Multi-Loop Systems, 4 Closed-Loop Drainback Systems · Drainback systems include a separate tank...

© Festo Didactic 52669-20 41

Drainback systems can consist of two closed-loop circuits. Each loop contains an active pump to circulate the solar and storage fluids independently. These systems can be used for heating water and space (air and/or surfaces).

When solar energy is available, the contents of a small storage tank, called a drainback tank are heated by the primary system loop. When the tank reaches the differential controller’s temperature setting, the circulator pump is turned off, and the water in the solar collector array drains backwards through the system into the drainback tank. For this reason, a check valve should not be installed in a drainback system loop. The system is simplified further by its low-pressure operation. An external heat exchanger (or an integrated one inside the tank), extracts heat from the drainback tank, as a secondary system loop pumps the heated fluid into a solar storage tank or a space heating element, such as a hydronic radiator or radiant flooring. The drainback tank must be installed below the collector and plumbing must be angled downward from the top of the collector to let gravity drain the fluid without any pump assistance. The pipes must also be well supported to minimize any sagging. A vacuum breaker valve must be installed at the top outlet of the solar collector to automatically permit air to enter the collector panel as it drains. This action prevents any potentially high vacuum from collapsing the collector panel, and also allows the collector to empty its contents completely. A high-head pump is often required to push the solar fluid upwards, back to the top of the collector, whenever the system turns on. Certain solar collectors cannot be used with drainback systems, such as flat-plate collectors with serpentine riser tubes and some evacuated-tube collectors.

Drainback tanks

Drainback systems include a separate tank (in addition to a storage tank), called a drainback tank. As shown in Figure 18, this tank is often smaller and is specifically intended to store the drained fluid from the solar collector array.

Closed-Loop Drainback Systems

Information Job Sheet 4

Job Sheet 4 – Closed-Loop Drainback Systems

42 © Festo Didactic 52669-20

Figure 18. Basic drainback system with external heat exchanger.

As shown in Figure 19, drainback tanks can have integrated heat exchanger coils, either internal to the tank or externally wrapped around the tank. Also, the thermal storage can be configured with multiple storage tanks.

Figure 19. Basic drainback system with integrated heat exchanger.

Drainback tank Storage tank

Pump

Heat exchanger

Pump

Solar collector

Drainback tank

Storage tank

Pump

Heat exchanger

Pump

Solar collector



The drainback tank is a thermally insulated, unvented vessel that often contains a sight glass (sight gauge) to visually monitor the fluid level within the tank. When commissioning a drainback system, the drainback tank must only be filled partially, well below the tank’s maximum capacity. Sizing the tank requires a simple determination of liquid volume within the solar collector array, including all piping that exists above the drainback tank level. Then, add 15.14 L for extra margin. Another rule-of-thumb is to simply double the solar fluid loop volume. Common tank sizes range from 18.93 to 113.56 L in 18.93 L increments. Standard electric water heaters range in size from 113.56 to 454.25 L, and may be used as large drainback tanks. Larger custom tanks are also available from 378.54 to 7 570.82 L. A pressure relief valve is often installed on the drainback tank, and is usually set for 344.74 kPa.

In the solar thermal training system, the storage tank is used as the drainback tank and a secondary storage tank is assumed.

Drainback pumps

Pump selection is critical for a drainback system to function properly. AC-powered, high-head, low-flow pumps are required to move the solar fluid back up to the solar collectors from the bottom of the drainback tank. The head for the pump is determined by adding four feet to the vertical distance between the bottom of the drainback tank and the uppermost part of the solar collector array. The extra margin helps to ensure that friction head is overcome.

System performance

This job sheet involves installing and operating a dual closed-loop water (DHW) and space (floor or air) drainback system with thermal storage, which has the following advantages and disadvantages.

System type

Active, indirect, closed-loop, unvented, water (DHW) and space (floor or

air) heating with storage.

Advantages

Efficient operation

Simple configuration reduces cost and lowers maintenance

Overheat-protection turns off pump when high temperature is reached

Can use water or sand for thermal storage

Limited freeze-protection provided without antifreeze

If used, antifreeze life is extended (compared to pressurized systems)



Disadvantages

Can freeze over extended cold periods, not for extremely cold climates

Additional tank or special tank needed

High-head pump needed

Collector array must be placed near and above storage tank

Drainback tank must be placed below collector array and above storage

tank

Only certain solar collectors that drain properly can be used

Collector piping must be sloped at least 1°, and cannot sag

Careful planning and installation is needed


In this job sheet, you will install a dual closed-loop water (DHW) and space (floor or air) drainback system that includes thermal storage and can use antifreeze as part of a solar thermal energy system. Four different solar heating systems can be installed. Three of these drainback systems use internal heat exchangers that are an integral part of the drainback tank: one for heating water, one for heating water and flooring, or one for heating water and space. A fourth example of a drainback system uses an external heat exchanger to heat water.

Equipment required

Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required for this job sheet.

Safety procedures

Before proceeding with this job sheet, complete the following checklist.

You are wearing safety glasses.

You are wearing safety shoes.

You are not wearing anything that might get caught such as a tie, jewelry, or loose clothes.

If your hair is long, tie it out of the way.

The working area is clean and free of oil.

The floor is not wet.

Your sleeves are rolled up.

The wheels of the system are locked in place.

Closed-Loop Drainback Systems

Job Sheet 4

OBJECTIVE

PROCEDURE



Installation

1. Figure 20 through Figure 23 show indirect solar heating systems with thermal storage for heating water and/or air. Choose one of those systems, then continue this procedure using the selected system.

Figure 20. Closed-loop, water-heating drainback (storage) system schematic.

Vacuum breaker

Expansion tank

Pressure-relief valve

TST

Heat exchangers

Storage tank

Ball valve

Funnel

Fill bowl

Shutoff valve

Ball valve

Solar collector

COL

TRF

Differential controller

Shutoffvalve

Storage tank

Pump

Pump

Ball valve

Hot water outlet(DHW to taps)

Cold waterinlet (WS)



Figure 21. Closed-loop, water/floor-heating drainback (storage) system schematic.

Vacuum breaker

Expansion tank


TST

Heat exchangers

Storage tank

Ball valve

Funnel

Fill bowl Shutoff valve

Ball valve

Solar collector

COL

TRF


Shutoff valve

Pump

Pump

Ball valve

Hot water outlet (to storage tank)

Cold water inlet (from

storage tank)

Radiant floor

Warm surface



Figure 22. Closed-loop, water/air-heating drainback (storage) system schematic.

Vacuum breaker

Expansion tank


TST

Heat exchangers

Storage tank

Ball valve

Funnel

Fill bowl

Ball valve

Solar collector

COL

TRF


Pump

Pump

Ball valve

Hot water outlet(to storage tank)

Cold waterinlet (from

storage tank)

Shutoff valve

Radiator

Warm air outlet

Fan

Cold airinlet (AS)

Power input (ES)

Shutoff valve



Figure 23. Closed-loop, water-heating drainback (storage) system schematic with external heat exchanger.

Vacuum breaker

Expansion tank


TST

Heat exchangers

Storage tank

Ball valve

Funnel

Fill bowl

Ball valve

Solar collector

COL

TRF


Pump

Pump

Ball valve

Hot water outlet(DHW to taps)

Cold waterinlet (WS)

Shutoff valve

Shutoff valve

Heat exchanger

Storage tank



2. Ensure that the following parts are mounted on the mobile workstation as shown in Figure 24 to Figure 27.

Solar collector

Circulator pumps (2)

Storage tank (used as drainback tank)

Plate heat exchanger

Differential controller (with 3 sensors)

Thermostat controller (with 1 sensor)

Expansion tank (1)

Radiator

Radiant floor

Thermometers (2)

Rotameters (2)

Pressure gauge

Pressure relief valve (1)

Vacuum breaker valve

Shutoff valves (2)

Fill bowl (1)

Electrical panel



Figure 24. Closed-loop, water-heating drainback (storage) system configuration.

Primary loop

Secondary loop

Hot flow

Cold return

Warm path

Hot flow

Cold return

Warm path



Figure 25. Closed-loop, water/floor-heating drainback (storage) system configuration.

Primary loop

Secondary loop

Hot flow

Cold return

Warm path

Hot flow

Cold return

Warm path



Figure 26. Closed-loop, water/air-heating drainback (storage) system configuration.

Primary loop

Secondary loop

Hot flow

Cold return

Warm path

Hot flow

Cold return

Warm path



Figure 27. Closed-loop, water-heating drainback (storage) system configuration with external heat exchanger.

3. Ensure that the electrical wiring is complete by referring to the electrical wiring instructions in Appendix F.

4. Adjust the solar collector tilt angle for 50° to ensure that the collector is above the drainback tank water level.

Be careful when rotating the solar collector. Improper use can lead to injuries.

5. Connect the necessary hoses as shown in Figure 20 and Figure 24 (or Figure 21 and Figure 25, or Figure 22 and Figure 26, or Figure 23 and Figure 27).

a Always use the minimum hose length required for each circuit connection.

6. Connect the training system to an ac power source.

Primary loop

Secondary loop

Hot flow

Cold return

Warm path

Hot flow

Cold return

Warm path



7. Partially fill the system with water by performing the System filling procedure in Appendix C, while adhering to the following note:

a Do not fill the drainback tank completely. Fill the tank to a depth of about 22.2 cm (measured at tank center).

Be careful to prevent water from being in contact with electrical components. Use

a bucket and mop if some leakage occurs.

8. Turn the power on by setting the circuit breaker to I.

9. Start the pumps by setting both override switches to Override.

Then, set the pumps to III.

10. Evacuate the air present in the system using the priming procedure (refer to Appendix C).

Commissioning

11. Set the valves as shown in Table 7.

Table 7. State of the different valves.

Valves States

Fill bowl ball valve closed

Solar collector ball valve open

Storage tank ball valve open

Shutoff valves (2) open

a Make sure the solar collector outlet hose (91.44 cm long) is connected directly to the top (return port) of the drainback tank, with no sagging. Also, ensure that the vacuum breaker valve is installed.

12. Turn the pumps on by setting both override switches to the Controller position.

13. If installed, turn on the radiator and set the fan to minimum speed (Switch settings: S1 = 2, S2 = ).



14. Use the differential controller (TDIC) to set the pump to AUTO mode. To do so, press and hold the + (right) button on the controller until a new menu appears. Then, use the + button to select the MAN-1 menu. Press the OK (center) button and use the + and - buttons to select the AUTO mode. Press the OK (center) button to confirm.

15. Use the thermostat controller (TH) to adjust the set-point temperature between 3°C to 6°C above the ambient temperature by using a rotating knob.

Ambient temperature °C

Set-point temperature °C

16. Measure (with the pump on) the different parameters listed in Table 8, then record the values in the “Initial” column of the table. After that, position and power up the two 500 W work lights so that most of the radiant energy floods the solar collector panel evenly.

a The pressure gauge port must be vertically level with the rotameter pressure port.

a If the rotameter does not indicate any flow, it means that the controller has turned the pump off.

a Use the magnetic surface thermometer to measure ambient air temperature in front of the radiator output.

The solar thermal training system should be operated under supervision at all

times. Never let the system operate unattended.

The surface of the work lights can become very hot. Whenever you manipulate

them, take great care to avoid direct contact with the skin.



Table 8. Test results.

Device Values for different measuring times

Units Initial At 15 min At 30 min

F1-1 (flow rate) L/min

PI-1 (pressure) kPa

F1-2 (flow rate) L/min

PI-2 (Pressure) kPa

TI (collector input) °C

TI (collector output) °C

TE-S1 (COIL) °C

TE-S2 (TST) °C

TE-S4 (TRF) °C

Floor temperature °C

Air temperature °C

17. After about 15 minutes, measure (with the pump on) the different parameters listed in Table 8, then record the values in the “At 15 min” column of the table.

18. After about 30 minutes, measure (with the pump on) the different parameters listed in Table 8, then record the values in the “At 30 min” column of the table.

19. Use the differential controller override switch to turn the primary pump off.

20. Use the thermostat controller override switch to turn the secondary pump off.

21. Turn off the work lights.



22. Measure and record the water levels in the storage tank (being used as a drainback tank) with the primary pump turned on and turned off. Measure the water depth at the tank center. See Figure 28.

a Wait at least 30 seconds with the pump off while draining ends.

Pump on: cm

Pump off: cm

Figure 28. Drainback (storage) tank water level.

23. Turn off the system and the radiator by setting the circuit breaker to O.

24. Disconnect the power cords and drain the system by performing the System draining procedure in Appendix C.



25. Use the data in Table 9 to determine the total volume of water that is contained in the solar collector and all tubing that is situated above the drainback tank water level. Do not include the volume listed for the storage tank.

Table 9. System component volume.

Item Description Volume

(L)

87042-10 1.82 m long hose with 1.27 cm inner diameter 0.23

87042-11 91.44 cm long hose with 1.27 cm inner diameter 0.12



89515 Solar collector 0.6

46508-1C Circulator pump 0.20

87318-10 Storage tank 51.1 L (37.85 L max.) 37.85

87318-10 Coil heat exchanger 0.30

46505 Plate heat exchanger 0.11 L (x2) 0.11

46509 Temperature sensor (RTD) 0.03

52181 Thermostat controller (sensor) 0.05

46510-10 Expansion tank 7.95 L (4.16 L max.) 0.05

6531-C0 Radiator 0.26

46512 Radiant floor 0.16

46501 Thermometer 0.04

6550-A0 Rotameter 0.09

6553-E0 Pressure gauge (60.96 cm long hose with 0.46 cm inner diameter)

0.01

46503 or 46504

Pressure relief valve 0.06

6520-A0 Shutoff valve 0.04

46507 Check valve assembly 0.12

46519-10 Fill bowl 0.05

46511 Automatic air vent 0.05

a The thermostat controller (52181) is part of the workstation (46500-2C).

a Each system component volume (in liters) includes the connection ports.



26. Based upon the estimation, how much water should drain into the drainback tank?

L

27. Calculate and record the difference in water levels ( height in cm) that you measured in the drainback tank with the primary pump turned on and turned off.

cm

28. Calculate the volumetric difference ( volume in L) between the two drainback tank conditions using the equation below.

b Use the inside dimensions of the drainback tank, or approximately 50.8 cm x 31.12 cm.

lcm

HeightWidthLengthVolume

/10003

29. Based upon the results, how much water is draining from the solar collector into the drainback tank?

L

30. Is this observed value similar to the estimated value (within 10%)?

Yes No

31. Ask the instructor to check your work.

1. Did the system work correctly?

Yes No

2. Describe and briefly explain the system behavior.

PROCEDURE QUESTIONS



3. Name at least two locations or devices in the system where thermal energy was transferred through a containment wall.

4. Where might a similar system prove to be effective or useful?

5. Was the drainback tank water level lower when the primary pump was running (on)?

Yes No

6. Based upon the measured values, what was the overall temperature rise of the water?

7. Based upon the measured values, what was the overall temperature rise of the floor or air?

Name: ______________________________ Date: ____________________

Instructor's approval: ______________________________________________

Multi-Loop Systems, 4 Closed-Loop Drainback Systems · Drainback systems include a separate tank...

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