A heat pump is a device that takes in low-temperature heat and upgrades it to a higher temperature to primarily provide process heat or space heating (Smith, 2010). The source of low grade heat can either be waste heat to be expelled to the atmosphere or drain, and can also include heat that is currently cooled by a refrigeration plant. As such, it has the potential to reduce the load requirement of a boiler and refrigeration plant for small-medium factories. In addition, reducing the load on refrigeration plants can lower the factory water consumption as refrigeration plants typically employ forced wet cooling towers to cool the heat discharge to atmosphere. However, the capital cost of HPs is high in comparison to a conventional combustion boiler and typically impose long economic payback period times. Despite this, there are different configurations of HPs with different COPs operating at different electricity costs that can be competitive with the aid of subsidies e.g. government grants like Renewable Heat Incentives (RHI) (IRENA, 2013).
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There are several commercial heat pump types available based on the mechanical vapour compression cycle or the absorption cycle (Wu et al, 2014a, Wu et al, 2014b), and new designs continue to emerge (Chua et al, 2010). A simple mechanical heat pump arrangement are shown in Figure 5.1 with performance equations below.
Figure 5. 1: Schematic of a simple heat pump (heat pump) and performance equations.
Stage 1 2: Evaporator: 𝑄̇𝑒= 𝑚̇𝑤𝑓(ℎ2− ℎ1) (𝐸𝑞 − 5.1) Stage 2 3: Compressor: 𝑊̇𝑐 = 𝑚̇𝑤𝑓(ℎ3− ℎ2)𝜂 (𝐸𝑞 − 5.2) Stage 3 4: Condenser: 𝑄̇𝑐= 𝑚̇𝑤𝑓(ℎ3− ℎ4) (𝐸𝑞 − 5.3)
Stage 4 1: Throttle: No enthalpy change Heat Pump efficiency:
𝐶𝑂𝑃 =𝑄𝑐 𝑊𝑐 (𝐸𝑞 − 5.4) where: 𝑄̇𝑒= 𝐻𝑒𝑎𝑡 𝑙𝑜𝑎𝑑 𝑜𝑓 𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑜𝑟; 𝑄̇𝑐= 𝐻𝑒𝑎𝑡 𝑙𝑜𝑎𝑑 𝑜𝑓 𝑐𝑜𝑛𝑑𝑒𝑛𝑠𝑒𝑟; ℎ = 𝐸𝑡ℎ𝑎𝑙𝑝𝑦; 𝑚̇𝑤𝑓= 𝑀𝑎𝑠𝑠 𝑓𝑙𝑜𝑤𝑎𝑟𝑒𝑡 𝑜𝑓 𝑤𝑜𝑟𝑘𝑖𝑛𝑔 𝑓𝑙𝑢𝑖𝑑; 𝑊̇𝑐= 𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 𝑏𝑦 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟; and 𝐶𝑂𝑃 = 𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 𝑜𝑓 𝑃𝑒𝑟𝑓𝑜𝑟𝑚𝑎𝑛𝑐𝑒.
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The working principal of a heat pump is as follows; the heat pump absorbs heat at a low temperature in the evaporator, consumes power when the working fluid is compressed and rejects heat at a higher temperature in the condenser. The condensed working fluid is expanded and partially vaporises. The cycle then repeats. The typical temperature lift can be anywhere up to 25°C and is rarely higher as the capital and operating costs of compressors with high discharge pressures i.e. above 25 bar impose long economic payback periods that are unattractive (Smith, 2010). This is because the construction materials of compressors required to withstand high pressures are expensive. Also, the energy consumption can be high as the power requirements are proportional to the differential pressure of the compressor which is dependent on the discharge temperature, for example the discharge pressure for Ammonia at 100°C is 62.5 bar compared to a discharge temperature at 50°C is 20.3 bar. To address the issue of high energy consumption, there are heat pump designs that incorporate multi-stage compressors with intercooling which can reduce the overall power requirements (Song et al, 2017b; Smith, 2010; Kristensen and Korfitsen, 1998).
Within the context of heat integration, there have been several attempts to integrate heat pump technology as part of a Pinch-based methodology (Oluleye et al, 2016; Liew et al, 2016; Kwak et al, 2014; Becker et al, 2012; and Kapustenko et al, 2008). However, the majority of these approaches are at a total site level where the integration of heat pump technology takes place at the utility level and not for specific plants or in between plants (Oluleye et al, 2016, Liew et al, 2016, and Kwak et al 2014). As such, the scope for heat pump integration is limited in current approaches, as it does not consider low-grade heat recovery within processes and between processes.
5.2.2.3. Organic Rankine Cycle (ORC)
An Organic Rankine Cycle (ORC) is similar in layout to a heat pump. However, the key difference is the compressor is replaced with a turbine. The working principal of an ORC is as follows; the ORC absorbs heat in the evaporator causing the working fluid to evaporate, the fluid then moves into an expander where the working fluid expands and drives a turbine generating electricity, the working fluid returns to the condenser as a liquid. Overall, the key difference is the generation of electricity rather than an upgraded stream at a higher temperature.
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Currently, ORCs are a technology which are still in development compared to heat pump technology which is more widely applied across various sectors and can be readily purchased as modular systems (STAR Refrigeration, 2017). Some of the factors which limit ORCs include the source of heat, working fluid types and mechanical efficiency of turbine to generate electricity (Quoilin et al, 2013). A comparison of different ORC technologies is shown in Table 5.1.
Table 5. 1: Survey of ORC manufacturers (Source: Quoilin et al, 2013).
Manufacturer Country Power range (kWe) Heat source temperature (°C)
ORMAT US 200 – 70,000 150 – 300 Turboden Italy 200 – 2,000 100 – 300 Adoratec Germany 315 – 1,600 300 Opcon Sweden 350 – 800 <120 GMK Germany 50 – 5,000 120 – 350 Bosch KWK Germany 65 – 325 120 - 150 Turboden PureCycle US 280 91 – 149 GE CleanCycle US 125 >121
Cryostar France n/a 100 – 400
Tri-o-gen Netherlands 160 >350
Electratherm US 50 >93
As in the case of heat pump, applying an ORC as part of a heat integration strategy has been explored by a few researchers (Yu et al, 2017; Gutierrez-Arriaga et al, 2015; Kapil et al, 2012). For example, Kapil et al (2012) adopted a TSA methodology to integrate an ORC for low-grade heat above 110°C. Gutierrez-Arriaga et al (2015) adopted a mathematical optimisation methodology to integrate an ORC for low-grade heat above 90°C. Yu et al (2017) adopted a mathematic optimisation methodology to integrate an ORC for low-grade heat between 120 – 200°C. However, the current examples are considered for the petrochemical sectors where the quantity of low grade is considerably higher than the food industry. As such, the economics of ORC technology is more viable since a high volume of heat recovery can be achieved.