lec8_9 - temperature Energy balance First Law of...

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MIT OpenCourseWare http://ocw.mit.edu 1 .020 Ecology II: Engineering for Sustainability Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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Lectures 08_8 & 08_9 Outline: Basic Thermodynamic Concepts, Building Energy Motivation/Objective Develop a model to determine how construction materials affect inside temperature and heat loss in a small house. Approach 1. Define system, identify unknown (interior temperature ) i T 2. Write energy balance equation (rate form) 3. Relate terms in balance eq to unknown temperature and other inputs 4. Specify heat capacities, thermal resistances, meteorological inputs, etc, solve energy balance eq. for unknown interior temperature (MATLAB) 5. Compute heat loss, examine impact of building material properties Concepts and Definitions Needed: Relevant thermodynamic properties: Mass, volume, internal energy, enthalpy, heat capacity,
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Unformatted text preview: temperature Energy balance, First Law of Thermodynamics for a closed system: Increase in system energy = Energy in across system boundary = Heat in - Work out: Incremental form: W Q U Δ − Δ = Δ Infinitesimal form: W Q dU δ − = Rate form: W Q dt dU & & − = Internal energy term: Write in terms of constant volume heat capacity dt dT C dt dU i vh = Heat mechanisms: Conduction, convection into system: ia i air h conv cond R T T A Q ) ( , − = & Outgoing radiation: 4 i rad T Q εσ = & Work done: Assume zero for house problem Complete balance eqs: Consider only conduction & convection through walls & roof (no net radiation into house): ia i air r ia i air w roof wall i vh R T T A R T T A Q Q dt dT C ] [ ] [ − + − = + = & & , specify air T Model Results Note energy saved as insulation is improved....
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lec8_9 - temperature Energy balance First Law of...

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