Combustion Technology & Emission ControlAssignment –Solar thermal technology& process heat

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Combustion Technology & Emission Control

Assignment – Solar thermal technology& process heat

Due: 11:59 pm. Tuesday 9th June 2020 – Online submission via MyUni.

This Assignment is an individual assessment and will contribute 10% of your total mark for

the course.

Over recent years the cost of natural gas has increased dramatically, causing greater interest

in the possibility of alternative energy resources to mitigate the costs, while also lowering net

CO2 emissions. On this basis, a company in South Australia, that converts agricultural residue

into valuable products through a distillery process, is seeking alternative energy options to

reduce its fuel consumption, which is currently mainly natural gas. Since the plant typically

operates for about six months of each year during summer (starting from October each year

and finishing in March) and the company has a vacant land of 2 hectares close to its plant site,

they are interested in using solar thermal energy to supply their process heat, using solar

troughs.

The energy demand of the plant is currently supplied via a low-pressure boiler that

consumes natural gas to produce saturated steam at 8 bar. As shown schematically in Figure 1,

the saturated steam (produced by the boiler) condenses within the distillery plant (in the

reboilers of the distillation columns) to produce their required process heat. The condensed

water is then pumped back into the boiler at a temperature of 70 °C and the process continues.

During the operational time, the plants requires the continuous supply of 6 tonnes steam per

hour. The company’s boiler has a maximum steam capacity of 8 tonnes steam per hour with a

turndown of 4:1. The turndown is the ratio between the maximum output of boiler and the

boiler output when operating at its lowest capacity. The boiler operates continuously. When

solar thermal energy is available, the load of the boiler can be decreased to its minimum

capacity. The boiler cannot be fully turned off because the company is not confident that they

would not lose the steam supply due to the solar resource variability (e.g. in case of cloudy

days). While also turning off the boiler might decrease its life.

Figure 1. A schematic diagram of the current process plant. 8 bar saturated steam, produced

by the boiler, condenses in the plant’s distillation columns to supply their heat demand. The

condensed water, produced within the plant, is then returned back to boiler and the process

continues.

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The company is investigating the potential for the solar hybridisation of their plant through the

use of sun-tracking ET-150 solar troughs with a heat transfer fluid (HTF) and a steam generator

heat exchanger, which is used to produce saturated 8 bar steam using the hot HTF. The plan is

to heat the HTF with the solar troughs and then use it for steam generation via a heat exchanger.

The efficiency of the ET-150 solar troughs is given by:

𝜂𝐸𝑇−150 = 0.75 − 0.000045 × (𝑇𝑖𝑛 − 𝑇𝑎𝑚) − 0.039 × ( 𝑇𝑖𝑛− 𝑇𝑎𝑚

𝐷𝑁𝐼 ) − 0.0003 × 𝐷𝑁𝐼 × (

𝑇𝑖𝑛− 𝑇𝑎𝑚

𝐷𝑁𝐼 )

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.

Here 𝑇𝑖𝑛 is the temperature of the inlet HTF into troughs, 𝑇𝑎𝑚 is the ambient temperature and

DNI is the direct normal irradiation. The mean ambient temperature at the plant site in summer

and winter are 30 ° and 10 °C, respectively. In addition, the ratio of land occupied by the ET-

150 troughs to their collection area is 1.30. For all assessments consider a temperature of 250

°C as the hot HTF leaving the troughs and an approach temperature of 10 °C for the steam

generator heat exchanger.

The company is to investigate two scenarios for the integration of solar thermal energy into

their process, namely solar thermal energy with storage and solar thermal energy without

storage. The case of with storage will be based on a two-tank configuration and is proposed to

maximise the capacity of using solar thermal energy. While in the case of without storage, solar

thermal energy is only used for diurnal steam generation when it is available.

1. Develop a process configuration for the solar hybridisation of the plant using the ET-150

solar trough for the two cases of: [10 Marks total]

(a) without storage; and

(b) with two storage tanks for cold and hot HTF.

The process configurations can be presented as block flow diagrams.

2. Select the appropriate HTF from the list provided in Table 1 and justify your selection.

[10 Marks total]

Table 1. Thermo-physical properties of heat transfer fluids.

Heat transfer fluid Density (kg/m3) Mean heat capacity

(J/kg. °C)

Therminol 55 737 2600

Therminol 66 863 2268

Hitec oil 847 2392

Dynalene SF 745 2698

3. Calculate the size of active solar collector field and comment on the preferred orientation of

the troughs, if any orientation is preferred. [5 Marks total]

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4. Calculate the variable flow rate of HTF to keep the temperature of the hot oil constant at 250

°C. [10 Marks total]

5. Calculate the spontaneous estimated-solar share for the case of hybridisation of the plant

without energy storage, based on the hourly averaged direct normal insolation (DNI) data over

the period of October to March for the site of plant (Table 2). [10 Marks total]

6. With the company’s available land for the solar troughs (2 hectares), is it advantageous to

employ any storage of solar thermal energy? If so, how much storage should be included in the

design? [10 Marks total]

7. The company is interested in expanding the available land for the solar troughs to achieve

12 hours of energy thermal energy storage. Calculate the size of land needed?

[15 Marks total]

Table 2. The hourly averaged normal direct insolation (DNI) at site of Company from October

to March.

Hours of day Hourly averaged Direct Normal Irradiance (W/m2),

from October to March

0:30 0

1:30 0

2:30 0

3:30 0

4:30 0

5:30 0

6:30 52.9

7:30 219.6

8:30 455.25

9:30 633.6

10:30 727.875

11:30 768.9

12:30 781.1

13:30 778.9

14:30 755.8

15:30 712.2

16:30 617.8

17:30 401.2

18:30 157

19:30 24

20:30 0

21:30 0

22:30 0

23:30 0

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Suggestions and potential useful information:

i. Refer to slides 17-20, 23 & 25 of Lectures 19& 20 and slides 12, 20& 21 of Lectures

21& 22. Information provided in other slides of the lectures are also useful.

ii. The flow rate of HTF can be varied in response to variations in the DNI, and hence net

solar thermal energy generated by the troughs, to keep the temperature of the outlet

hot HTF constant. The hot HTF can be used for steam production through steam

generator heat exchanger. If the amount of produced hot HTF is more than that

required for steam production the excess hot HTF can be stored in the hot storage

tank.

iii. The lowest capacity of the boiler can be calculated from its turndown ratio.

iv. The spontaneous solar share is defined as the contribution of the solar thermal

energy to the net energy input to the process.

𝜒𝑠𝑜𝑙 = contribution of solar energy to the process heat

contribution of solar energy to the process heat + contribution of fuel energy to the process heat

v. Methane as the main component of natural gas can be used as its surrogate for the

assessment.

vi. Saturation temperature of steam at 8 bar is ~170 °C.