacity.

Using the electrical definitions of capacity and demand, energy can be defined as the demand times time-in-use (D × t), or capacity times time-in-use (C × t). These definitions are used when calculating the total consumption. For example, the United States consumed about 4 trillion kWh of energy in 2005.² That equates to about 13 MWh per year per person. The peak electrical demand for the United States is about 760 GW.

When scientists talk about global or national energy use, the term quad is frequently used. A quad is equal to one quadrillion BTUs. A quad is also approximately equal to the energy stored in 180 million barrels of crude oil. One quad equals 293 billion kWh. The total global primary energy production in 2004 was 446 quads.

Avoided costs are the incremental costs to an electric utility to generate electrical energy. These costs generally include fuel costs for generation, operational costs of transmission, and operational distribution costs per kWh. Fixed costs (e.g., capital equipment improvements, and franchise fees) are not included in avoided costs.

Net metering generally refers to buying and selling energy at the same rate. Net metering applies to small (<10 kW) systems, where the financial impact is relatively negligible to the utility. Under net metering rules, a renewable energy (RE) system is allowed to sell its generated energy at the same rate it is buying energy from the utility. However, utilities and public utility commissions set rules for net metering, which vary by jurisdiction. The utility may require the renewable energy entity to pay avoided costs for excess energy generated each month. More commonly, the utility will pay avoided costs for excess generation at the end of the year. Xcel Energy currently has an option to bank excess energy indefinitely. We will discuss the financial details of net metering in the economics chapter....