CEM Certified Energy Manager (CEM) Questions and Answers

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All electrical energy consumed by the lamp is converted into heatwithin 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 isC. 100%.

To calculate the additional cost due to a power factor of less than 100%, follow these steps:

Step 1: Convert Real Power (kW) to Apparent Power (kVA)

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Step 2: Compute Additional kVA Due to Power Factor

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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 EnergyManager (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⋅ΔT 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 coolingcapacity. 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.

To determine the annual energy cost savings from switching to LED luminaires:

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APitot tubemeasuresflow rateby determining thepressure differentialin a fluid.

Why is a Pitot Tube the Correct Answer?

It measuresdynamic pressureandstatic pressurein a moving fluid.

Using Bernoulli’s equation, it calculatesvelocityandflow rate.

Analysis of Other Options:

Psychrometer (A):Measures humidity, not flow rate.

Infrared camera (B):Detects temperature differences, not pressure.

Hot-wire anemometer (C):Measures air velocity using heat dissipation, but not pressure differential.

Ultrasonic flow meter (E):Measures flow using sound waves, not pressure.

Thus, the correct answer isD. Pitot tube.

Monitoring energy consumption is a critical component of effective energy management. It serves multiple purposes that collectively enhance an organization's ability to control energy use, identify inefficiencies, and implement improvements. These purposes include:​

Establishing a Basis of Management Control:By systematically tracking energy consumption, organizations create a foundation for managing energy use effectively. This data enables the setting of benchmarks and performance indicators, facilitating informed decision-making and strategic planning.

Identifying Deviations in Energy Consumption Patterns:Continuous monitoring allows for the detection of anomalies or unexpected changes in energy usage. Recognizing when and why these deviations occur is essential for diagnosing potential issues, such as equipment malfunctions or operational inefficiencies, and addressing them promptly.

Supporting Analysis of Energy Information:Comprehensive energy data collection supports detailed analysis, aiding in understanding consumption trends, evaluating the impact of energy conservation measures, and identifying opportunities for further efficiency improvements.

Given that monitoring energy consumption fulfills all the above functions, the correct answer is D. All of the above.

Net metering allows customers with renewable energy systems to receive credit for excess electricity fed back to the grid, offsetting their consumption over a billing period. This mechanism encourages renewable energy adoption by providing financial benefits to consumers.

Step 1: Purpose of Thermal Scans

Thermal scans (infrared thermography) detecttemperature variationsin electrical components.

Loose connectionsincrease resistance → causeoverheating, which isvisible in thermal scans.

Step 2: Analysis of Each Option

A. Locating areas of high power factor❌Incorrect→Power factor is measured with power meters.

B. Reading current-carrying capacity of conductors❌Incorrect→Measured using ammeters and engineering data.

C. Locating a loose electrical connection✅Correct→Loose connections causeheat buildup, easily detected with thermal scans.

D. Measuring the load on a refrigerant system❌Incorrect→Refrigerant loads are measured usingpressure sensors and flow meters.

Thus, the correct answer isC. Locating a loose electrical connection.