Hybrid PV and solar-thermal systems for domestic heat and power provision in the UK: Techno-economic considerations

A techno-economic analysis is undertaken to assess hybrid PV/solar-thermal (PVT) systems for distributed electricity and hot-water provision in a typical house in London, UK. In earlier work (Herrando et al., 2014), a system model based on a PVT collector with water as the cooling medium (PVT/w) was used to estimate average year-long system performance. The results showed that for low solar irradiance levels and low ambient temperatures, such as those associated with the UK climate, a higher coverage of total household energy demands and higher CO2 emission savings can be achieved by the complete coverage of the solar collector with PV and a relatively low collector cooling flow-rate. Such a PVT/w system demonstrated an annual electricity generation of 2.3 MW h, or a 51% coverage of the household’s electrical demand (compared to an equivalent PV-only value of 49%), plus a significant annual water heating potential of to 1.0 MW h, or a 36% coverage of the hot-water demand. In addition, this system allowed for a reduction in CO2 emissions amounting to 16.0 tonnes over a life-time of 20 years due to the reduction in electrical power drawn from the grid and gas taken from the mains for water heating, and a 14-tonne corresponding displacement of primary fossil-fuel consumption. Both the emissions and fossil-fuel consumption reductions are significantly larger (by 36% and 18%, respectively) than those achieved by an equivalent PV-only system with the same peak rating/installed capacity. The present paper proceeds further, by considering the economic aspects of PVT technology, based on which invaluable policy-related conclusions can be drawn concerning the incentives that would need to be in place to accelerate the widespread uptake of such systems. It is found that, with an electricity-only Feed-In Tariff (FIT) support rate at 43.3 p/kW h over 20 years, the system cost estimates of optimised PVT/w systems have an 11.2-year discounted payback period (PV-only: 6.8 years). The role and impact of heat-based incentives is also studied. The implementation of a domestic Renewable Heat Incentive (RHI) at a rate of 8.5 p/kW h in quarterly payments leads to a payback reduction of about 1 year. If this incentive is given as a one-off voucher at the beginning of the system’s lifetime, the payback is reduced by about 2 years. With a RHI rate of 20 p/kW h (about half of the FIT rate) PVT technology would have approximately the same payback as PV. It is concluded that, if primary energy (currently dominated by fossil fuels) and CO2 emission minimisation are important goals of national energy policy, PVT systems offer a significantly improved proposition over equivalent PV-only systems, but at an elevated cost. This is in need of careful reflection when developing relevant policy and considering technology incentivation. Currently, although heat outweighs electricity consumption by a factor of about 4 (by energy unit) in the UK domestic sector, the support landscape has strongly favoured electrical microgeneration, being inclined in favour of PV technology, which has been experiencing a well-documented exponential growth over recent decades

Distributed heat conversion technologies based on organic fluid cycles for a high-efficiency and sustainable energy future

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.This paper presents and discusses the emergence of two distinct classes of energy conversion systems based on thermodynamic vapour-phase heat engine cycles undergone by organic working fluids, namely organic Rankine cycles (ORCs) and two-phase thermofluidic oscillators (TFOs). Each type of system has its own distinctive characteristics, advantages and limitations. ORCs are a more well-established and mature technology, are more efficient, especially with higher temperature heat sources and at larger scales, whereas TFOs have the potential to be more cost-competitive, in particular at lower temperatures and at smaller scales. Specifically, ORC systems are particularly well-suited to the conversion of low- to mediumgrade heat (i.e. hot temperatures up to about 300 – 400 °C) to mechanical or electrical work, and at an output power scale from a few kW up to 10s of MW. Thermal efficiencies in excess of 25% are achievable at the higher temperatures, and efforts are currently in progress to develop improved ORC systems by focussing on advanced architectures, working fluid selection, heat exchangers and expansion machines. Correspondingly, TFO systems are a more recent development aimed at the affordable conversion of low-grade heat (i.e. hot temperatures from 20 – 30 °C above ambient, up to about 100 – 200 °C) to hydraulic work for fluid pumping and/or pressurisation. Ultimately, TFOs could emerge at scales of up to a few hundred W and with a thermal efficiency of the order of a few % points. The two energy conversion systems are complementary, and together have a great potential to be used for distributed power generation and improved energy efficiency, leading to primary energy (i.e. fuel) use and emission minimisation. Relevant applications and fields of use include the recovery of waste heat and conversion to useful work including mechanical, hydraulic or electrical energy, or the effective utilisation of renewable energy sources such as geothermal, biomass/biogas and solar energy.dc201

A UK-based assessment of hybrid PV and solar-thermal systems for domestic heating and power: System performance

AbstractThe goal of this paper is to assess the suitability of hybrid PVT systems for the provision of electricity and hot water (space heating is not considered) in the UK domestic sector, with particular focus on a typical terraced house in London. A model is developed to estimate the performance of such a system. The model allows various design parameters of the PVT unit to be varied, so that their influence in the overall system performance can be studied. Two key parameters, specifically the covering factor of the solar collector with PV and the collector flow-rate, are considered. The emissions of the PVT system are compared with those incurred by a household that utilises a conventional energy provision arrangement. The results show that for the case of the UK (low solar irradiance and low ambient temperatures) a complete coverage of the solar collector with PV together with a low collector flow-rate are beneficial in allowing the system to achieve a high coverage of the total annual energy (heat and power) demand, while maximising the CO2 emissions savings. It is found that with a completely covered collector and a flow-rate of 20L/h, 51% of the total electricity demand and 36% of the total hot water demand over a year can be covered by a hybrid PVT system. The electricity demand coverage value is slightly higher than the PV-only system equivalent (49%). In addition, our emissions assessment indicates that a PVT system can save up to 16.0tonnes of CO2 over a lifetime of 20years, which is significantly (36%) higher than the 11.8tonnes of CO2 saved with a PV-only system. All investigated PVT configurations outperformed the PV-only system in terms of emissions. Therefore, it is concluded that hybrid PVT systems offer a notably improved proposition over PV-only systems

Simultaneous capacitive probe and planar laser-induced fluorescence measurements in downwards gas-liquid annular flow

Various experimental techniques are available to analyse two-phase flows. The measurement concept and the applicability can however vary greatly. Prime examples from the opposite spectrum are planar laser-induced measurements (PLIF) versus capacitive probes. PLIF is an optical technique, it is non-intrusive but optical access is necessary. PLIF based measurements are known for their high temporal and spatial resolution but require a costly set-up. In contrast, the capacitive probe is another non-intrusive technique but doesn’t require optical access. It is fairly easy to set up, robust, and is cheap to construct. To rigorously compare both techniques, simultaneous PLIF and capacitive probe measurements are made in this work. As the void fraction is one of the key parameters to classify flow regimes, both techniques are compared on the determination of the void fraction. This is done for a limited set of six annular flows. The experiments were performed in a downward annular-flow facility with demineralized water - air as working medium. The first results indicate that both techniques give similar volume averaged void fractions. The mean absolute percentage error and the maximum relative error between both techniques are 0.30% and 0.54%, respectively. The PLIF measurements confirm however to have a better spatial resolution

ORC cogeneration systems in waste-heat recovery applications

The performance of organic Rankine cycle (ORC) systems operating in combined heat and power (CHP) mode is investigated. The ORC-CHP systems recover heat from selected industrial waste-heat fluid streams with temperatures in the range 150°C-330°C. An electrical power output is provided by the expanding working fluid in the ORC turbine, while a thermal output is provided by the cooling water exiting the ORC condenser and also by a second heat-exchanger that recovers additional thermal energy from the heat-source stream downstream of the evaporator. The electrical and thermal energy outputs emerge as competing objectives, with the latter favoured at higher hot-water outlet temperatures and vice versa. Pentane, hexane and R245fa result in ORC-CHP systems with the highest exergy efficiencies over the range of waste-heat temperatures considered in this work. When maximizing the exergy efficiency, the second heat-exchanger is effective (and advantageous) only in cases with lower heat-source temperatures (< 250°C) and high heat-delivery/demand temperatures (> 60°C) giving a fuel energy savings ratio (FESR) of over 40%. When maximizing the FESR, this heat exchanger is essential to the system, satisfying 100% of the heat demand in all cases, achieving FESRs between 46% and 86%

Off-design comparison of subcritical and partial evaporating ORCs in quasi-steady state annual simulations

The subcritical ORC (SCORC) is considered the industry standard due to its simple configuration, acceptable efficiency and low costs. However, it is known that alternative ORC configurations have the potential to increase efficiency. A cycle modification which closely resembles the SCORC is the partial evaporating ORC (PEORC), where a two-phase mixture of liquid-vapour enters the expander instead of superheated vapour. In theoretical studies at design conditions, higher power outputs are achieved for the PEORC compared to the SCORC. This work aims to go a step further by investigating the performance of the SCORC and PEORC under time-dependent operating conditions. A direct comparison between the SCORC and PEORC is made for identically sized systems using as input the waste heat stream of a waste incinerator plant and the changing ambient conditions. Performance maps of both cycle configurations are compiled and the benefit of an expander operating at variable speed is briefly discussed. The results indicate that for the specific case under investigation, the PEORC has an increased annually averaged net power output of 9.6% compared to the SCORC. Use of annually averaged input conditions results in an overestimation of the net power output for both the SCORC and PEORC, and furthermore, the relative improvement in power output for the PEORC is reduced to 6.8%. As such, the use of time-averaged conditions when comparing cycle architectures should preferably be avoided

Off-design operation of ORC engines with different heat exchanger architectures in waste heat recovery applications

Organic Rankine cycle (ORC) engines in waste-heat recovery applications experience variable heat-source conditions (i.e. temperature and mass flow rate variations). Therefore maximising the ORC system performance under off-design conditions is of key importance, for the financial viability and wider adoption of these systems. In this paper, the off-design performance of an ORC engine with screw expander and two heat exchanger (HEXs) architectures is investigated, while recovering heat from an internal combustion engine (ICE). Firstly, nominal system sizing results indicate that the screw expander isentropic efficiency exceeds 80%, while the plate HEXs (PHEXs) heat transfer area requirements are 50% lower, than the respective ones for double pipe (DPHEX) design. Next, the ORC engine operation is optimised at part-load (PL) ICE conditions. Although, the HEXs heat transfer coefficients decrease with part-load, the total HEX effectiveness increases, due to higher temperature difference across the working fluids. Findings also reveal that the PHEX performance is less sensitive to the off-design operation. Off-design power output maps indicate that the optimised ORC engine PL reduces to 72%, for ICE PL of 60%, while ORC engines with PHEXs generate slightly more power, for the same heat source conditions. Overall, the modelling tool developed can predict ORC performance over an operating envelope and allows the selection of optimal designs and sizes of ORC HEXs and expanders

Combined PLIF-IR thermal measurements of wavy film flows undergoing forced harmonic excitation

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.A combined PLIF/IR thermography technique was developed and employed towards the measurement of unsteady and conjugate heat transfer in thin, gravity-driven falling liquid film flows (with and without flow pulsation) over an inclined heated metal foil. Simultaneous, local film thickness, film and substrate temperature, heat flux exchanged with a heated foil and heat transfer coefficient results are reported for a range of electrically applied heat input values, flow Reynolds (Re) numbers and flow pulsation frequencies. Moreover, interfacial wave velocities were calculated from cross-correlations across successive thickness profiles. Results concerning the instantaneous and local heat transfer coefficient variation and how this is correlated with the instantaneous and local film thickness variation (waves) suggest that the heat transfer coefficient experiences an enhancement in thinner films. The particular observation is most probably attributed to a number of unsteady flow phenomena within the wavy fluid films that are not captured by the steady analysis. At low flow Re number values the mean Nusselt (Nu) was around 2.5, in agreement with laminar flow theory, while at higher Re values, higher Nu were observed. Finally, lower wave amplitude intensities were associated with higher heat transfer coefficient fluctuation intensities.cf201

A framework for the analysis of thermal losses in reciprocating compressors and expanders

Paper presented at the 9th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Malta, 16-18 July, 2012.The establishment of a reliable methodology for the design of a host of practical thermal and thermodynamic systems demands the development of a framework that can accurately predict their performance by accounting formally for the underlying unsteady and conjugate heat transfer processes that are undergone as part of their operation. The purpose of this paper is to present a framework that includes such a description and to show results from its application to the characterisation of a reciprocating compression and expansion process. Specifically, an unsteady and conjugate heat transfer model is proposed that solves the one-dimensional unsteady heat conduction equation in the solid simultaneously with the first law in the gas phase, with an imposed heat transfer coefficient taken from relevant experimental studies in the literature. This model is applied to the study of thermal losses in gas springs. Beyond the explicit inclusion of conjugate heat transfer, the present model goes beyond previous efforts by considering the case of imposed volumetric compression and by allowing the resulting gas pressure to vary accordingly. Notable effects of the solid walls of the gas spring are revealed, with worst case thermodynamic cycle losses of up to 14% (relative to equivalent adiabatic and reversible processes) for cases in which unfavourable solid and gas materials are selected, and closer to 11% for more common material choices. The contribution of the solid towards these values, through the dimensionless thickness of the gas spring cylinder wall, is about 8% and 2%, respectively, showing a non-monotonic trend with the thermodynamic losses; increasing with increased solid thickness, reaching a maximum and then decreasing again. These results suggest strongly that, in designing highefficiency reciprocating machines, the full conjugate and unsteady problem must be considered and that the role of the solid in determining the performance of the cycle undergone by the gas cannot, in general, be neglected.dc201

Potential of Organic Rankine Cycles (ORC) for waste heat recovery on an Electric Arc Furnace (EAF)

The organic Rankine cycle (ORC) is a mature technology to convert low temperature waste heat to electricity. While several energy intensive industries could benefit from the integration of an ORC, their adoption rate is rather low. One important reason is that the prospective end-users find it difficult to recognize and realise the possible energy savings. In more recent years, the electric arc furnaces (EAF) are considered as a major candidate for waste heat recovery. Therefore, in this work, the integration of an ORC coupled to a 100 MWe EAF is investigated. The effect of working with averaged heat profiles, a steam buffer and optimized ORC architectures is investigated. The results show that it is crucial to take into account the heat profile variations for the typical batch process of an EAF. An optimized subcritical ORC (SCORC) can generate an electricity output of 752 kWe with a steam buffer working at 25 bar. However, the use of a steam buffer also impacts the heat transfer to the ORC. A reduction up to 61.5% in net power output is possible due to the additional isothermal plateau of the steam