The difference between energy demand and peak loads. How to estimate the electrical demand and energy demand for hot water calculations.
Energy flow rates vs energy amounts. Flow rate, e.g. what comes from a tap. Amount, a flow rate times a duration. Expressed in Joule.
1 J/s = 1 Watt
1 J = 1 Watt second
1 Wh = 3600 J
In this course, W and Wh are used because they make it easy for hourly estimates.
These are synonyms to energy flow rate: power, load, capacity.
These are synonyms to energy amounts: energy, energy demand, energy use.
Energy design of buildings: 1) Choose the right size of the HVAC equipment. 2) Determine and minimize the annual energy demand.
'Size' means maximal power/load/capacity: kW. Also called nominal power/load/capacity. Relates to physical size. Relates to costs.
Two methods to size:
Method 1: heating equipment should deliver heat for the highest demand hour of the year. Cooling should do the same.
May lead to oversized equipment.
Method 2: Use horly meteorological data over a whole typical year. A balance is made for each hour of the year.
How the energy balance helps determine annual demand.
Again, interested in choosing right size of heating/cooling equipment and how to minimize the energy demand.
Three methods:
First. Use Equivalent Full Hours. Based on data from comparable buildings, FLH = (1/n) <em> sum(energy demand / nominal power) for each building. Then E_year = Q_nom FLH.
Second. Degree Days. Based on data about outdoor temperatures. Uses reference temperature about when people start heating/cooling (country dependent). Sometimes criticized because solar radiation is not accounted for.
Disadvantages of both methods is inaccuracy and the fact that the characteristics of the specific building is not accounted for. Still good for quick estimates and comparisons.
Third. Hourly calculations using a typical climate year. Make the energy balance on all hours of a year and sum for all 8760 hours (use software). This is what building simulation software like energy+ does. Better than a single year is to use a TMY year (closer to long term averages).
For accuracy, hourly calculations are the preferred method.
Very dependent on the function of the building. e.g. offices often don't use a lot of hot water, but houses use a lot. As houses get more efficient the percentage of energy going to hot water increases.
Need to heat at least to 60 - 65 deg C to prevent bacteria.
How to estimate the electrical demand for a building.
Mechanical ventilation, lighting, appliances (electrical heating/cooling is a thermal demand still, not accounted for here).
Again, highly dependent on function and use of building.
Sum up the separate electrical needs to calculate.
Assume a ventilator is on for 70% of the time for quick calculations.
Offices often have more electric lighting than houses.
A model in an excel file for calculating energy demand.