CEM Certified Energy Manager (CEM) Questions and Answers

All electrical energy consumed by the lamp is converted into heat within the space, either as direct heat or as waste heat from the light source.

In a windowless office, no energy escapes as light. The light energy that is emitted eventually turns into heat through absorption by surfaces.

Thus, the correct answer is C. 100% .

Electric utility companies transmit power at high voltages primarily to reduce transmission power losses, which are proportional to the square of the current (I²) multiplied by the resistance (R) of the transmission lines. This relationship is expressed by the formula for power loss: P_loss = I²R.​

Key Points:

Power Loss and Current Relationship: Power losses in transmission lines are directly proportional to the square of the current flowing through them. Reducing the current decreases these losses significantly.

High Voltage Transmission: By increasing the transmission voltage, the current required to deliver the same amount of power decreases. This is because power (P) is the product of voltage (V) and current (I): P = VI. For a given power level, increasing voltage allows for a corresponding decrease in current.

Reduced I²R Losses: Lower current results in reduced I²R losses, enhancing the efficiency of power transmission over long distances.

Conclusion:

Transmitting power at high voltages minimizes transmission losses, making it the most efficient method for long-distance electrical power delivery. Therefore, the correct answer is C. Transmission power losses (I²R) are less.

The Institute of Electrical and Electronics Engineers (IEEE) Standard 519, titled " Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, " sets forth guidelines to maintain the quality of electrical power by limiting harmonic distortions. According to IEEE 519, for general low-voltage systems (defined as systems with a voltage less than 600 volts), the Total Harmonic Distortion (THD) of the voltage waveform should not exceed 5%. This threshold ensures the reliable operation of electrical equipment and minimizes potential issues such as overheating or malfunctioning caused by excessive harmonics. Therefore, the correct answer is C. Less than 5%.

To determine the equivalent price of natural gas per gigajoule (GJ) for a natural gas water heater, we compare the cost of electricity and natural gas on an energy-equivalent basis.

Step 1: Convert Electricity Cost to Cost per GJ

Electricity cost: $0.10/kWh

1 GJ = 277.8 kWh

1) Define what is being asked (CEM context)

For a chilled-water thermal energy storage (TES) system, AEE CEM guidance states that storage capacity is determined from:

Where:

= stored thermal energy

= mass of water

= specific heat of water

= temperature difference between supply and return

This equation is explicitly used in CEM thermal energy storage sizing problems (AEE CEM Body of Knowledge – Cooling Systems & Thermal Storage).

2) Given data

Required cooling storage:

Chilled water supply temperature = 4°C

Chilled water return temperature = 14°C

Temperature difference:

Specific heat of water:

Density of water (CEM standard assumption):

3) Solve for required water mass

4) Convert mass to volume

Using water density:

1) Definition of Commissioning (CEM Standard)

According to the AEE Certified Energy Manager (CEM) Body of Knowledge and the ASHRAE/AEE commissioning guidance referenced in CEM training , commissioning (Cx) is a systematic quality assurance process applied throughout the design, installation, startup, and operation phases of a facility to ensure that building systems perform as intended by the owner and designer .

2) Evaluation of Each Option

A. Ensures a system is designed to operate as intended

Commissioning includes design review to verify that the design intent, owner’s project requirements (OPR), and basis of design (BOD) are aligned.

CEM materials explicitly include design-phase commissioning as a benefit.

B. Ensures a system is installed to operate as intended

Construction and installation verification is a core commissioning activity .

Functional performance testing confirms systems are installed and perform per design.

C. Ensures staff know how to operate a system as intended

Commissioning requires operator training , review of control sequences, and system demonstrations.

AEE CEM guidance highlights improved operations staff competence as a major benefit.

D. Ensures staff know how to maintain a system as intended

Commissioning includes delivery and review of O & M manuals , maintenance procedures, and hands-on training.

This reduces long-term energy waste and equipment degradation.

3) CEM Exam Key Concept

Commissioning improves energy performance, system reliability, occupant comfort, and staff capability by verifying design intent, installation quality, and operational readiness.

1) Life-Cycle Cost Reduction via Commissioning (CEM Guidance)

The AEE CEM Body of Knowledge emphasizes that commissioning reduces total life-cycle cost , not just first cost, through multiple pathways.

2) Evaluation of Each Option

A. Reduce the cost of change orders during construction

Early detection of design and coordination issues reduces rework.

B. Improve the performance of energy systems

Ensures systems operate at intended efficiency.

C. Improve the satisfaction of building occupants

Better comfort reduces complaints and operational disruptions.

D. Improve the effectiveness of operations and maintenance

Training and documentation improve long-term reliability and efficiency.

All are documented commissioning benefits in AEE CEM training materials .

To determine which advantage makes an ice Thermal Energy Storage (TES) system more preferred over a water TES system for a load shifting strategy, we need to evaluate each option based on the principles of thermal energy storage as outlined in the Association of Energy Engineers (AEE) Certified Energy Manager (CEM) training materials. Load shifting involves storing energy (cooling capacity) during off-peak periods and releasing it during peak demand, making storage efficiency and capacity critical. Let’s analyze each option step-by-step.

Step 1: Understand Ice TES vs. Water TES in Load Shifting

Ice TES : Uses the latent heat of fusion of water (ice melting) to store cooling energy. Ice is formed during off-peak hours (e.g., overnight) and melted during peak hours to provide cooling.

Water TES : Uses the sensible heat capacity of water, storing chilled water (typically 4–6°C) to provide cooling.

Load Shifting Goal : Maximize cooling storage in minimal space and cost, shifting electrical demand from peak to off-peak periods.

CEM Reference : CEM materials in the " Thermal Energy Storage " section highlight ice TES for its high energy density and compact storage, contrasted with water TES for simpler operation but larger volume requirements.

Step 2: Evaluate Each Option

Option A: Ice-storage systems operate with a higher coefficient of performance (COP)

Analysis :

COP Definition : COP = (Cooling Output) / (Energy Input). For TES, this relates to the chiller’s efficiency.

Ice TES : Requires chillers to operate at lower temperatures (e.g., -5°C to 0°C) to freeze water, which typically reduces chiller COP (e.g., 3–4) compared to water TES chillers operating at 4–6°C (COP ~5–6).

Reality : Ice TES systems often have a lower COP due to the additional energy needed for phase change, though total system efficiency may improve with load shifting benefits.

CEM Reference : CEM notes that ice TES energy input is higher per unit of cooling due to lower evaporating temperatures, contradicting a " higher COP " claim.

Conclusion : This statement is incorrect and not an advantage for ice TES in load shifting.

Option B: Ice-storage systems require smaller storage tanks since ice has a higher energy storage density

Analysis :

Energy Storage Density :

Ice TES : Relies on latent heat of fusion = 334 kJ/kg (80 kcal/kg or ~144 Btu/lb). This is the energy absorbed/released when water freezes/melts, far exceeding sensible heat.

Water TES : Relies on sensible heat = cp ⋅ Δ T c_p \cdot \Delta T cp​ ⋅ ΔT, where cp=4.18 kJ/kg\cdotp°C c_p = 4.18 \, \text{kJ/kg·°C} cp​=4.18kJ/kg\cdotp°C (1 Btu/lb·°F). For a typical ΔT=10°C\Delta T = 10°CΔT=10°C (e.g., 4°C to 14°C), energy stored = 4.18×10=41.8 kJ/kg 4.18 \times 10 = 41.8 \, \text{kJ/kg} 4.18×10=41.8kJ/kg (~20 Btu/lb).

Comparison : Ice stores ~8 times more energy per kg than water for a 10°C range (334 vs. 41.8 kJ/kg).

Volume Impact : Ice’s density (~917 kg/m³) is slightly less than water (~1000 kg/m³), but the latent heat advantage dominates, reducing required tank volume significantly.

Load Shifting : Smaller tanks mean less space and potentially lower capital costs, a key advantage for peak load management.

CEM Reference : CEM training emphasizes ice TES’s high energy density as a primary reason for its preference in space-constrained load shifting applications.

Conclusion : This statement is correct and a clear advantage for ice TES.

Option C: Water-storage systems require smaller storage tanks since water has a higher density than ice

Analysis :

Density : Water = 1000 kg/m³; Ice = 917 kg/m³. Water is denser, but density alone doesn’t determine storage size in TES.

Energy Storage : As calculated, water’s sensible heat capacity (e.g., 41.8 kJ/kg for 10°C) is much lower than ice’s latent heat (334 kJ/kg). To store the same cooling capacity, water TES requires ~8 times more mass and thus larger tanks (even accounting for density differences).

Implication : Water TES tanks are larger, not smaller, contradicting the statement.

CEM Reference : CEM materials note water TES’s larger volume requirements as a disadvantage compared to ice TES.

Conclusion : This statement is incorrect and not an advantage for ice TES (it favors water TES incorrectly).

Option D: Ice-storage systems require lower maintenance due to lower pumping volume

Analysis :

Pumping Volume : Ice TES often uses glycol or brine solutions to transfer heat at lower temperatures, requiring pumps sized for smaller volumes due to concentrated cooling capacity. Water TES circulates larger volumes of chilled water. However, " lower pumping volume " doesn’t directly translate to " lower maintenance. "

Maintenance : Ice TES systems are more complex (ice-making equipment, heat exchangers), potentially increasing maintenance (e.g., defrost cycles, corrosion from brine). Water TES is simpler, often with lower maintenance needs.

CEM Reference : CEM discusses ice TES complexity as a trade-off for its density advantage, not a maintenance benefit.

Conclusion : This statement is questionable and not a primary advantage for load shifting.

Step 3: Identify the Key Advantage for Load Shifting

Load Shifting Context : The goal is to store maximum cooling capacity efficiently during off-peak hours. Option B (smaller tanks due to higher energy storage density) directly supports this by reducing space and installation costs, a critical factor in TES design per CEM guidelines.

Elimination :

A: Incorrect (lower COP, not higher).

C: Incorrect (water TES tanks are larger).

D: Weak (maintenance isn’t clearly lower; not the primary driver).

B: Correct and relevant.

Commissioning documentation, per AEE CEM and ASHRAE Guideline 0 , focuses on ensuring systems are designed, installed, operated, and maintained as intended .

Included commissioning documentation typically consists of:

Original facility and equipment design intent (OPR/BOD)

As-built drawings

Equipment control sequence documentation

Operations & Maintenance (O & M) manuals

Financing payment structure reports are related to project finance or performance contracting , not commissioning.

To calculate the cost of heating water using natural gas:

Step 1: Compute Energy Required to Heat the Water

Given:

Mass of water = 500 L = 500 kg

Temperature rise (ΔT) = 50°C - 18°C = 32°C

Specific heat of water (Cp) = 4.2 kJ/kg-°C

Energy Required (Q)=m×Cp×ΔT

=500×4.2×32

=67,200 kJ=67.2 MJ