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.Lectures 08_10 Outline: Radiative Energy Transfer, Building Energy Motivation/Objective Extend house energy model to account for radiation to and from roof. Review effect of building materials on energy loss. Approach 1. Revise system definition to include two parts: 1) interior house space, walls, floor and 2) roof. Identify unknowns (interior temperature and roof temperature ). iTrT2. Write coupled energy balance equations (rate form) for interior and roof 3. Relate terms in the energy balance eqs to unknown interior and roof temperatures and other inputs. 4. Specify heat capacities, thermal resistances, meteorological inputs, etc, solve energy balance eqs. for unknown interior and roof temperatures (MATLAB) 5. Compute heat loss, examine impact of building material properties Concepts and Definitions Needed: Interior energy balance: Change in interior internal energy = Conduction/convection between interior & outside Roof energy balance: Change in roof internal energy = Conduction/convection between interior & roof + Conduction/convection between roof & outside + net incident radiation ( ) radQ& Net SW and LW radiation into roof: 44)(rrskyskygrroutingrrradTTSALWLWSAQσεσεαα−+=−+=& rα= roof SW absorptivity, rskyεε,= sky & roof LW emissivities This approach (with radiation) gives same results as Lectures 8&9 (no radiation) only when and the radiation terms cancel so . airiTT =0=radQ& Complete balance eqs: riirrisisoilfiaiairwivhRTTARTTARTTAdtdTC][][][ −+−+−= Interior 44][][rrskyskygrrrarairrririrrvrTTSARTTARTTAdtdTCσεσεα−++−+−= Roof Obtain from meteorological records. gskyairSTT ,, Modeling Example -- Building Energy Note differences in interior temperature and energy consumption when radiation is included. Also, note effects of diurnal and seasonal variation in incident