Energy Saving from Higher Temperature Setting

Objective

To estimate the energy savings from an AC by maintaining the thermostat at a higher temperature.

Outcome

Students will realise that considerable energy savings can be achieved just through setting higher temperature in cooled spaces.  

Requirements

Psychrometric chart

Prerequisites

Conceptual knowledge of psychrometry.

Given Data:

Design Conditions (peak summer)

1. Ambient condition : DB = 40°C, RH = 60%
2. Desired condition for scenario 1 : DB = 22°C, RH = 50%
3. Desired condition for scenario 2 : DB = 25°C, RH = 50%
4. Density of air : 1.12 kg/m³
5. 4 air changes per hour
6. 120 sq. ft. room, 10 ft. ceiling height

Conversion Factors:

1. 1 kW = 1 kJ/s
2. 1 hr = 3,600 s
3. 1 m³ = 35.32 ft³
4. 1 TR = 3.517 kW of cooling

Procedure

Step 1: On the psychrometric chart, estimate Enthalpy (H) of ambient air condition and desired conditions for both scenarios (kJ/kg) and find the differences (∆H₁ & ∆H₂).

Step 2: Estimate required volumetric air flow rate (m³/hr).

Step 3: Convert volumetric air flow rate (m³/hr) to mass flow rate (kg/hr).

Step 4: Estimate cooling required for scenario 1 (kJ/s).

Step 5: Estimate cooling required for scenario 2 (kJ/s).

Step 6: Estimate the percentage energy savings.

Calculations:

Step 1: Enthalpy of

1. Ambient condition (H_a) = 114kJkg
2. Desired condition 1 (H_d1) = 42kJkg
3. Desired condition 2 (H_d2) = 50kJkg

ΔH₁ = H_a – H_d1 = 114-42 = 72kJkg 

ΔH₂ = H_a – H_d2 = 114-50 = 64kJkg

Step 2: volumetric air flow rate (v_a) (m3hr) = air changes per hour (1hr) volume of the room (ft3)35.32 (ft3m3)
= 4 120 1035.32  
= 135.84m3hr 

Step 3: mass flow rate (m_a) (kghr) = v_a (m3hr) x density of air (kgm3)

= 135.8 x 1.12

= 152.1kghr

Step 4: Cooling required for case 1 (L₁) (kJs) =_ma (kghr) ΔH1(kJkg)3600 (shr) 

=152.14 723600 

= 3.043kJs

Step 5: Cooling required for case 2 (L₂) (kJs) =_ma (kghr) ΔH2(kJkg)3600 (shr) 

=152.14 643600 

= 2.705kJs

Step 6: Percentage energy savings = L1 – L2L1x 100

= 3.043 – 2.7053.043

=  11.1% 

Image:

References

None

Source of Idea