unit 4 solar energy heat

8/14/2019 Unit 4 Solar Energy Heat

1/46

Introduction to Alternative EnergiesIntroduction to Alternative EnergiesUnit 4Unit 4 Solar Energy (Heat)Solar Energy (Heat)

1


2/46

The enormous amount ofsolar energyavailable today is used primarily for electricity

or heat

From the Burgan article Is There Enough Solar

Energy in O io Ohio receives 1.4 megawatt hours per square

meter (MWh/m2) in an average year

Annually residents of Ohio use 4.5 MWh for

electricity and 7.6 MWh for heat

2


3/46

Is There Enough Solar Energy in Ohio? Solar energy collection systems currently have

overall efficiencies of about 15% for electricity

and 70% for heat Taking into account these efficiencies, each

erson would re uire 21.4 s uare meters for

electricity and 7.8 square meters for heat

This unit covers the systems used for solar heatwith the next unit covering solar electricity orsolar photovoltaic

3


4/46

After completing this unit you will

Be aware of the amount of energy available

from the sun

Be able to explain how the potential energy

Be able to determine the amount of energy

from solar collectors

Be able to explain other methods of acquiring

energy from the sun on a larger scale

4


5/46

What is the

amount of energyavailable from the

5


6/46

Insolation

The power density of solar radiation is

referred to as Insolation

This power density is about 1360 watts/m-2, a

The solar constantvaries throughout the year

depending on the earths distance to the sun,

and the fact that not all of this energy makes itto the earths surface

6


7/46

A minimal amount of energy is absorbed bythe upper atmosphere and a large portion is

absorbed by the ozone

We need to look at the power density at the

Earths surface or at sea level

T e constant or t is va ue, power ensity atsea level, is 1000 W m-2 also referred to as

one sun

This value is when the sun is perpendicular tothe Earths surface on a perfectly clear day. As

the sun moves, this value is reduced7


8/46

Insolation depends on two items1. Theposition of the sun, given by two

variables

a. Thezenith angle which is the angle between aline created from the observer and the sun andthe local vertical

b. The azimuth which is measured clockwise fromthe north

Both of these variables are functions of the local time ofday, the day of the year, and the latitude of the observer

2. And the transparency of the atmosphere

8


9/46

There are calculations to determinethe insolation of different areasand times for optimum solar

energy For general purposes though, many

of these calculations have been

done for major cities in the U.S.which can be found in varioustables and maps

These typically will give theminimum, maximum, and averageinsolation for the city or area

9


10/46

Average Insolation of the United States

www.goldenstateenergy.com/images/Ridge3.jpg

10


11/46


12/46

How can the

potential energy ofthe sun be used for

12


13/46

Solar Collectors

With the solar energy available as shown, it

must be collected for practical use

Different methods of collecting solar energy

In order to achieve the maximum energy and

efficiencies, the proper architecture is

required There are many variables in solar architecture;

following are some of the more important13


14/46

Exposure controlis needed to optimizethe amount of sunlight received

Building orientation, therefore, must be in

such a manner to conform to localinsolation conditions

Also with the site location trees can be

used to shade the building from heat in thesummer, yet when leaves fall in the cooler

seasons allow more sunlight to hit the

building Along with trees, shrubs and other natural

plants can be used to insulate the building

14


15/46

Heat storage is needed to retain excessheat received during the day for use

through the evening hours, a different

day, or even a different location Different mechanisms could be used for this

structure or a roof pond

These mechanisms need not be complex as

noted by the example in the text of using

stacks of soda cans full of water

15


16/46

With the storage of solar energy, ameans ofcirculation is also needed

Heat transfer can be accomplished naturally

through convection currents with the use ofvents

Circulation can also be accom lished b

simple economic pumping systems

It is also important to have circulation of

fresh air in the system to remove

undesirable gases and noxious chemicalsthat may accumulate in the building

16


17/46

Insulation must also be considered With the storage and circulation of energy

or heat, you need to take precautions not to

lose any from radiation The efficiency of a system is relevant to the

amount of heat or energy supplied to the

Therefore, insulation is used to ensure thatthe heat generated by the solar powerremains in the system, not radiating to the

atmosphere The insulation of a system is based on the

thermal conductivity of the materials used

17


18/46

Flat collectors Primarily for low

temperature heat

used for a residence,hot water systemsand swimming pools

The important factorin using these is tohave a small

temperature changefor a large volume ofwater

18


19/46

Flat collectors Simple in design composed of

an insulated shallow box

In the box is an absorberplate with coolant channelsbuilt in for fluid to flow

The area below the absorbingplate is insulated to reduceheat losses

Above the absorber plate isan air gap and then a glassplate sealing the box

19


20/46

Solar radiation goes through the glass into theabsorber plate heating the flowing fluid

20


21/46

Evacuated tubes Conceptually work

the same as flat

collectors with the

difference in the

These tubes are

usually placed in an

array with areflective surface

behind them21


22/46

Evacuated tubes Comprised of two concentric

tubes, an inner tube in which

the fluid flows and an outer

tube made of glass or similar

There is an air gap between the

glass tube and the fluid tube

allowing the solar radiation toheat the fluid and insulate from

heat loss due to the air gap22


23/46

A diagram of an evacuated tube solar collector

www.hillssolar.com.au

23


24/46

How do we

determine the

amount of energy

collectors

24


25/46

To determine the amount ofenergy from a solar collectorsystem is relatively simple

You will need the following

a. The average insolation foryour area

b. The absorber area of the

solar collectorc. The efficiencyof the solar

collector

25


26/46

The average insolation As previously mentioned this can be found

from various internet sites such as: http://www.solar4power.com/solar-power-insolation-window.html

http://www.sundancepower.com/pdf/solarInsolation.pdf

From the earlierexample, you findthe average daily

insolation forColumbus Ohio tobe 4.15 kWh/m2

26


27/46

The Absorber Area Not the surface area of the collector itself but

of the effective absorbing area

Varies depending on the type of collector, flat

or evacuated tube

Will typically be found in the modelspecifications for a given solar collector as seen

on the web page:

http://www.apricus.com/html/solar_collector_technical_info.htm

From Apricus, a manufacturer of both flat and

evacuated tube collectors

27


28/46

The Absorber Area From the Apricus

web page

For model AP-20

the absorber area

is 1.6 m2

28


29/46

Efficiency The rated efficiencyfrom the supplier may be

listed at more than 90%

This efficiency is not practical though, as thereare losses due to weather and the fact of the

For estimating, use the following efficiencies

60% for cold weather days

70% for hot weather days 80% for Maximum Power Point Tracking

(MPPT) collectors

29


30/46

To roughly determine the availablepower(kWh) of a solar collector system, use the

following equation

Output (kWh)= IxA x e x n

Where :I is the average daily insolation (kWh/m2)

A is the Absorber area (m2)

e is the efficiency in decimal form, 75% = 0.75

n is the number of collectors in the system

30


31/46

For exampleLiving in Columbus, Ohio, what amount of

energy per day, on the average, could you get

from a system using (5) of the Apricus, modelAP-20 solar collectors using efficiency of 65%

Output (kWh) = IxA x e x n

= 4.15 x 1.6 x 0.65 x 5

= 21.6 kWh

Or annually21.6x365 days = 7884 kWh31


32/46

What are other

methods of

acquiring energy

larger scale

32


33/46

Concentrators

General solar

collectors are usedfor low temperature

volumes of fluid

Higher temperature

differentials requirethe use of

concentrators

33


34/46

Used to focus the solar radiationachieving greater temperature

differentials

Used for applications such aselectricity generators

Basica y a system to ocus t e so arradiation to a given point similar to

using a magnifying glass to focus

the sunlight to a fine point burninga hole in paper

34


35/46

There are two different types of concentrators1. A 2-D focuses the light into a line

2. A 3-D focuses the light into apoint

And two different parameters to consider

1. The concentration can be defined as either the

ra o o e aper ure area o e rece ver area or

the ratio of the power density of the aperture to

the receiver

2. The acceptance angle is the angle through which

the system can be misaligned and still achieve

the desired result

35


36/46

Solar Plant Configurations With the enormous amount of free energy

in solar radiation, there has been considerableresearch in different configurations of facilitiesfor power generation

One type of plant is the high temperaturesolar heat engine

A relatively simple method that uses solar

energy through concentrators developing hightemperatures to drive steam generators

36


37/46

The basic concept of a solar plant shownschematically

37


38/46


39/46

Compared to the cost of $1000/kWfor fossil fuel plants, this seems

very expensive

The important thing to notethough is that there is no cost for

Therefore, the only cost is the

initial build and maintenance of

the facility Only time will tell if this is a viable

source of power39


40/46

There has already been a second plant, SolarTwo, built with the same power output ratings

as Solar One

The cost of this plant

.

Bringing the cost of

power to around

$4000/kW

40


41/46


42/46

The air under the tent is heated from thegreenhouse effect and rises up through the

chimney

As the heatedair flows up

,

it drives a

wind turbine

generatingelectricitycommons.wikimedia.org

42


43/46

In early models of anactual solar chimney,the cost to produceelectricity was alsoaround $4000/kW

As technology

improves, this cost willmost certainlydecrease making this avery viable possibilityfor future electricitygeneration

43


44/46

Solar Ponds A very simple concept and although very low

efficiency, again, the fuel is free as with othersolar plant configurations

that occurs in shallow ponds reducing the

temperature gradient and dirt and growth

that develops

44


45/46

One possible solution to overcome thesedifficulties is to have some sort of a cover

for the pond, similar to a pool cover

With large ponds, where a plastic cover maynot be practical there have been some studies

would float on the top of the pond

In order for the gel to be applicable, it must be

transparent, stable under ultraviolet radiation,insoluble from water, non toxic, and

inexpensive

45


46/46

Work CitedDa Rosa, A. V. (2005). Fundamentals of Renewable Energy Processes. Burlington, MA,

USA: Elsevier Inc.

Is There Enough Solar Energy in Ohio?, www.GreenEnergyOhio.org, 22-23

- - -

http://www.sundancepower.com

46

unit 4 solar energy heat

Documents