The optimal design and operation of cogeneration and trigeneration systems for buildings applications is a complex issue. Analysis' methods oriented to achieve maximum energy savings or minimum exergy destruction have arisen several controversial issues; both the most common operation strategies, namely heat-tracking and electricity-tracking, have advantages and drawbacks in terms of operating results and may lead the plant designer either to undersize or oversize the CHP unit. After pointing out the limits of heuristic approaches based on demand duration curve, an integrated design optimization is proposed, assuming a flexible management criterion which represents a good compromise between profit and energoenvironment oriented solution. It results from a brief thermoeconomic analysis of the profit formation process and is based on the definition of few operating modes to be scheduled depending on energy price profiles. The method, which takes into account technical limits of engines, is applied to a large hotel in Italy. Large benefits are achieved in terms of Net Present Value due to a more effective use of the installed capacity during peak hours.
Despite the recognized potential for polygeneration plants in buildings, these systems cover a negligible share of the installed capacity, even in large buildings of the tertiary sector where a major profitability could be achieved. Economics of polygeneration depends on several variables, i.e. plant lay-out, size of components, management strategy, control system effectiveness and energy prices; consequently, the optimization of small scale cogeneration and trigeneration systems is a complex issue, which may be based on thermodynamic analysis (Yilmaz, 2004), energy consumption calculations (Kong et al., 2005) and linear programming models (Rong and Lahdelma, 2005). Heuristic approaches have been also adopted, based on cumulative curve of energy demand or on enhanced immune algorithms and oriented to determine “physically meaningful” near-optimal solutions. The objective function should represent a good compromise between the most profitable design and the solution achieving maximum social- benefits, i.e. primary energy saving or reduction in pollutant emissions. Furthermore, being assessed as “high efficiency CHP” is very important under the growing regulatory framework promoting cogeneration (Directive 2004/8/EC); depending on the incentives, this aspect could play a primary role even for profit-oriented optimizations, thus becoming evident the need for multi-objective decision functions or constrained optimizations (constraints expressed by minimum savings to be assessed as environment-friendly system). Methodologies: 1. Design of grid connected CHCP systems covering a variable energy demand cannot be effectively optimized with no regard to the optimization of plant operation; 2. Variability of internal demand plays a primary role in CHP applications for buildings; 3. Being electricity price highly variable on hourly basis, in order to maximise profit a flexible management strategy (based on appropriate indicators) should be pursued; 4. A technically feasible operation should be ensured and physical limits of components kept into account. Heuristic methods based on duration curve (Cardona and Piacentino, 2003) must be adjusted; 5. Dispatching priority for CHP electricity and the opening of free electricity markets suggest to conceive CHCP systems as power producer and not only as “auto-production units”. This approach, usually adopted for large industries with low demand variability,can also extend CHP viability for small systems in case of favourable tariff context; 6. The system should be assessed as “highly efficient” according to the legislation in force.
|Effective start/end date||1/1/06 → …|
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