# Biomass District Heating Systems Based on Agriculture Residues
## 3 Methodology
#### 5.3.3 Straw
According to [24], network investment can be evaluated as follows:  
\[CAPEX_{N}=(C_{1}+C_{2}\cdot d_{a})\cdot L_{P}=(C_{1}+C_{3}\cdot P_{th}^{0.5}) \cdot L_{P} \tag{6}\]  
The use of plastic pipes in heating networks has reduce the costs at levels of \(C_{1}=120\)\(\upxi\)/m and \(C_{3}=750\)\(\upxi\)/m\(\cdot\)W\({}^{0.5}\) evaluated from real installation within the scope of this research. _(ii) Reference Baseline definition._ The baseline (\(L_{base}\)) in a BioDH project, which replaces heating and ACS generation from individual or collective boilers in buildings, is the unit cost of the useful energy, plus the depreciation and maintenance costs of the individual boiler (Equation (7)) [8]. \[L_{base}>Price_{kWh}>\frac{C_{fixed}^{BioDH}+C_{a}^{central}\ +C_{a}^{network}\ +C_{E}^{network}+C_{fuel}^{ central}}{E_{sold}} \tag{7}\]  
where \(C_{fixed}^{BioDH}\) is the network fixed cost (\(\upxi\)), \(C_{a}^{central}\) and \(C_{a}^{network}\) are the central and network amortisation costs respectively, \(C_{E}^{network}\) is the network energy cost, \(C_{fuel}^{central}\) is the fuel cost and \(E_{sold}\) is the total energy sold (kWh). The reference baseline cost for the current heating system is based on the use of individual or collective diesel boilers, with a cost of 45 \(\upxi\)/MWh. If considering also maintenance costs and amortization of the individual boilers are added and combined with a seasonal average yield of 70%, the reference baseline cost is 85 \(\upxi\)/MWh. _(iii) Incomes._ Incomes are calculated as follows:  
The price should be clearly below than the baseline price to make it attractive to the consumer avoiding to introduce additional costs to the end user. \[I_{n}=D\cdot Price_{kWh} \tag{8}\]  
where \(I_{n}\) are the incomes (\(\upxi\)) and \(D\) the energy sold (kWh). To promote the subscription and to give a clear benefit to the end user the sale price for the case study includes a discount on the current cost of heating of 20%, resulting in a selling price of 68 \(\upxi\)/MWh. _(iv) OPEX._ Operational Expenditures (OPEX) in the cost-benefit analysis are given by biomass cost and conveying energy consumption cost, which depends linearly on the heating demand. They must be included together with the fixed operation costs. \[OPEX=C_{fuel}^{central}+C_{E}^{network}+C_{fixed}^{BioDH} \tag{9}\]where \(C_{fuel}^{central}\) is the product of the mean price of biomass \(\overline{P}_{B}\), of each BioDH by the energy consumed by the system. Pumping cost depends on the energy demand and to a lesser extent on the topology of the network. A estimation for fixed costs \(C_{fixed}^{BioDH}\) is 5.5% of the investment [25]. _(v) Viability assessment and investment return_. Due to the infrastructure characteristics and the involved equipment a time horizon of thirty years is considered [5; 8]. The Internal Rate of Return (\(IRR\)) is used as main parameter for the evaluation and comparison between projects of the interest of investment. \[-CAPEX+\sum_{i=1}^{30}\frac{I_{ni}-OPEX_{i}}{\left(1+IRR\right)^{i}}=0 \tag{10}\]  
Step 7. Evaluation of the potential of a BioDH system and environmental assessment  
To determine the real potential of biomass district heating in an area o region a minimum viability threshold must be set, It will depend on the characteristics of the investor (i.e.,: if it is public or private). Once defined the viability criteria, for all viable BioDH networks, the avoided emissions can be evaluated by applying the conversion factor for fuel oil [26]:  
\[\Delta_{CO_{2}}=CO_{2}^{Boil-dom}=\frac{D}{\overline{\eta}_{ domestic\ boil}}\cdot GHG_{Diesel} \tag{11}\]  
where \(\overline{\eta}_{ domestic\ boil}\) is the individual boiler efficiency replaced by the BioDH system, and \(GHG_{Diesel}\) is the conversion factor for diesel fuel.