The calculation model developed in the research provides an energy and economic analysis of the operation of trigeneration systems natural gas feed (or renewable sources) through the creation and development of sub-models for the definition of the consumption curves for the users, from data collected by means of an energy audit platform.

The first step of the model is the application of the typical process of "energy certification" of the user, with the aim to acquire in a standardized way the characteristics of the building energy need (both thermal and electric). In fact, the energy audit process of buildings used for the "energy certification" is a systematic set of survey, collection and analysis of parameters for the consumption and the facilities operating conditions evaluation.

In the second step, the model estimates the energy consumption profile for heating and cooling, also giving a basic sizing of HVAC systems through the evaluation of the heat load of the building in critical conditions. Similarly, the survey data of electric installations are used to define the power load curves in order to estimate energy consumption for lighting, and power for other electric appliances. It seems clear, therefore, that the load curves (both thermal and electric) are based on an energy survey that must follow “official procedure” (like the energy certification one) and it has to be carefully planned in order to give a complete energy profile of the user.

The final result of model process is the creation of thermal and electric hourly-load curve of users and depending on the current season.

At present, there are three modelling simulations depending upon the seasons (summer, winter, and spring/autumn), while the operating hours can be freely varied (the default value is 12 hours from 8 a.m. to 8 p.m.) [5].

The simulation model refers to a trigeneration plant based on conventional CCHP scheme (Figure 1), consisting of an internal combustion engine fuelled with natural gas or vegetable oil, a power generator, a heat recovery generator (capable of producing hot water from the waste heat of the engine), an absorber powered by the engine exhaust gas (for the cooling load demand) and an auxiliary boiler fuelled by natural gas integration to cover peak-demand of heat or cooling (feeding the absorption chiller). The system is also provided with an external heat exchanger, in order to ensure a stable operation when the user requires low values of thermal energy [6].

Figure 1.

Trigeneration system scheme and percentage of energy involved

The simulation is independent of the type of absorption system, (ammonia or lithium bromide, single or double stage) and of the working conditions (temperature of the fluid). The basic parameters of the system such as power, COP, and therefore power consumption is sufficient for the creation of consumption profiles during periods of its operation.

Regarding the dimensioning of the parameters of the whole system, some of them are freely adjustable such as the sizing of the cogenerator thermal power, the heat recovery rate and the absorber COP related to the product chosen. Consequently, other parameters like the cogeneration electric power obtained as a ratio of the thermal power, the gas consumption, etc. are directly fixed to prevent sizing inconsistencies.

The objective is to ensure adequate freedom in designing the system and then simulating multiple scenarios, while maintaining high correspondence with existing systems on the market [7].

The initial audit process allows users to build an energy model that, in fact, consists of hourly power consumption curves for each energy use (heating, cooling, lighting and other services).

Then the model gives a preliminary choice of a set of the most appropriate trigeneration systems, showing the main parameters: thermal power and cooling capacity, performance, etc.

The choice of the CCHP system working curve can be "pre-set" (fixed curve) or "tracking" (following the user consumption). This variable is particularly important and in the case of the second option, it gives an operation optimized according to the real user consumption [8].

The first step of evaluation allows the construction of a second set of curves of heating supply, power production and cooling capacity from the trigenerator.

By superimposition of these curves with the absorption curves mentioned above, it is possible to evaluate the system operation scenarios, to quantify the self -consumption of energy, the surplus of power production (sold back to the grid), and the need of integrating auxiliary boilers or auxiliary heat exchangers for cooling.

The assessment of energy saving in the model is essentially carried out through the evaluation of the benchmark between the current situation of consumption, both electrical and thermal, and the CCHP system operation (in the best operation hypothesis).

The research aims at the technical and economic evaluation of cogeneration solutions financed through a TPF mechanism. Therefore, once the CCHP peak power of the plant has been fixed, the calculation model optimizes the economic gain obtainable from the sale of electricity. So, the optimum sizing is generally obtained referring to the heat load, leaving aside the evaluation of the auto-consumption of the electric energy. The surplus can however be sold to the network and highly incentivized through some financial mechanisms, analysed later in the paper [9].

Depending on the choice, the system proposes an evaluation of the power production. Excluding the case where all the energy produced is used entirely to cover the self-consumption, the first analysis assesses the percentage of non-coverage needs that will buy directly from the grid with prices varying according to model input data. In the next step the opposite situation is evaluated when the power surplus is directly sold back to the grid through a contract of “Ritiro Dedicato” (Dedicated Withdrawal - DW).

The value of electricity sold to the electric grid is presented with different features depending on the regime chosen by the plant managers. Here are the options available, depending on the plant size and plant configuration.

White Certificates: the incentive scheme known as "White Certificates System" (Energy Efficiency Certificates - TEE in Italy) has been operated since 2005. It is a market-based mechanism of the bonds to be paid by distributors of electricity and natural gas, issued pursuant to the achievement of energy efficiency target with energy end users. In addition to mandatory parties, other energy market actors (e.g. Energy Services Companies – ESCos) can access the TEE system and manage them as a business in case of energy saving measures on an adequate scale.

Green Certificates - GC: the power producers from Renewable Energy Sources (RES), owners of qualified RES plants, may access the Green Certificates system as a form of incentive of the power production from RES. In Italy, as required by law 244/07, the power production from RES or retrofitting of power plants operating from April 1^st, 1999 to December 31^st, 2007, is considered renewable generation for the first twelve years of operation.

All-Inclusive Tariff - AIT: in Italy, the Ministerial Decree of 18 December 2008 has introduced a new incentive mechanism for power production from RES as an alternative to the Green Certificates system for the small plants with a power not exceeding 1 MW (0.2 MW for wind energy) in operation after December 31^st, 2007. This system consists of a simplified tariff regime for the power sold to the grid with a unique fixed price. The all-inclusive tariff depends on the energy source, and is fixed for a period of 15 years. This fee excludes any other remuneration for electricity sold to the network (including DW) [10].

The EPC is a contract which obliges "the supplier" (usually an ESCo) to fulfil by its own funds or funds of others a series of integrated services and interventions aimed at upgrading and improving energy efficiency of a system owned by another (the beneficiary). The contract also fixes an amount of energy savings (previously identified in the feasibility analysis phase) obtained as a result of the energy consumption optimizing measure identified in the contract for the system. In relation to risk-sharing, to cover the financing and the remuneration of the ESCo, the energy performance activities may lead to the following types of EPC:

• First out”

The ESCo provides the capital project (also using other third-party funding). The energy savings achieved are entirely used to repay the loan of the intervention and to reward the activities of the ESCo. The contract usually lasts for about 3-5 years. At maturity the savings should be entirely in favour of the customer who takes ownership of the facilities and the completed works.

• Shared Savings”

The “Shared Savings” is similar to the “First Out” scheme, but the parties shall agree on the allocation of savings income in the first year. The contracts have a duration of 5-10 years in view of the fact that only a portion of the savings contributes to the recovery of the initial investment. During the execution of the contract, the ownership of the plants and works is retained by the ESCo and the contractual deadline moves to the customer.

• Guaranteed Savings”

In this kind of contract, the financing is provided by a third party different from the ESCo and the customer provides financial guarantees. The ESCo takes on an organizational role to ensure a high level of performance under which the compensation is received by the customer. The contract lasts about 4-8 years. According to this formula, therefore, the ESCo is committed primarily to ensure that savings are not inferior to an agreed minimum, established on the basis of the feasibility study.

• First in”

In “First in” contract, a certain reduction of energy costs incurred in previous year’s historic intervention is guaranteed to the user. The cost savings achieved as a result of the intervention made by ESCo - responsible for the plant, which will retain ownership and management until the conclusion of the contract - is in the responsibility of the ESCo for the entire contract period, in way to determine the number of years necessary for ESCo to cover the investment and to achieve more profit for the company, according to forecasts of energy saving project.

• Chauffage”

In the “Chauffage” contract, the customer entrusts the management of their facilities to the ESCo, which provides the energy expenditure for the duration of the contract, upon the payment of a fee equal to the energy bill that the customer paid before the entry into force of the contract, less an agreed discount. In essence, with the Chauffage (asset ownership) a sort of outsourcing is put in place, namely operation where the customer entrusts the conduct of an activity (carried out independently in the past) to a third party.

Table 1 summarizes the contractual arrangements described, showing the duration of the contract, appliances, management and funds obtained and the percentage of savings for the client during the contract period.

The model validation was performed on a user of the large retail sector (shopping centre) located in the central part of Italy. The case study has a general interest because of the plants and building structures analysed represent the typical elements of the large retail sector and the results can be easily applied to other users of the sector.

The shopping centre has two levels with a total area of 24,430 m² with 1 hypermarket, 58 shops of various sizes and 6 restaurants. The building has a very simple concrete structure, the windows are double glazed and the ceiling of the common areas is covered by pyramid-shaped windows that provide natural day-light.

The HVAC system consists of electric heat pumps placed at the top of the building. Table 2 gives an example of the data collected during the audit phase with regard to heat pumps data.

Once the audit of the thermal systems and building structures is performed, the model algorithm estimates the behaviour of thermal energy loads on the basis of preset cycles.

As a first validation of the model, some measurements were performed on the heat pumps at the primary circuits with the aim to extrapolate the time-curve of thermal power load. Figure 2 shows a comparison between the simulated curves and the measurement performed on the machines in winter.

Figure 2.

Simulation and measurement of heat power (winter operation daily hours)

The results show an error rate in terms of thermal power output in the range 6÷9.5%. The error over an annual basis is in the order of 6.25%. So, the simulation model has a reliability of more than 93% and, therefore, it’s possible to set the “tracking” program (above described) for the model operation [11].

In addition to the basic parameters of a trigeneration plant (as discussed above), the model can also elaborate the input data as a consumption curve and verify the filling ratio between the consumption curves of the users and the supply curve of the trigenerator.

All the scenarios created require an assessment, because in the function that evaluates DW or AIT, the constraints vary in accordance with national regulations. Therefore the model indicates to the operator the possibility to change some values for each scenario.

As a result, it is possible that some optimized solutions present different production curves related to the choice of the regime of supply. For example, in the "DW with guaranteed minimum prices”, the system automatically limits the electric power system forcing up to 1 MW_e, and the operator may choose an engine size lower than other cases. It also follows a lower heat output, with its reduced load charts.

For the regime of “DW” and “DW with guaranteed minimum prices”, a CCHP system with internal combustion engine powered by natural gas is chosen; for the other cases: “green certificates”, “all-inclusive tariff with traceable vegetable oils” and “all-inclusive tariff”, the engine is fed by vegetable oil.

Regarding the choice of vegetable oil, from a technological point of view, the soy is preferable because it is the only one that doesn’t introduce critical aspects in the system, it has a low content of phospholipids and low iodine value and thus cures less than others.

The model output consists of the CCHP power supply curve overlapping the consumption curve of the user for a fixed period (day, week, month, etc.). The Figures 3 and 4 show the daily curves in the winter time for a system operating in DW. The model can evaluate each scenario that combines different systems for electricity exchange with the grid, with all the eligible incentives and contractual options [12].

Figure 3.

Electric curves overlapping (typical winter day)

Figure 4.

Thermal curves overlapping (typical winter day)

The comparison of these scenarios considering the respective economic values, incentives, royalties from EPC contracts, financial rates, depreciation, etc., the model extrapolates the optimum scenario of the project from the ESCo-side and for the client-side for entire duration of the contract.

The model operates on a maximum time of 15 years, which is the duration of the typical plant warranty and the period in which the green certificates are guaranteed. This period of time is subdivided, depending on the chosen contract, into a part of ESCo-management and the remaining when the management is given directly to the customer.

Among the output data, there are the values of profit after tax and the corresponding tax (zero in case of negative profit) for all 15 years of expected life of the plant. Financial ratios are also calculated: NPV (Net Present Value), IRR (Internal Rate of Return), IP (Index of Profitability: an index that attempts to identify the relationship between the costs and benefits of a proposed project), and ADSCR (Average Debt Service Coverage Ratio: the ratio between operating cash flow and debt service during any one-year period).

Regarding the client-side, some data are shown: the purchase price for natural gas, the price trends of energy sold to the grid, the average price of electricity supply and the prices for “vegetable oil”; in the “tax exemption for methane purchase” area it is possible to calculate precisely the unit price of natural gas (€/Nm³); moreover, in the mask “MT Network Connection” the cost for the network connection (in Medium Voltage) is estimated; finally, in the “financial evaluation” mask, the pattern of Euribor rates of the last 10 years are recorded.