*4.2. Energy Systems Optimisation Modelling (ESOM) Results*

A refurbished detached house with a mediumweight thermal mass construction material and PUZ112 heat pump type with 35 ◦C flow temperature is selected as the baseline scenario. As the water flow runs in low temperatures, the stored water will be heated to 60 ◦C one hour once a week to avoid legionnaires' disease based on HSE [61] guidance. Five types of tariffs (Standard, E7, Comfy Heat, E12, and E20) and four tank sizes (250 L, 500 L, 750 L, and 1000 L) are compared with two different settings (Gas boiler as a backup heater and no backup heater) for the optimisation.

The amount of energy produced by the heat pump and backup heater varies between 16,500 and 17,500 kWh based on TES tank sizes. Figure 6 illustrates the energy input for optimisation scenarios for the heat pump and boiler. When the gas boiler is used as a backup heater, Standard tariff results show the lowest energy consumption values because the heat pump is running at full capacity assuming that there is no peak or off-peak time tariff, so the gas boiler usage is minimised. The boiler is only running when the tank size is 250 L. Even though the consumption figure is 6015 kWh with a 250 L tank size, energy input values occur as 3794 kWh, 3879 kWh, and 3973 kWh for larger tank sizes respectively. The 250 L tank size is not enough to replace the gas boiler in the Standard tariff. E20 tariff results show similar but slightly higher results than the Standard tariff. The consumption increases to 6553 kWh with 250 L tank size. However, when the tank sizes increased, the consumption figures decreased in the E20 tariff. So, increasing the tank size has a positive impact on this tariff.

**Figure 6.** Energy input of backup scenario analysis for heat pump and backup heater during off-peak and peak times.

The highest consumption occurs in the E7 tariff with 250 L tan size (11,064 kWh). This is mainly because the low number of off-peak hours leads to natural gas usage during peak time and decreases electricity usage to 16% of total energy consumption. Higher tank sizes help to decrease energy consumption to 9159 kWh, 7925 kWh, and 6243 kWh with 500 L, 750 L, and 1000 L tank sizes, respectively. Higher tank sizes offer higher electricity usage in off-peak times and help to reduce natural gas usage. Even though the share of electricity in total energy consumption is 16% in 250 L tank size, this could be increased to 24%, 33%, and 51% with higher tank sizes, respectively.

Comfy Heat and E12 tariffs have 12% and 25% lower results than E7 tariff on average. When the number of off-peak hours increases energy consumption reduces because of a higher heat pump fraction in energy generation. This reduction is greater in higher tank sizes. However, the trend in natural gas usage is similar. When tank sizes are increased, energy consumption reduces. E12 tariff with a 1000 L tank size can reduce the consumption to 5116 kWh, which is 25% higher than the lowest energy consumption in all scenarios. This is relatively small when compared with the 9638 kWh consumption of a 250 L tank size.

When the backup heater is not in operation, all energy demand is provided by the heat pump. Energy consumption figures are very similar in all scenarios ranging between 3446 kWh and 3763 kWh with a 250 L tank size. When the tank size increases, energy consumption could reach 3791–3973 kWh, but the difference is still small when compared with the backup heater operation. Even though the total energy consumption values are similar, the fraction of peak and off-peak times changes for E7, Comfy Heat, and E12 tariffs. The peak time usages in E7 tariff are 49%, 47%, 46%, and 43% for 205 L, 500 L, 750 L, and 1000 L tank sizes. However, these numbers reduce to 28%, 24%, 19%, and 16% for Comfy Heat tariff and 21%, 17%, 14%, and 13% for E12 tariff. These differences could result in higher heating costs because of higher peak time tariffs, but this will be covered in the heating cost discussion.

Figure 7 illustrates the total heating cost results for both heating technologies during peak and off-peak times. Even though there is no differentiation between off-peak and peak time costs for natural gas, the results are presented in the same format as electricity. This approach helps to understand the optimisation of the scenario by reducing gas usage and maximising electricity in off-peak time.

**Figure 7.** Heating cost of backup scenario analysis for heat pump and backup heater during off-peak and peak times.

When the backup heater is in operation, the Standard tariff has the highest cost with £792.9, £798.5, £812.5, and £828.1 with 250 L, 500 L, 750 L, and 1000 L tank sizes respectively. Increasing the tank size creates higher heating costs as there is no off-peak time strategy in this tariff. E20 tariff expects an average 14% reduction in total heating cost. However, the highest standing charge occurs in this tariff with £171.2 followed by £87.1, £86.8, £74.1, and £65.6 in E7, Standard, Comfy Heat, and E12 tariffs, respectively. Therefore, the reduction in the total cost is limited in the E20 tariff. Another reason for the high heating cost is that even though the 20 h of off-peak rate is very competitive, the highest electricity off-peak rate among other tariffs also exists in this tariff.

E7 tariff shows around 15% lower results than the Standard tariff. The cost of the scenario with a 250 L tank size reduces to £709.7. Electricity peak time cost dominates off-peak time results with £145.9 and £97.2 respectively. A similar trend occurs for larger tank sizes. However, the electricity share increases from 53% with 250 L tank size to 44%, 37%, and 26% with larger tank sizes. Even though the reduction of total heating cost is not significant when the tank size increases, the contribution of the heat pump increases from 47% to 74%.

Comfy Heat tariff has an average of 37% lower results than the Standard tariff, but the lowest reduction occurs in the E12 tariff with around 39% in all scenarios. Off-peak time electricity cost dominates peak time results, and the difference is greater with larger tank sizes. Peak time electricity share in total electricity cost is 23% in both tariffs with 1000 L tank size. Although half of the cost in scenarios with 250 L tank size comes from natural gas, this contribution reduces to 25% with 1000 L tank size.

When the backup heater is not in operation, heating costs in all scenarios reduce. The main reason for that is there is no standing charge for the gas boiler in this setting, so it creates a benefit. The high efficiency of the heat pump could eliminate increases coming from electricity costs depending on different electricity rates in different tariffs. The lowest reductions occur in Standard, E7, and E20 tariffs with an average of 12%, 14%, and 16% decreases. The highest reduction can be achieved with the Comfy Heat tariff with a 31% reduction followed by the E12 tariff with 26%. The total cost of a heating bill can be achieved as £450 with the Comfy Heat tariff. The weather conditions and having no extreme conditions provide higher COP values which could compete with gas prices and make standalone operations financially feasible.
