Chapter 6 Solar Energy

Objectives State why solar energy is one of the long term options for energy independence. Describe 3 basic types of active solar collectors. Give examples of 3 types of passive solar collectors. Perform calculations to determine the payback period of solar collecting equipment.

Collecting Solar Energy The current methods used to collect the sun’s energy are insufficient and expensive compared to other energy sources. It is also difficult to store solar energy. Solar research began to fade in the 1980s as tax credits for the installation of solar devices expired and were not renewed.

Solar Energy Collection Concepts Active solar energy collection systems use circulating pumps and fans to collect and distribute heat. Passive solar energy collection techniques do not make use of any externally powered, moving parts, such as circulation pumps, to move heated water or air.

Open loop solar collection In an open loop solar collection system, the heated water or air is directly distributed for use. Any heat collected will result in less consumption of conventional energy to heat the water to optimum temperature. Both energy and money will be saved.

Closed loop solar collection In a closed loop solar collection system, a collection medium is used to collect the heat and transfer it to water or air for end use. The liquid in the closed loop simply circulates from the collector to the heat exchange tank and then back up to the collector to collect more heat. It never comes in direct contact with the water being heated for end use in your home. A closed loop system can use water-alcohol antifreeze solution to avoid freezing.

Types of Active Solar Energy Collection 1. Flat Plate Collectors: The flat plate collector is typically stationary, mounted on a rooftop, facing south in the northern hemisphere. Simplicity and reliability are distinct advantages of a flat plate collector. Flat plate collector is the most popular collector even though it has a relatively low collection ratio for the space it occupies. Light colors tend to reflect light and dark colors tend to absorb light and reradiate it in the form of heat.

Types of Active Solar Energy Collection 2. Linear Concentrating Parabolic Collectors: Parabolic collectors track the sun from east to west in what is known as the azimuth path (fig 6-10, page 129). This type of collector collects much more energy than a flat plate collector while occupying less space. 3. Parabolic Dish Collectors: this type of collector has tremendous collection ratio. It requires a sophisticated tracking mechanism. It is used in some countries along the equator to collect heat for the production of steam to generate electricity. It is not for residential use.

Types of Passive Solar Collection 1. The direct gain approach: Basic requirements of passive solar architecture are good southern exposure, and very few windows facing north (fig 6-13, page 131). Another concept is to plant deciduous trees that maintain their leaves through the warm months and shed their leaves annually each fall or winter.

Types of Passive Solar Collection 2. The indirect gain approach: makes use of a storage medium to store heat for later use. The storage medium may be rocks, concrete, or water. 3. The isolation gain approach: the solar collector is typically located next to or beneath the home. The system relies on convection to carry heated air up to the structure (fig 6-15, page 134).

Calculating the Payback Period The payback period is also called the breakeven point. Step 1: Determine how much energy is needed to heat the water for 1 day. Step 2: Determine how much it will cost to heat the water for 1 year using conventional energy source. Step 3: Determine what % of hot water will be saved using a solar collection system – 40% for Midwest. Step 4: Determine dollars saved by installing a solar collection system. Step 5: Determine the payback period for solar collection system installation. The example in the text has a payback period of 16 years (page ). Generally, payback period should be less than 10 years. In Albuquerque, NM the payback period would be 8 years which would be acceptable. Remember to factor in maintenance costs and life cycle renewal of parts.

Photovoltaic Cells Photovoltaic cells emit electrons when exposed to light which results in the flow of electricity. Since the electricity produced by photovoltaic cells is direct current (DC) it has to be inverted to alternating current (AC) for residential or commercial use. Solar panels can also be used for cooling purposes.

Summary The current methods used to collect the sun’s energy are insufficient and expensive compared to other energy sources. Active solar energy collection systems use circulating pumps and fans to collect and distribute heat. Passive solar energy collection techniques do not make use of any externally powered, moving parts, such as circulation pumps, to move heated water or air. In a closed loop solar collection system, a collection medium is used to collect the heat and transfer it to water or air for end use. A closed loop system can use water-alcohol antifreeze solution to avoid freezing. Flat Plate Collectors: The flat plate collector is typically stationary, mounted on a rooftop, facing south in the northern hemisphere. Simplicity and reliability are distinct advantages of a flat plate collector. Photovoltaic cells emit electrons when exposed to light which results in the flow of electricity.

Home Work 1. What is the main difference between active and passive solar collection techniques? 2. Give an advantage of a closed loop system? 3. What are the advantages of a flat plate collector?