TECH 581 – Solar Energy Systems Summer 2009 Module 2-5 – Solar Thermal Applied

Solar Heating/Cooling/Dehumidifier Systems

Energy for heating buildings and hot water consumes approx. 1/4 th of the annual energy production in USA.

Please read Sec. 5.1 for the calculation of heating and hot water loads in buildings.

Solar Water Heating Systems

Basically two types:

a) Thermo-syphon - passive systems (natural circulation): in non-freezing climates

b) Active systems (forced circulation): in freezing climates and commercial and industrial process heat.

Solar Energy Systems/ J.P. Agrawal / M2_5- page 2 Thermo-syphon (Natural Circulation) Systems

The less dense fluid rises above a more denser fluid. This phenomenon can be utilized to create fluid motion in a collector. As water is heated in the collector, it rises to the tank, and the colder water in the tank moves to the bottom of the collector.

Solar Energy Systems/ J.P. Agrawal / M2_5- page 3 Natural Circulation Systems…

LHggdxxgh

PP

Out

L

Stor

BouyantFlow

0

)(

H: Height of the legsL: Height of the collector(x): local collector fluid densityStor : tank fluid densityOut : collector outlet fluid density

PFlow : Flow pressure drop fluid loop, due to friction in tubings and joints

PBouyant : Bouyant force “pressure” in the Caused by the differing fluid densities of the hot and cold legs of the fluid loop.

2)( KVhhdP averageloopLaverageloop

Loop

LFlow

Where the pressure drop in the loop is:

K: Sum of the component loss velocity factors in the fluid loopV: Flow velocity

Solar Energy Systems/ J.P. Agrawal / M2_5- page 4

• Generally, the thermosyphon systems do not use a pump, therefore, the plumbing fixtures are selected to be more than normal (one size larger is sufficient) to reduce the friction losses.

• The hot water system loads change very little during a year. Hence, the tilt angle is made equal to the latitude, = Latitude angle.

• Tempwater outlet -Tempwater inlet 8-11C during the middle of a sunny day.

• To prevent the reverse flow in the night time, the top of the collector must be at least 30 cm below the cold leg fitting on the storage tank., else a check valve must be used.

• An electric heater is used as the back-up during the cloudy days, located near the top of the tank. This location helps in creating and enhancing the temperature stratification inside the tank.

Compact model tank and collectorassembly

Solar Energy Systems/ J.P. Agrawal / M2_5- page 5 Example:Determine the “pressure difference” available for a thermo-syphon system with a 1 m high collector and 2 m high legs. The water temperature input to the collector is 25C and the collector output temp is 35C. if the overall system velocity factor K is 15.6, estimate the system flow velocity.Solution:

Solar Energy Systems/ J.P. Agrawal / M2_5- page 6

Solar Energy Systems/ J.P. Agrawal / M2_5- page 7 Forced -Circulation Systems

1. Open loop 2. Closed loop3. Closed loop with drainback

Open LoopThe solar loop is at the atmospheric

pressure, therefore, the collectors are empty when they are not providing useful heat.

Disadvantage: high pumping power is required to pump the water into the collector when it becomes too hot.

Solar Energy Systems/ J.P. Agrawal / M2_5- page 8

Closed LoopThe solar loop remains filled with water

under pressure.The pump has to overcome only the

resistance of pipes.The solar loop consists of a small 2 gallon

tank to allow thermal expansion of water from heating, and a pressure relief valve.

Because water always stays in the collector of the system, antifreeze (propylene glycol or ethylene glycol) is required in freezing conditions.

During the summer months, the temperature in the collector can become very high, leading to high pressure in the loop. This can cause leaks in the loop unless some fluid is allowed to escape through a pressure-release valve (achieved in the system with closed loop with drainback.

Please read more details on page 227 of the text.

Solar Energy Systems/ J.P. Agrawal / M2_5- page 9 Solar Air-Heating SystemsApplication: space heating and heating in small building complexes

Please see Table 5.6 for advantages and disadvantages of air v. liquid heating systems.

Mainly, the air systems do not have freezing, pipe corrosion, no leak or pressure problems, require no heat exchanger between the collector-storage and storage build-up loops.

Flat-plate collectors are ideally suited for the air heating systems.

Solar Energy Systems/ J.P. Agrawal / M2_5- page 10

Please read the details in Chapter 4 of the text.

Solar Energy Systems/ J.P. Agrawal / M2_5- page 11 Solar Cooling SystemsThe seasonal variation of solar energy is extremely well suited to the space-cooling requirements of buildings. The highest solar insolation occurs in the warmest seasons of the year when the demand for comfort cooling and refrigeration are the highest.

The cooling system consists of a refrigerant that cycles through major components:

a) compressor, b) condenser, c) expansion valve and d) evaporator. The refrigerant of choice is

Chlorofluorocarbon (CFC)

The refrigerant enters the compressor as a cold low-pressure vapor.Compressor pumps the vapor to high pressure. When compressed, the temperature and pressure of

the vapor increases.The hot vapor enters condenser coil, where it transfers the heat to the environment (air/water) in the

right (condenser) chamber. The hot vapor releases latent heat and converts to liquid.The liquid refrigerant then passes through an expansion valve, pressure reduces and the resulting

low-pressure mix of liquid and vapor refrigerant enters the evaporator coil. The refrigerant vaporizes in the evaporator coil by taking the heat from the environment (air/water) in

the left (evaporator) chamber.

Solar Energy Systems/ J.P. Agrawal / M2_5- page 12

The basic refrigeration cycle can be used to do heating as well as cooling.

Heat pumps have the ability to reverse.

The W is provided from the solar thermal system.

1

11

RHP

LLHHP

COPCOPW

Q

W

WQ

W

QCOP

The efficiency of the refrigeration cycle is calculated as,

W

Q

inputWork

outputDesiredCOP L

R

Solar Energy Systems/ J.P. Agrawal / M2_5- page 13 Absorption CoolingThe mechanical compressor in the conventional vapor compression system is replaced with components that perform the same function but do so thermochemically.

A refrigerant like water vapor or lithium bromide (LiBr) from the (thermochemical) generator enters thecondensor under high pressure. The vapor changes state to liquid in the condenser, releases heat to the environment. The condenser and evaporator combination works a in the conventional system. The refrigerant entering the absorber is re-pressurized using heat rather than the compressor. The absorber contains lithium bromide (LiBr) into which the water vapor dissolves. A very small pump shuttles the water_LiBr solution from the absorber to the generator. The heat from the solar thermal pressurizes the dissolved water vapor and drives it out of the solution. The LiBr returns to the absorber.

Gen

LR Q

QCOP

Solar Energy Systems/ J.P. Agrawal / M2_5- page 14