2.6 Tools and Other Algorithms

This section contains a description of some tools and algorithms that are not central to the RETScreen CHP model. Some subsections detail algorithms that are part of the Energy Model or Load and Network worksheets; other refer to auxiliary algorithms found in the Tools worksheet.

The values in Table 5 are estimates only; the building types may be described as follows:

• Office:
Government, data processing, financial centre, post office, office with retail (except food), real estate, computer centre, etc.


• Retail:
Strip mall, hardware store, department store, furniture store, drugstore, car dealership, multi retail buildings.


• Restaurant:
Full service, cafeteria, carry out, food related sales and service.


• Warehouse:
Storage, agricultural storage, stand alone barns, etc.


• School:
Educational buildings, colleges, Universities, etc.


• Health / Medical:
Medical clinic, dental clinic, veterinary clinic, out-patient care, rehabilitation centre.


• Hospital:
Medical care hospital, mental care facility.


• Hotel:
Motel, hotel, short-term residential, tourist home.


• Residential:
Apartments, condominiums, (may be used for single family homes).


• Food / Grocery:
Retail food, supermarket, farmer’s market, specialty food stores.


• Miscellaneous:
Fire/police station, library, religious assembly, amusement arcade, museum, art gallery, concert hall, theatre, gas station, jail, shelter home, civic assembly, passenger terminal, etc.

2.6.3 Calculation of base case fuel consumption

To evaluate the financial benefits of the proposed system, one has to calculate the quantity of fuel that would be used if the proposed system were not installed. This is what is called the alternative fuel consumption, or what is referred to as the base case system.

Units used to measure fuel consumption and heating values depend on the type of fuel used. Table 6 summarizes the units and heating values for the different fuel types in RETScreen.

where M_HAFC is the heating alternative fuel consumption [units: m³, L or MWh, as per Table 6], η_hs,se is the heating system seasonal efficiency [expressed without units] entered by the user, C_f is the heating value for the selected fuel type [kWh/unit, as per Table 6], and Q_H is the heating energy use of the building or cluster of buildings [expressed in kWh]. The same formula applies to the calculation of the cooling alternative fuel type consumption M_CAFC :

where η_cs,se is the cooling system seasonal efficiency [expressed without units] entered by the user, and Q_c is the cooling energy use of the building or cluster of buildings [expressed in kWh].

When the electricity is used for heating or cooling, the net amount of electricity used on a monthly basis is added to the amount of electricity used for heating or cooling purposes in order to calculate the gross monthly average power load. This can be shown mathematically as:

where P_p,i,gross is the gross monthly average power load, _p,i is the monthly power net average load specified by the user, P^e_H,i is the monthly power load for the heating system, _H,i is the monthly average heating load using electricity as a fuel, η_hs,se is the heating system seasonal efficiency, P^e_c,i is the monthly power load for the cooling system, _c,i is the monthly average cooling load using electricity as a fuel and η_cs,se is the cooling system seasonal efficiency.

2.6.4 Calculation of fuel heating value of biomass fuels

The calorific value or heating value of fuel is the measure of heat released, per unit weight of fuel, during the complete combustion of the fuel. The higher heating value (also referred to as HHV or gross heating value) refers to the maximum energy that can be released, per unit weight of dry fuel, from burning dry fuel (see note 10). The lower heating value (also referred to as LHV or net heating value or as fired heating value) of the fuel subtracts the energy in the water vapour produced from the water in the fuel and in the water vapour produced from the hydrogen in the fuel; it is expressed per unit weight of wet fuel.

High moisture content biomass fuel reduces system efficiency, because the vaporization of water to steam requires heat. As flue gases are rarely condensed in small biomass system, this energy which otherwise would be useful in heat production is thus diverted to drying the wood fuel in the combustion system prior to actually burning it. Higher moisture content in the fuel means lower net heating value of the fuel. Typical as fired heating values for biomass range from 10,800 to 15,900 MJ/tonne of biomass on a wet basis.

The heating value of biomass fuels depends on the nature of the fuel considered. In the CHP heating model of RETScreen, the user selects the type of biomass fuel from a list, and specifies the moisture content. The moisture content of biomass fuel on the wet basis is the weight of water in a wood sample divided by the total weight of the sample:

where HHV_Biomass is the higher heating value [MJ/kg], and C, H₂, O₂, N₂ and S are the weight percentage for carbon, hydrogen, oxygen, nitrogen, and sulphur respectively in the dry fuel. The corresponding lower heating value LHV (as fired), in MJ/kg, is given by:

The Tools worksheet contains a tool allowing the user to enter a user defined solid or gas fuel. The fuel entered in these tools will appear in the fuel database available through a drop-down list in the software.

The higher heating value for fossil fuels may be approximated by Dulong's formula:

where HHV_FossilFuel is the higher heating value [kJ/kg], and C , H₂ , O₂ and S are the mass percentage for carbon, hydrogen, oxygen, and sulphur respectively in the fuel.

The higher heating value for gas fuels is also calculated using Dulong’s formula (equation (65)). The Tools worksheet formula is considering the amount of methane CH₄ , ethane C₂H₆ , carbon-dioxide CO₂ , oxygen O₂ and nitrogen N₂ in the gas. As an example, for a gas that has 50% methane and 50% carbon-dioxide, the mass percentage for carbon, hydrogen, oxygen are:

where X is the total mole mass, x_co2 and x_CH4 are the mass percentage of carbon-dioxide and methane respectively and C , H₂ , O₂ , N₂ and S are the mass percentage for carbon, hydrogen, oxygen, nitrogen, and sulphur respectively in the fuel.

2.6.5 Network design

District heating and cooling network design is included in the RETScreen CHP model so that the user can do a preliminary sizing of the pipes and cost the installation. The calculation takes place in the Load and Network worksheet; however its results have no influence on the energy calculation part of the worksheet.

A district heating/cooling piping distribution system consists of an underground hot/cold water distribution network with supply and return line pipes in a closed circuit. Each building is connected to the network via a building heat/cooling transfer station that regulates and measures the energy taken from the distribution system. The network comprises a main distribution line which connects several buildings, or clusters of buildings, to the heating/cooling plant, and secondary distribution lines which connect individual buildings to the main distribution line. The pipe network is usually oversized to allow a future expansion of the system. In RETScreen the over-sizing factor is specified by the user.

For preliminary sizing of the network pipes a simplified method has been used in the RETScreen CHP model. It has been assumed that the head loss is not to exceed 20 mm H₂O or 200 Pa per meter of pipe; and for dimension larger than 400 mm a maximum velocity of 3 m/s is to be used. Standard formulae (Avallone & Baumeister, 1987) for pressure head loss in pipes as a function of water velocity and pipe diameter have been used to calculate maximum flow values shown in Table 8.

The total heating load carried in a pipe in the main distribution line can be calculated as:

where V is the volumetric flow of water, ρ the density of water, c_p its specific heat (set to its value at 78°C, 4,194 J/(kg °C) for heating pipes and at 5°C, 4.205 J/(kg °C) for cooling pipes), and ∆T_s-r is the differential temperature between supply and return, specified by the user. This relationship can be inversed to find, given the peak load of the building cluster (quantity P_H,j from equation (10)), the volumetric flow of water that the pipe will be required to carry:

The actual formula used in RETScreen includes a factor for pipe over-sizing; if κ is the main pipe over-sizing factor, expressed in %, entered by the user, equation (71) becomes:

Then, a lookup in Table 8 provides the desirable pipe size given the flow. In the case where several clusters of buildings are served by the same main distribution line pipe, the load in equation (72) should naturally be replaced by the sum of the relevant loads.

Finally, a similar relationship holds for secondary distribution lines piping. The denominator of equation (72) is then replaced with a load P'_H,j given by:

where κ' is the secondary pipe network over-sizing factor specified by the user, and N_j is the number of buildings in the cluster.

2.6.6 Landfill gas generation

The Tools worksheet in RETScreen includes a section to estimate landfill gas availability. This section explains the models used for this calculation.

Landfill gas is generated by the biological decomposition of wastes placed in a landfill. The composition of landfill gas is highly variable and depends on a number of site-specific conditions including solid waste composition, density, moisture content, and age. The specific composition of landfill gas varies significantly from landfill to landfill and even from place to place within a single landfill. However, landfill gas is typically comprised of methane and carbon dioxide, approximately 50 percent each by volume with trace quantities of other compounds including hydrogen sulphide, mercaptans, and non-methane organic compounds (NMOC).

Two other gases that may be present in landfill gas in varying quantities are nitrogen and oxygen. The presence of nitrogen and oxygen in a sample of landfill gas is often an indication that there is intrusion of ambient air into the landfill gas collection or delivery system. When in the presence of methane or other combustible gases, a gas mixture containing oxygen is considered to be explosive when the concentration of oxygen is between 5 and 15 percent by volume. A suitable source of landfill gas for flaring or utilization purposes attempts to maximize the concentration of methane and minimize the amount of oxygen and nitrogen in the mixture. For the purposes of a technical feasibility assessment of the use of landfill gas in a combined heat and power project as a fuel source, the concentration of oxygen and nitrogen in the gas fuel are assumed to be zero. Methane is the primary component of landfill gas which contributes to the gas’s heating value. The heating value of methane is typically defined on a volume basis.

Modeling landfill gas generation

There are numerous models available for estimating rates of landfill gas generation, however accepted industry standard models are generally first order kinetic models that rely on a number of basic assumptions. These models are used to predict the variation of landfill gas generation rates with time for a typical unit mass of solid waste. This generation rate curve is then applied to records, or projections, of solid waste filling at a site to produce an estimate of the site’s landfill gas generation over time.

The Scholl Canyon model, with defined default parameters is the empirical, first order decay model most widely accepted and used by industry and regulatory agencies, including Environment Canada and the United States Environmental Protection Agency (USEPA). There are many more detailed models available for the estimation of landfill gas generation rates, however, these models require more specific knowledge of the waste quantities, waste composition, and landfilling practices associated with the site than is generally available, especially for older landfill sites where such records were not required.

The Scholl Canyon model is based on the assumption that there is a constant fraction of biodegradable material in the landfill per unit of time, and is an estimate of the generation of methane from this biodegradable material. The first order equation is given below:

where Q_CH4,t is the volume of methane produced in year i from a section of waste, k the methane generation constant, L₀ the methane generation potential, m_i the waste mass disposed of in year i , t the years after closure of landfill.

It is typical practice to assume that the volume of landfill gas generated consists of 50 percent methane and 50 percent carbon dioxide so that the total volume of landfill gas produced is equal to twice the volume of methane calculated from equation (74). The volume of landfill gas estimated may be adjusted for any concentration of methane in the same manner.

The k constant represents the first order biodegradation rate at which methane is generated following the placement of biodegradable wastes. This constant is influenced by moisture content, the availability of nutrients, pH, and temperature. The moisture content of the waste within a landfill is one of the most important parameters affecting the landfill gas generation rate. The moisture content of waste within a landfill is influenced primarily by the infiltration of precipitation through the landfill cover, the initial moisture content of the waste, the design of the leachate collection system, and the depth of waste in the site. Typical values for k range from 0.02 for dry sites to 0.07 for wet sites.

The methane generation potential, L₀ , represents the total yield of methane expressed in units of m³ of methane per tonne of waste. The value of L₀ is dependent on the composition of the waste, in particular, the fraction of organic matter present in the waste. The value of L₀ may be estimated based on the carbon content of the waste, the biodegradable carbon fraction, and a stoichiometric conversion factor. Typical values for L₀ range from 125 m³ of methane/tonne of waste to 310 m³ of methane/tonne of waste. The default value for L₀ of 170 m³ of methane/tonne of waste recommended by the USEPA in their New Source Performance Guidelines (NSPS Tier 1 default, 1994), is considered to be a fairly conservative value, which is representative of a majority of domestic and municipal solid waste landfills in the United States. Selection of a different value for the methane production potential L₀ should be based on the users specific knowledge and experience with the landfill site that is being assessed.

The quantity (in tonnes) of typical waste landfilled in a particular year is represented by variable m_i in equation (74). In landfills where there are good data indicating a significant portion of inert or non-decomposable waste, such as construction and demolition debris, this parameter may be reduced to represent only the amount of waste that is not inert. However, in many cases there is insufficient data to make this determination. A specific reduction of m_i should only be made if there is a readily discernible portion of the waste that is different from the typical waste received at most conventional mixed solid waste landfills. The default assignment of L₀ already recognizes that there is a mixture of decomposable organic wastes and inorganic wastes being deposited in a typical fill site.

Another important factor is the assumed lag time between the placement of waste in a landfill and the beginning of the anaerobic decomposition of the waste mass, i.e., production of landfill gas. A typical lag time between the placement of waste and the start of methane generation is 1 year.

Figure 17 provides a landfill gas generation curve produced using the Scholl Canyon model with the conservative USEPA default values used for a preliminary site assessment for a relatively dry site (less than 625 mm (25 inches) of rain), with a constant fill rate of 500,000 tonnes per year for 25 years (from 1990 to 2015), using a value for k of 0.05, L₀ of 170 m³ of methane per tonne of waste. The figure shows two curves, the theoretical total amount of landfill gas generated using the Scholl Canyon model and the landfill gas collected assuming a typical collection system efficiency of 75 percent.

It is important to note that the results obtained from these models represent estimated production rates. Actual recovery rates will vary as dictated by the actual landfill gas generation rate and by the recovery efficiency of the landfill gas collection system. Reported recovery efficiencies range from 60 to 80 percent, and 75 percent is generally assumed in the absence of site-specific data.