Energy Balance Flashcards
What is the primary purpose of calculating building heat flow rates under steady-state conditions?
To set the size of the heating generation system (kW) and heating terminal devices such as radiators, fan coils, and radiant floors
Define transmission heat flow rate.
It is the heat flow rate through the building’s opaque and transparent envelope due to the temperature difference between the indoor and outdoor environment
What is the formula for the transmission heat flow rate?
Q=U.A.(Ti-to)
What is the significance of solar heat gains in a building’s energy balance?
Solar heat gains are the heat energy entering a building through windows or other transparent elements, which can reduce heating needs in winter but may cause overheating in summer
How is ventilation heat flow rate calculated?
Q=V.p.Cp.(Ti-To)
What are the components of the steady-state energy balance equation for a thermal zone?
What is the RC model in energy balance?
The RC model simplifies dynamic energy balance by using resistances and capacitances to represent heat transfer and storage, respectively, in building elements
What does “steady-state balance of a building” mean in energy analysis?
It is the assumption that all heat gains and losses are balanced, with no energy stored in or released from building components over time24_10_17_EnergyBalance
What are the two major types of heat gains in a building?
Internal heat gains (from occupants, lighting, and appliances) and solar heat gains (from sunlight entering through windows)
What is the equation for thermal transmittance U?
How do you calculate the overall U-value for a building envelope with different materials?
What are the key components of a building’s heat balance under dynamic conditions?
Heat flow rate through the envelope, ventilation and infiltration heat flow, internal heat gains, solar heat gains, and heat stored or released in building elements
What is the role of thermal mass in dynamic energy balance?
Thermal mass stores and releases heat, smoothing out temperature fluctuations and reducing heating/cooling demands during peak times
What is the difference between Qt and Qv?
Qt is the heat flow rate due to transmission through the building envelope, while Qv is the heat flow rate due to ventilation and infiltration
Why is it essential to include internal heat gains in energy balance calculations?
Internal heat gains from occupants, appliances, and lighting contribute to reducing heating demand in winter and increasing cooling demand in summer
What is the steady-state assumption in energy balance?
It assumes no changes in energy storage, meaning all incoming and outgoing energy flows are equal at any given time
How is solar heat gain through glazing calculated?
What is meant by the term “heat loss coefficient”?
It represents the total heat loss per degree of temperature difference between the inside and outside, combining transmission and ventilation losses24_10_17_EnergyBalance
Why is energy balance critical for sustainable building design?
It ensures optimal sizing of heating/cooling systems, enhances energy efficiency, and minimizes environmental impact by reducing energy consumption
What is the primary advantage of dynamic over steady-state energy modeling?
Dynamic modeling captures variations over time, providing a more accurate depiction of energy usage and occupant comfort
What are common sources of internal heat gains in buildings?
Occupants, lighting, electrical appliances, and equipment
What factors influence ventilation heat losses?
Air change rate, indoor and outdoor temperature difference, air density, and specific heat of air
What is the significance of the thermal zone in energy balance?
It represents a part of the building where heat flow rates are analyzed to optimize thermal comfort and energy efficiency
How is heat loss due to infiltration typically minimized?
By improving building envelope airtightness and using controlled mechanical ventilation systems
What does η represent in the context of energy efficiency?
represents the overall efficiency of a building system, including generation, distribution, emission, and control efficiencies
What are the main components of building energy consumption?
Heating, cooling, ventilation, lighting, and equipment/appliance usage
Why is infiltration often problematic in energy balance?
Infiltration is uncontrolled and leads to unpredictable heat losses or gains, reducing energy efficiency and occupant comfort
What are the typical air change rates for residential buildings?
Air change rates typically range from 0.3 to 0.5 per hour for residential buildings, depending on the ventilation system and building airtightness
What is the role of thermal resistance in energy balance?
Thermal resistance determines how effectively building materials resist heat flow; higher resistance means lower heat transfer
What is linear thermal transmittance (ψ)?
It measures the rate of heat transfer per unit length of a thermal bridge, usually expressed in W/m·K
How does occupant behavior influence energy balance?
Occupant behavior affects energy usage through thermostat settings, ventilation practices, lighting usage, and appliance operation
How do dynamic conditions affect internal gains?
Internal gains vary with occupancy schedules, equipment usage, and lighting demand, requiring time-based modeling in dynamic simulations
What factors influence the performance of solar heat gains?
Window orientation, glazing type, shading devices, and solar radiation intensity
How is overall building efficiency (η) calculated?
What are the typical thermal transmittance (U) values for windows and walls?
Walls: 0.2–0.3 W/m²·K (insulated); Windows: 1.0–1.6 W/m²·K (double-glazed)24_10_17_EnergyBalance
What are typical values for specific heat (𝑐𝑝 ) and air density (ρ) used in energy balance?
Specific heat (𝑐𝑝 ): 1.005 kJ/kg·K; Air density (ρ): 1.2 kg/m³ at standard conditions
What assumptions are made in the RC model for energy balance?
Assumes the building behaves as an electrical circuit, with thermal resistances and capacitances representing heat transfer and storage
What is the impact of shading devices on energy balance?
Shading devices reduce solar heat gains, helping to control overheating in summer and reduce cooling loads
What is the purpose of dynamic simulation tools like EnergyPlus?
To model building performance under variable conditions, providing detailed insights into energy consumption and thermal comfort
What are two methods to calculate infiltration losses?
Using empirical formulas based on leakage areas or pressure testing and detailed airflow modeling
What does the term “operational energy” mean?
Operational energy refers to the energy used for heating, cooling, lighting, and equipment during the building’s use
How does thermal bridging influence energy performance?
Thermal bridging creates localized areas of high heat transfer, increasing overall heat losses and reducing energy efficiency
Why is the specific heat of materials important in energy balance?
Materials with higher specific heat can store more heat, moderating temperature fluctuations and reducing peak energy demands
What is the difference between sensible and latent heat?
Sensible heat changes the temperature of air or materials, while latent heat involves phase changes, such as moisture evaporation
How does air infiltration differ from ventilation?
Infiltration is uncontrolled air movement through cracks and gaps, while ventilation is intentional air exchange designed for IAQ and comfort
What factors contribute to solar heat gain coefficients (SHGC)?
SHGC depends on window glazing type, frame materials, shading devices, and the angle of solar incidence
What is the importance of surface heat transfer coefficients?
Surface heat transfer coefficients represent convective and radiative heat exchange between surfaces and surrounding air
What are common methods to improve energy balance in buildings?
Enhancing insulation, reducing infiltration, optimizing solar gain, and implementing efficient HVAC systems
What is the difference between heating degree days (HDD) and cooling degree days (CDD)?
HDD measures heating demand by summing temperature differences below a setpoint, while CDD measures cooling demand by summing temperature differences above a setpoint
What is latent heat recovery in ventilation?
Latent heat recovery captures moisture from exhaust air, preconditioning incoming air and improving efficiency
How can dynamic energy balance improve sustainability?
By optimizing system performance and reducing energy waste, dynamic energy balance promotes sustainable and cost-effective building operation24_10_17_EnergyBalance
What is the heat transfer equation for multi-layered walls?
Why are U-values lower for well-insulated materials?
Lower U-values indicate higher resistance to heat transfer, which reduces heat loss through the material
What is thermal lag in building materials?
Thermal lag is the delay between heat absorption and release in materials, which helps moderate indoor temperatures
What are common methods to reduce solar heat gain through windows?
Use of reflective coatings, low-emissivity glazing, external shading devices, and optimized window orientation
What is the purpose of thermal zoning in buildings?
Thermal zoning groups areas with similar heating and cooling needs, optimizing HVAC performance and energy use
Why are air infiltration rates higher in older buildings?
Older buildings often have less airtight construction and more gaps or cracks in the envelope
What is the primary goal of low-energy building design?
To minimize energy consumption while maintaining occupant comfort and meeting regulatory standards
What is the relationship between ventilation rate and indoor air quality (IAQ)?
Higher ventilation rates generally improve IAQ by diluting indoor pollutants
