Types of performance prediction
Utilisability // Empirical Correlation // Simplified Analytical Approach // One-day repetitive.
Where is utilisability used?
Mainly for industrial applications with large store.
What is the condition for which utilisability can be used for closed loop systems?
If the input to the heat exchanger is known and constant.
What is the critical threshold?
The threshold above which radiation is useful
what is hourly utilisability?
the fraction of hourly radiation that can be converted into useful heat by an optically ideal collector
What are empirical correlations based on?
simulations in the UK using a reference system
Assumptions for empirical correlations?
- direct system with preheat
- preheat vessel with a volume > 0.8 x load
- load 100 l/day at 55
- cold water at 14 degrees
- flow rate 0.02 kg/sm^2
- pump is on when DT > 1.5C of off when < 1C
- weather is based on london
Describe the simplified analytical method
There is a single expression for annual output
Where are simplified analytical methods useful?
Industrial applications
- inlet temp and load known and constant throughout year
- open lop systems where Tci and Txi are known
- the system runs when there is sunshine and doesnt when there isnt
What is good about the one-day repetitive method?
Very flexible - most types of systems can be modelled
what are the two approaches to the one-day repetitive method?
- Typical meteorological day
- MIRA
What criteria for a Typical Meteorological day method does a cumulative frequency distribution mirrored about noon satisfy ?
- has the correct statistical distribution of all hourly radiation on the collector.
- has the correct mean daily radiation
- is symmetrical about solar noon
- reaches a maximum at solar noon
- is zero at collector sunrise and sunset
- cumulative frequency distirbution satisfies
typical meteorological day method:
- cumulative frequency distribution generated and mirrored about noon to form TMD
- an initial storage temp is assumed
- system is simulated over the TMD repeatedly until the storage temp at the end of the day converges with the storage temp at the beginning
MIRA method:
- MIRA uses a typical diurnal variation in irradiance
- The method uses the hourly utilisability concept to find the store temperature for each hour through the day
- Start with an assumed store temperature as with the TMD method
Explain f chart (4 points)
- based on empirical correlation from a large number of detailed simulations
- only one calculation for each month
- only applicable for a few standard systems
- one equation represents fm the fraction of the load met by solar energy
what is the Φ f-chart?
- similar to f-chart but for industrial applications - it uses utlisability
Key points on Φ f-chart?
- any water inlet temp allows
- any water outlet temp allowed
- constant load over the whole month
- no load dumping
