Building Load Profiles Research Articles

An analysis of solar assisted ground source heat pumps in cold climates

Exploiting renewable energy sources for air-conditioning has been extensively investigated over recent years, and many countries have been working to promote the use of renewable energy to decrease energy consumption and CO2 emissions. Electrical heat pumps currently represent the most promising technology to reduce fossil fuel usage. While ground source heat pumps, which use free heat sources, have been taking significant steps forward and despite the fact that their energy performance is better than that of air source heat pumps, their development has been limited by their high initial investment cost. An alternative solution is one that uses solar thermal collectors coupled with a ground source heat pump in a so-called solar assisted ground source heat pump.A ground source heat pump system, used to heat environments located in a cold climate, was investigated in this study. The solar assisted ground source heat pump extracted heat from the ground by means of borehole heat exchangers and it injected excess solar thermal energy into the ground. Building load profiles are usually heating dominated in cold climates, but when common ground source heat pump systems are used only for heating, their performance decreases due to an unbalanced ground load. Solar thermal collectors can help to ensure that systems installed in cold zones perform more efficiently. Computer simulations using a Transient System Simulation (TRNSYS) tool were carried out in six cold locations in order to investigate solar assisted ground source heat pumps. The effect of the borehole length on the energy efficiency of the heat pump was, in particular, analyzed. Finally, a suitable control strategy was implemented to manage both the solar thermal collectors and the borehole heat exchangers.

Generalised water flow rate control strategy for optimal part load operation of ground source heat pump systems

Abstract A control strategy was developed to predict optimal ground source heat pump water flow rates under part load operation. Using this strategy, optimal flow rates are calculated from available design data and minimal on-line measurements. The optimal control strategy was evaluated with validated single speed and tandem speed ground source heat pump simulation models in both heating and cooling mode. A range of building load profiles were implemented when analysing heat pump system performance. The optimal strategy was shown to result in an improvement in system performance when compared to nominal flow rate operation, particularly for low load conditions. Due to the prevalence of low load operation over a heating or cooling season, the seasonal system performance (SPF3) was shown to increase by between 20% and 40% when using the optimal strategy.

Designing coaxial ground heat exchangers with a thermally enhanced outer pipe

Ground heat exchangers installed in boreholes are an expensive component of a ground-coupled heat pump system, where minimizing the borehole length with appropriate materials and configuration can reduce the overall cost of the system. Design calculations performed analytically indicate that the coaxial pipe configuration can be more advantageous than the single U-pipe configuration to reduce the total borehole length of a system. A decrease of the borehole thermal resistance and an increase of the thermal mass of water contained in the coaxial exchanger helped to reduce borehole length by up to 23% for a synthetic building load profile dominated by cooling. The decrease of the borehole thermal resistance was achieved with an outer pipe made of thermally enhanced high-density polyethylene, where the thermal conductivity is 0.7 W m-1 K-1. The coaxial configuration requires further investigations of the technical barriers related to the installation of ground heat exchangers in the field.

Changes in heat load profile of typical Danish multi-storey buildings when energy-renovated and supplied with low-temperature district heating

Denmark has a long-term objective of being free of fossil fuels by 2050, with the energy supply mix for buildings being fossil-free by 2035. Energy consumption for existing buildings needs to be decreased concurrent with the conversion from fossil-fuel supply to renewable-energy (RE) supply. When end-use savings are implemented in buildings concurrent with the application of low-temperature district heating (LTDH), the heat profiles of the buildings will change. Reducing peak loads is important, since this is the dimensioning foundation for future district heating systems. To avoid oversized RE-based capacity, a long-term perspective needs to be taken. Applying LTDH in existing buildings without changing the heating system implies reduced radiator performance, so it is of great importance that acceptable comfort temperatures can still be provided. The results indicate that it is possible to apply LTDH most of the year without compromising on thermal comfort if energy renovation is also implemented.

Study on the Optimization of Multi-Factor Influencing Ice Thermal Storage Air Conditioning System Economy

On the basis of dynamic hourly cooling load, an optimal target function was taken as the minimization of sum of system fist cost and operation cost etc., within life cycle for ice thermal air conditioning system, the mathematics model of system optimization was established, and solved based on the external point penalty function method with step accelerate algorithm, using VB.NET for writing calculation program, taking ice thermal air conditioning systems for typical public buildings (an office building, a 4 star level hotel, a large scale store) as study objectives, the optimal calculations and analysis were carried out to multi-factor such as building load profile characteristics, discharge rate, and electrical price structure. Results in this paper provide technical information and reference for designer and operator.

A Study on the Optimization of Control Strategy of a Thermal Energy Storage System for Building Air-Conditioning

Many new buildings in Malaysia are installed with a centralized air-conditioning system integrated with thermal energy storage system. Building owners are motivated to reduce the operational costs and are being encouraged by the attractive tariff offered by the utility power provider. However, without proper control the operational costs can be even more than the conventional system due to the higher maximum demand charge. This paper describes the results of an investigation on the actual operating data of a centralized air-conditioning plant integrated with thermal energy storage system operated under two different control strategies. Comparisons on the electrical power consumption, overall plant efficiency and chiller coefficient of performance were carried out by using two identical building load profiles. The results show that the supply pressure control is better than delta-temperature control. The electrical power consumption, overall plant efficiency and chiller coefficient of performance for supply pressure control are 30.1%, 33.4% and 33.5% better than the delta-temperature control, respectively. Also from the general observation on the data it was found out that the storage tank is under utilised. The charging capacity is usually less than the nominal storage capacity. In order to further increase the performance characteristic the plant operation schedule need to be revised.

A stochastic method for battery sizing with uninterruptible-power and demand shift capabilities in PV (photovoltaic) systems

This paper presents a stochastic simulation using Monte Carlo technique to size a battery to meet dual objectives of demand shift at peak electricity cost times and outage protection in BIPV (building integrated photovoltaic) systems. Both functions require battery storage and the sizing of battery using numerical optimization is popularly used. However, the weather conditions, outage events and demand peaks are not deterministic in nature. Therefore, the sizing of battery storage capacity should also be based on a probabilistic approach. The Monte Carlo simulation is a rigorous method to sizing BIPV system as it takes into account a real building load profiles, the weather information and the local historical outage distribution. The simulation is split into seasonal basis for the analysis of demand shifting and outage events in order to match the seasonal weather conditions and load profiles. Five configurations of PV (photovoltaic) are assessed that cover different areas and orientations. The simulation output includes the predicted PV energy yield, the amount of energy required for demand management and outage event. Therefore, consumers can base sizing decisions on the historical data and local risk of outage statistics and the success rate of meeting the demand shift required. Finally, the economic evaluations together with the sensitivity analysis and the assessment of customers’ outage cost are discussed.

Influence of the power contribution of a grid-connected photovoltaic system and its operational particularities

This paper shows the relationship between the energy contribution of an 11.07 kWp grid-connected photovoltaic system and the particularities of its operation in parallel with a conventional electric distribution grid. Taking the experimental results as a starting point, it was verified that the contribution to the building's annual consumption is about 55% of the total load. The experimental results show how this kind of device influences the power quality parameters of the building: total harmonic distortion (THD), power factor (FP), and RMS voltage. The building power factor can be altered in a significant way depending on the system's production capacity and on the building load profile. Therefore, if a great number of PV systems are connected to the distribution grid, the reverse power flow coming from those systems can cause an increase in the voltage of a feeder and eventually cross the allowed upper threshold.

Optimising the economic viability of grid-connected photovoltaic systems

The impact of photovoltaic (PV) array size, orientation, inclination, load profile, electricity buying price, feed-in tariffs, PV/inverter sizing ratio (‘sizing ratio’) and PV/inverter cost ratio (‘cost ratio’) on the economic viability of a grid-connected PV system was investigated using a validated TRNSYS simulation model. The results showed that the fractional load met directly by a PV system depends on matching between PV supply and building load profile, sizing ratio and PV inclination. The profitability of a grid-connected PV system increases if the PV system is sized to reduce excess PV electrical energy fed to the grid when the feed-in tariff is lower than electricity buying price. The effect of feed-in tariffs on PV saving for selected European countries has been shown. The cost of the PV electricity depends on sizing ratio, PV and inverter lifetimes, cost ratio, PV inclination and financial parameters. The effect of cost ratio on the optimum PV/inverter sizing ratio is less significant when the cost ratio lies within 7–11. The minimum PV electricity cost at low and high insolation conditions were obtained for sizing ratios of 1.6 and 1.2, respectively. The lowest PV electricity cost was found for surface slopes within 30–40° for the selected European locations. The PV electricity cost for cost ratio of 5 and 13 varied from 0.44–0.85 €kWh −1 to 0.38–0.76 €kWh −1, respectively within high to low insolation conditions when the PV module unit cost, market discount rate, PV size, PV lifetime and inverter lifetime were assumed to be 6.5 €W p −1, 3%, 13 kW p, 20 years and 10 years, respectively.

Environmental performance and economic analysis of all-variable speed chiller systems with load-based speed control

There are increasing views on implementing all-variable speed chiller plants in place of conventional constant speed plants. Supporters of these views claim that all-variable speed chiller systems can operate much more efficiently at part load in response to changes in building cooling load. This paper introduces load-based speed control for all-variable speed plants to optimize their environmental performance. Thermodynamic-behaviour chiller system models were developed to perform environmental assessment (in terms of annual electricity and water consumption) for typical constant speed and all-variable speed chiller systems operating for the cooling load profile of a local office building. Operating cost differences between the two systems were calculated and compared in an economic analysis. Applying load-based speed control to the variable speed chiller plant can decrease the annual total electricity use by 19.7% and annual water use by 15.9% relative to the corresponding constant speed plant. The significance of this study is to provide more insights into how to make chiller systems more sustainable.

Improved energy performance of air cooled centrifugal chillers with variable chilled water flow

This paper considers how to apply optimum condensing temperature control and variable chilled water flow to increase the coefficient of performance (COP) of air cooled centrifugal chillers. A thermodynamic model for the chillers was developed and validated using a wide range of operating data and specifications. The model considers real process phenomena, including capacity control by the inlet guide vanes of the compressor and an algorithm to determine the number and speed of condenser fans staged based on a set point of condensing temperature. Based on the validated model, it was found that optimizing the control of condensing temperature and varying the evaporator’s chilled water flow rate enable the COP to increase by 0.8–191.7%, depending on the load and ambient conditions. A cooling load profile of an office building in a subtropical climate was considered to assess the potential electricity savings resulting from the increased chiller COP and optimum staging of chillers and pumps. There is 16.3–21.0% reduction in the annual electricity consumption of the building’s chiller plant. The results of this paper provide useful information on how to implement a low energy chiller plant.

Applications of one-cycle control to improve the interconnection of a solid oxide fuel cell and electric power system with a dynamic load

Adding distributed generation (DG) is a desirable strategy for providing highly efficient and environmentally benign services for electric power, heating, and cooling. The interface between a solid oxide fuel cell (SOFC), typical loads, and the electrical grid is simulated in Matlab/Simulink and analyzed to assess the interactions between DG and the electrical grid. A commercial building load profile is measured during both steady-state and transient conditions. The load data are combined with the following models that are designed to account for physical features: a One-Cycle Control grid-connected inverter, a One-Cycle Control active power filter, an SOFC, and capacitor storage. High penetration of DG without any power filter increases the percentage of undesirable harmonics provided by the grid, but combined use of an inverter and active power filter allows the DG system interconnection to improve the grid tie-line flow by lowering total harmonic distortion and increasing the power factor to unity.

Modelling Transient Occupant Loads for Offices

Occupant load profile of the space is an essential parameter for planning facilities, operations, water and energy consumptions of a building. In view of growing concerns on the effectiveness of building designs, in terms of space utilization, equipment sizing, energy performance and consumption, occupant loads have been specified in some building regulations, design guides and codes of practice. However, an occupant load would be varied from time to time and difficulties would be encountered in determining it to certain accuracy at the building design stage. This paper proposes a mathematical model to generate time-variant design occupant loads using Monte-Carlo simulations, with the model parameters determined from a year-round occupant load assessment in a typical high-rise office building in Hong Kong. Example occupant load profiles for some offices were generated to demonstrate the application of the model. The model would be useful in determining the occupant load profiles of some office buildings in Hong Kong.

Low-energy design for air-cooled chiller plants in air-conditioned buildings

Chillers are widely used for cooling buildings in the subtropical regions at the expense of considerable energy. This paper discusses how the number and size of air-cooled chillers in a chiller plant should be designed to improve their energy performance. Using an experimentally verified chiller model, four design options were studied for a chiller plant handling the cooling load profile of an office building. Using chillers of different sizes is desirable to increase the number of steps of total cooling capacity. This enables the chillers to operate frequently at or near full load to save chiller power. Pumping energy can also be saved because of the improved control of chilled water flow whereby the chilled water supplied by the staged chillers can match with that required by air side equipment for most of the operating time. It is estimated that the annual electricity consumption of chiller plants could drop by 9.4% with the use of unequally sized chillers. The findings of this research will offer guidance on how to select chillers of different sizes for a low-energy chiller plant.

Future economics of the fuel cell housing market

This paper examines how a market of small-scale stationary fuel cells of up to 20 kW could look like, if, costs of stationary fuel cell systems allow market entry. This paper analyses what the market potential for this technology would be, what types of residential buildings might be most attractive, and what would be the quantitative changes in the fuel and the power market. Finally, does the perspective of stationary fuel cells offer a business opportunity for power and gas distribution companies? The methodology of this paper differs from that of other studies in that we model the operation of stationary fuel cells on the basis of 15 min power load profiles of individual buildings. From these we draw synthetic functions describing the fuel cell power output/natural gas input, as a function of a number of specific properties of individual buildings. We then develop a statistical distribution of these properties of the residential building stock in Germany (15 million units), finally using a Monte Carlo simulation the relevant market shares are calculated. The methodology that is developed here has an advantage in that it is flexible and can be applied for different population of buildings. We know, for example, that the results would differ between rural and urban areas. The model may reflect these differences thus allowing deeper insights into future fuel cell housing markets.

Sensitivity analysis of energy performance of office buildings

The sensitivity of energy performance of office buildings in Hong Kong is examined. Basic principles of sensitivity methods for the study of building energy performance are explained. The DOE-2 building energy simulation program is used on a generic model of an office building to generate data for the study. Important input design parameters are identified and analysed from points of view of annual building energy consumption, peak design loads and building load profiles. It is believed that sensitivity techniques are useful for assessing thermal responses of buildings and data variability in building energy simulation. However, the results should be interpreted in context with clear understanding of the implications and limitations.

96/00814 The control of ice-storage systems

The tradeoffs between chiller and tank capacity for different load profiles and two storage strategies are evaluated in this article. Air-conditioning systems that employ ice storage incorporate equipment that produces ice during one period and melts it in another period to provide cooling for the building. In designing such systems, there are two basic strategies to consider: full-load and partial-load. In a full-load strategy, the entire daytime cooling energy is met using only cooling supplied by the ice storage tank. In a partial-load strategy, the chiller and the storage are used simultaneously to meet the load. With a full-load strategy, the tank capacity must be sufficient to meet the entire energy requirement, and the chiller capacity must be sufficient to recharge the tank during the night-time. In a partial-load strategy, a smaller chiller and tank than that for a full-load strategy are required, and there are many combinations of the two that will meet a given building load. The design and sizing of the components of an ice storage system depend not only on the desired strategy and total daily cooling energy but also on other factors. The maximum load dictates the amount of cooling required at any time. Themore » cooling rate provided by an ice filled tank is not constant, but decreases as the ice inventory drops, and the time that the maximum load occurs is important. Thus, there is an interaction between the building load profile and tank size. In addition, the flow rate of the circulating fluid through the tank and the cooling coil may limit the supply air temperatures that may be reached. Thus, the circulating fluid flow rate must be sufficient to provide the desired rates of cooling to the building. The challenge to the design engineer is to size the components to meet the building load at all times at the lowest system cost. Effective designs must acknowledge the dynamic performance of the ice storage system.« less

Validation of a computer aided design tool for assessing building thermal performance characteristics

Computer aided design (CAD) tools of various complexities are being used extensively to predict the energy performance of buildings. A simple user-friendly CAD software was employed to estimate the cooling load profile of an energy efficient institutional building as a case study. At the same time, its air conditioning load pattern was measured and was found to be in good agreement with the predicted values. Parametric studies were carried out to assess the contribution of the various features incorporated in this energy efficient building. The results show that, by using such a simple tool, it is possible to identify the modifications which contribute to appreciable energy and demand reductions, and to gauge their economic viability. Such CAD tools are suitable not only for analysing new constructions at the design stage, but also for retrofitting of existing buildings.

The Utility Looks at Load Profiles of All-Electric Buildings

It is becoming the standard both to heat and to cool occupied space in buildings. Electricity is used almost universally for cooling, and increasingly it is also used to heat. The integration of these loads with other power requirements is supplied by the utility. The magnitude and profile of the load determines the facilities needed by the utility. This paper supplies considerable data on the characteristics of electric heating and cooling loads. Due to the large internal heat gain of most commercial buildings, cooling must be supplied during relatively low outside temperatures. Correspondingly, heating is necessary over a relatively narrow temperature range. This factor results in considerable diversity between these loads and the utility system load. This diversity varies with both the hours of the day and days of the week. The geographic location and the magnitude of system heating and cooling loads will determine how representative the data presented are for any specific area. 37190.