Performance Assessment Metrics

3.2.5.1 Energy Analysis

Aggregated annual, monthly, daily and half-hourly energy consumption figures (in kWh or MWh) were utilised in this research project. These were acquired via periodic meter readings or by AMR. These figures were used to assess the quasi-dynamic and seasonal performance of a DE system and were calculated using Equation (3. 5). This equation divides the total energy generated by the primary energy consumed from the system or sub-system. The primary energy does not include parasitic electricity consumption.

𝜼 = 𝑻𝒐𝒕𝒂𝒍 𝒆𝒏𝒆𝒓𝒈𝒚 𝒐𝒖𝒕

𝑷𝒓𝒊𝒎𝒂𝒓𝒚 𝒆𝒏𝒆𝒓𝒈𝒚 𝒊𝒏=𝑬⁻+ 𝑾⁻ + 𝑸⁻

𝑬⁺+ 𝑾⁺ + 𝑸⁺ (3. 5)

3.2.5.2 CO

Emissions Analysis

In the UK, natural gas is commonly the primary fuel used for the heating sector as individual natural gas boilers are usually used to heat dwellings or offices. Building regulations Part L 2013 acknowledges this, so when a DH system is assessed, the practitioner compares its operation to a baseline, which assumes a similar amount of consumed heat to be generated by individual natural gas boilers operating with a seasonal efficiency of 85%. If the DH energy plant also generates some electricity, the equivalent amount of electricity is assumed to be provided from the national grid. The baseline does not consume electricity to pump the heating flow and does not have any heat losses to the environment as shown in Figure 3- 2.

As grid electricity in the UK has a high carbon intensity factor, operating an energy plant in cogeneration while consuming natural gas reduces the national CO2 emissions. To calculate the DH system CO₂ emissions and possible CO₂ reduction, SAP 2012 carbon factors were used and are given hereafter (DECC, 2013d):

 Natural gas: 0.216 kg CO2/kWh

 Wood pellet: 0.039 kg CO2/kWh

 Elec: 0.519 kg CO2/kWh

Figure 3- 2 DH system CO2 emission compared to the UK baseline

3.2.5.3 Financial Analysis

The financial analysis of a DH system is dependent on the following three parameters. Firstly, the cost of financing new equipment varies with every contract and also depends on the repayment term. Secondly, a particular technology at a given time may benefit from

resource and is in competition to the baseline that operates natural gas boilers. Taking into consideration these factors, two different types of financial analysis were undertaken to assess the viability of a DE technology:

a) The operation cost saving b) The payback

a) Operational cost saving

The operational cost saving is calculated with Equation (3. 6). It is calculated by subtracting the scenario operational cost from the baseline. Note that the baseline will be different for an existing and a new DH system as described below:

𝑪_{𝑺𝒂𝒗𝒊𝒏𝒈} = 𝑪_{𝑩𝒂𝒔𝒆𝒍𝒊𝒏𝒆}− 𝑪_{𝑺𝒄𝒆𝒏} (3. 6)

Baseline operational cost

 For an existing DH system, the baseline is the current operation of the plant.

Operational cost is calculated with Equation (3. 7). This equation includes the energy resource consumed to generate the electricity and/or heat, the energy plant maintenance cost and the income from exporting electricity.

𝑪_{𝑩𝒂𝒔𝒆𝒍𝒊𝒏𝒆} = (𝑪_𝑹𝒆𝒔)_{𝑩𝒂𝒔𝒆𝒍𝒊𝒏𝒆}+ (𝑪_{𝑴𝒂𝒊𝒏𝒕})_{𝑩𝒂𝒔𝒆𝒍𝒊𝒏𝒆}− (𝑪𝑰𝒏𝒄𝒐𝒎𝒆_𝒆𝒍𝒆𝒄)_{𝑩𝒂𝒔𝒆𝒍𝒊𝒏𝒆} (3. 7)

 For a new DH system, the baseline is based on Part L 2013 (DECC, 2013d): It

assumes the heat to be generated by individual natural gas boilers operating with a seasonal efficiency of 85% and the electricity is provided by the grid. As the individual boilers maintenance costs are neglected in this thesis, Equation (3. 7) reduces to Equation (3. 8).

𝑪_{𝑩𝒂𝒔𝒆𝒍𝒊𝒏𝒆} = 𝑪_{𝑹𝒆𝒔𝒐𝒖𝒓𝒄𝒆}

(3. 8)

Scenario operational cost

The operational cost for a new and/or existing but upgraded DH energy plant is calculated with Equation (3. 9). It includes the annual cost of the primary energy resource consumed, the maintenance cost, the financing cost and if relevant the income from exporting electricity.

𝑪_{𝑺𝒄𝒆𝒏}= (𝑪_𝑹𝒆𝒔)_{𝑺𝒄𝒆𝒏}− (𝑪𝑰𝒏𝒄𝒐𝒎𝒆_𝒆𝒍𝒆𝒄)_{𝑺𝒄𝒆𝒏}+ (𝑪_{𝑴𝒂𝒊𝒏𝒕})_{𝑺𝒄𝒆𝒏}+ (𝑪_{𝑹𝒆𝒑𝒂𝒚})

𝑻𝒆𝒄𝒉 (3. 9)

b) Payback assessment

A DH operator may decide to invest significantly in purchasing a new technology for a reduction in heating cost. For this reason, before investing in a new technology, a DH operator assesses its viability by calculating the potential operational cost saving and the payback of the new technology. The payback in number of years is calculated using

Equation (3. 10) and divides the capital cost of the new technology by the annual associated cost savings using Equation (3. 11).

𝑷𝒂𝒚𝒃𝒂𝒄𝒌 = 𝑪𝒂𝒑𝒆𝒙

𝑪_{𝑺𝒂𝒗𝒊𝒏𝒈} (3. 10)

𝐶_{𝑆𝑎𝑣𝑖𝑛𝑔} = 𝐶_{𝐵𝑎𝑠𝑒𝑙𝑖𝑛𝑒}− (𝐶_𝑅𝑒𝑠)_{𝑆𝑐𝑒𝑛}+ (𝐶_{𝐼𝑛𝑐𝑜𝑚𝑒𝑒𝑙𝑒𝑐})_{𝑆𝑐𝑒𝑛}− (𝐶_{𝑀𝑎𝑖𝑛𝑡})_{𝑆𝑐𝑒𝑛} (3. 11)

An Energy Service Company (ESCo)

Operators usually do not have the capital to purchase DE technologies such as CHP engines and may prefer to make an alternative funding arrangement with an ESCo. Although ESCo arrangements vary greatly, they are based on supplying heat and electricity to the operator for a fixed term (typically 10 to 15 years). Usually, the operator becomes the owner of the plant at the end of the agreement period. The drawback of having such an arrangement with an ESCo is that the benefits are shared because two parties are involved. As the ESCo purchases, installs and maintains the DE technology the net operating income cannot be assessed in the same way for the two parties. In the case of a CHP engine, the operator can quantify its operational cost savings with Equation (3. 12). This equation calculates the resource energy consumption saved, the income for the electricity generated and subtracts the agreed ESCo cost. The agreed ESCo cost can be calculated with Equation (3. 13) or Equation (3. 14).

Equation (3. 13) calculates the cost of operating the energy plant with a CHP engine, the repayment cost for the installation cost of the CHP engine and the additional cost from the service provided by the ESCo, whereas Equation (3. 14) calculates this cost as provided by the ESCo agreement contract. As an ESCo agreement contract guarantees the operation of the

CHP engine to generate a fixed amount of electricity that the ESCo will invoice; the additional electricity generation is usually free of charge for the operator. This fixed amount of electricity that an ESCo invoices is referred as the “intake”.

𝑪_{𝑺𝒂𝒗𝒊𝒏𝒈} = 𝑪_{𝑩𝒂𝒔𝒆𝒍𝒊𝒏𝒆}− (𝑪_𝑹𝒆𝒔)_{𝑬𝑺𝑪𝒐_𝑺𝒄𝒆𝒏}+ 𝑪_{𝒈𝒆𝒏_𝒆𝒍𝒆𝒄}− (𝑪𝑨𝒈𝒓_𝒄𝒐𝒏𝒕𝒓𝒂𝒄𝒕)_{𝑬𝑺𝑪𝒐} (3. 12)

(𝑪𝑨𝒈𝒓𝒆𝒆.𝒄𝒐𝒏𝒕𝒓𝒂𝒄𝒕)_{𝑬𝑺𝑪𝒐} = (𝑪_{𝑴𝒂𝒊𝒏𝒕})_{𝑷𝒍𝒂𝒏𝒕}+ 𝑪_{𝑹𝒆𝒑𝒂𝒚𝒎𝒆𝒏𝒕}+ 𝑨𝒅𝒅𝒊𝒕𝒊𝒐𝒏𝒂𝒍 (3. 13)

(𝑪𝑨𝒈𝒓𝒆𝒆.𝒄𝒐𝒏𝒕𝒓𝒂𝒄𝒕)_{𝑬𝑺𝑪𝒐} = 𝑨𝒏𝒏𝒖𝒂𝒍 𝒊𝒏𝒕𝒂𝒌𝒆 𝒐𝒇 𝒆𝒍𝒆𝒄𝒕𝒓𝒊𝒄𝒊𝒕𝒚 𝒈𝒆𝒏𝒆𝒓𝒂𝒕𝒊𝒐𝒏 (3. 14)

As it is the ESCo that invests in the DE technology and maintains it rather than the operator, the payback is calculated from the perspective of the ESCo using Equation (3. 15). This equation divides the installation cost by the agreed annual intake from the CHP engine electricity generated reduced by the maintenance costs. The agreed intake is usually equivalent to a CHP engine operating at maximum load for approximately 4,000 hours. The conference paper in Appendix B discusses this and also expands on the maintenance cost.

𝑷𝒂𝒚𝒃𝒂𝒄𝒌 = 𝑪𝒂𝒑𝒆𝒙

(𝑪𝑨𝒈𝒓_𝒄𝒐𝒏𝒕𝒓𝒂𝒄𝒕)_{𝑬𝑺𝑪𝒐}− 𝑪_{𝑴𝒂𝒊𝒏𝒕} (3. 15)

3.2.5.4 Exergy Analysis

Exergy is defined as the theoretical maximum work which can be obtained from each energy unit and the overall system, (Borel and Favrat, 2010). The sources of energy can be divided into two groups: High and low grade energy. The conversion of high grade energy to shaft work such as electrical energy is exempted from the limitations of the second law of thermodynamic that states that in a natural thermodynamic process, there is an increase in the sum of the entropies of the participating systems (Borel and Favrat, 2010). Josiah Willard Gibbs (1839 – 1903) was accredited with being the originator of the availability of energy concept by indicating that the environment plays an important part in evaluating the available energy (Nag, 2002) and according to Favrat et al. (2008) the exergy approach allows us to quantify in a coherent way, both the quantity and the quality of the different forms of energy considered.

Compared to other definitions of efficiency, the exergetic efficiencies are never bigger than 100% and are compatible with all cases of energy conversions for all energy services. It indicates the relative quality of the conversion and therefore different technologies can be compared. As illustrated in Figure 3- 3, a clear set of system boundaries needs to be drawn in order to define the exergy indicator.

Figure 3- 3 DH system boundaries at Tref and Pref (Favrat et al., 2008)

Assuming a constant atmospheric temperature, the overall exergy efficiency of the defined systems can be calculated using the definition Equation (3. 16), where 𝜂 is the exergy efficiency, 𝐸̇ is the mechanical and electrical work, 𝐸̇_𝑞 is the heat exergy and 𝐸̇_𝑦 is the transformation exergy.

𝜼 = _{∑ 𝑬̇}^{∑ 𝑬̇}₊⁻_{+ ∑ 𝑬̇}^{+∑ 𝑬̇𝒒−+∑ 𝑬̇𝒚−}

𝒒++∑ 𝑬̇_𝒚⁺ (3. 16)

In Equation (3. 16), all terms are positive, differentiating between the terms entering the system with “+” sign and the positive terms (services) delivered by the system with a “-“

sign.

 𝐸̇_𝑞⁺ is the heat exergy and can be calculated with Equation (3. 17). 𝜃 is the Carnot Factor and is equal to (1 −^𝑇_𝑇^𝑎) and 𝑄̇ is the rate of heat transfer.

𝑬̇_𝒒⁺ = ∫ 𝜽𝜹𝑸̇⁺ = ∫ (𝟏 −𝑻_𝒂

𝑻) 𝜹𝑸̇⁺ (3. 17)

 The transformation exergy 𝐸̇_𝑦⁺groups the transformation exergy of material flows entering or leaving the system. This term groups the network of fluids which come into direct contact with each other. Heat exchangers link two networks and therefore involve two transformation exergy terms. The exergy value of the fluid in the heat distribution is calculated with Equation (3. 18).

𝑬̇_𝒚⁺ = 𝑸̇⁺(𝟏 − 𝑻_𝒂 𝑻̂_{𝒍𝒏 𝒇𝒍𝒖𝒊𝒅}) With 𝑻̂_{𝒍𝒏 𝒇𝒍𝒖𝒊𝒅}= ^{𝑻𝒐𝒖𝒕−𝑻𝒊𝒏}

𝒍𝒏(^𝑻𝒊𝒏

𝑻𝒐𝒖𝒕) in Kelvin. (3. 18)

The subsystem boundaries from Figure 3- 3 can each have their exergy efficiency calculated with Equation (3. 19).

𝜼_𝒊 =∑ 𝑬̇_𝒒𝒊⁻

∑ 𝑬̇_𝒚𝒊⁺ (3. 19)

After calculating individual exergy efficiencies for subsystem 1 to 4, identified in Figure 3- 3, the overall exergy efficiency can be calculated with Equation (3. 20).

𝜼 = 𝜼_𝟏∗ 𝜼_𝟐∗ 𝜼_𝟑∗ 𝜼_𝟒 (3. 20)

4 THE RESEARCH UNDERTAKEN

Following the methodology described in Chapter 3, four case studies of DE and DH systems were analysed and their operational performance assessed. This included investigating the management and performance of energy plants, the heat losses to the soil from DH networks and the exergy breakdown from the DH network to the end-users. As discussed in Section 1.3, a DH energy plant generates electricity and/or heat while consuming a primary fuel or electricity, but parasitic electricity and additional electricity are also consumed to operate the supply units and to pump the heat to the consumers. Hence, the following four investigations were assessed:

 The supply units dynamic and seasonal performance

 The thermal storage use and operating efficiencies

 The parasitic electricity consumption to operate the supply units

 The additional electricity consumption to pump the DH network flow

These investigations assisted undertaking CO₂ emissions analysis, financial assessment and the exergy efficiency study. Finally, using these investigations, an optimisation method was developed to devise the guidelines to optimise a DH system. This paper is given in Appendix E and Table 4- 1 outlines the investigations undertaken on each case study.

Table 4- 1 Research undertaken