MODULE 1 Introduction to Mechanical and Electrical Systems:
Energy, Sustainability, and Economics
The calculation examples are shown in an accordion-style display. Some questions are accompanied by equations and/or tables for quick reference. Solve the problem on your own and then click on the question to view the solution. This will help you make sense of the work as you read through the problem-solving process.
1.
If lighting load for a 10,000 sq.ft. building is estimated at 1 Watt/sq.ft., what will be the resulting heat generated by lighting in units of MBtu for 3,000 hours of lights on?
The total electrical power will be 10,000 sq.ft. x 1 Watt/sq.ft., or 10,000 Watts (10kW). The conversion factor from electricity to heat is 3,413 Btuh/kW, so the heat generated will be 10 kW x 3,413 Btuh/kW = 34,130 Btuh. Heat energy for 3,000 hours will be 3,000 hours x 34,130 Btuh = 102 million Btu.
2.
Question 2.a Question 2.b
2.a
How much heat (Btu’s) will be stored in a 100 sq.ft. concrete wall 1 ft. thick if it is warmed from 65 deg. F to 85 deg. F by exposure to sunlight?
Use the equation Q = M x C x TD with information from Table 1-2. Mass will be 100 cu.ft. x 144 lb./cu.ft., or 14,400 lbs.
Specific heat of concrete is 0.156.
Q = 14,400 x 0.156 x (85-65) = 44,900 Btu
2.b
What is the value of the heat in Question 2.a compared with gas at $1.00 per therms burned in a boiler at 85% efficiency?
One therm is 100,000 Btu. If the boiler is 85% efficient, one therm will produce 85,000 Btu of output for the $1.00 worth of gas. The value of 44,900 Btu is, therefore (44,900 / 85,000) x $1.00, or $0.53.
3.
If the lighting load were increased, what would be the effect on other building systems in a Midwestern U.S. climate? Would you increase the capacity of the heating system? The cooling system? What would the energy impact of higher lighting loads be on gas for heating, electric for cooling, and overall electric usage?
Heat from lights adds to air conditioning load. Increasing lighting load will increase the required capacity of air conditioning equipment; but heating systems are designed to heat with the lights off, so there will be no effect on design of heating capacity. Heat from lights might decrease energy requirements for building heating, increase electric for cooling, and overall electric usage will increase.
4.
How much CO2 will be liberated to the atmosphere in a year’s time due directly to lighting operation in the building of question 1? What will the relative impact be on CO2 for heating and for cooling?
From Table 1-4, we find data on CO2 production resulting from power generation. Assuming a coal fired plant, 2.4 lbs. of CO2 will be produced per kWh. The building is Question 1 uses 10 kW for 3,000 hours, which is 30,000 kWh. At 2.4 lbs. CO2 per kWh, the annual production is 2.4 x 30,000 = 72,000 lbs. CO2. Heat from lighting reduces the amount of gas used for heating, so will reduce the CO2 emissions from gas. Heat from lighting increases the amount of electric used for cooling, so will increase the resulting CO2 emissions.
5.
An energy conservation option has a first cost of $50,000. It requires $4,000 per year maintenance and saves $10,000 per year in utilities. What is the simple payback period for the option?
The net annual saving for the investment is $6,000, which is utility saving less maintenance cost. Simple payback is $50,000 investment divided by $6,000 savings, or about 8 years.