Solar driven water heating systems for medium-rise residential buildings in urban mediterranean areas

Highlights

• Solar driven water heating systems were compared with an electric water heater.

• Hot water provision for an urban medium-rise residential building was investigated.

• Annual net electricity consumptions and life-cycle costs were assessed.

• The PV/T and PV-HPWH systems have net positive electricity production.

• The solar thermal and the PV/T water heating systems have lower life-cycle costs.

Abstract

International Energy Agency reported that buildings are accountable for one-third of the global final energy demand in 2017. On-site renewable energy generation can reduce buildings' grid electricity consumptions. Medium-rise buildings located in urban areas have limited available rooftop or facade surfaces, thus solar driven technologies such as solar thermal, photovoltaics (PV) or photovoltaics/thermal (PV/T) are in competition for the available space. This investigation aims to compare available solar driven water heating systems in the market, suitable to replace the conventional electric water heater for a multi-residential building. Under the present study, solar PV electric water heating system (S1), solar thermal water heating system with electric boosting (S2), solar PV/T water heating system with electric boosting (S3) and integrated solar PV and heat pump water heating system (S4) are investigated. The performance parameters compared are the annual net electricity consumption from the grid and the net present value of life-cycle cost (LCC) for 20 years life. Results reveal that S3 and S4 have ‘net’ positive electricity production but higher initial costs, compared to the other systems. For buildings located in colder climates, S2 has lower LCC compared to S3 but for warmer climates the LCC of S3 is the lowest.

Introduction

Buildings are responsible for 38% of the total final energy consumption in the European Union (EU), two-thirds of which is for residential buildings [1,2]. They present a great opportunity and at the same time, they are a significant barrier towards the 2030 climate target of the EU for 40% cuts in greenhouse gas (GHG) emissions, compared to 1990 levels. To meet the target, special consideration must be given to multi-residential buildings, due to the limited unshaded rooftop and façade areas for the installation of renewable energy systems (RES).

The commonly applied technologies for on-site production of renewable energy in buildings are the solar thermal collectors and the photovoltaic (PV) panels. Solar thermal collectors convert solar radiation into usable heat, with a typical efficiency of up to 80% [3], while solar PV panels have an energy conversion efficiency of 12%–18% [4]. Due to higher efficiencies, solar thermal technologies are applied for space and water heating, which are the most energy-intensive end-uses in residential buildings [1]. Many support schemes for PV development have been launched in most European countries, leading to rapid growth over the last decades. Even though photovoltaics/thermal (PV/T) systems provides combined efficiencies they are not commercially attractive solutions yet [5].

Various studies compared the performance and costs of various water heating systems, or combination with electricity production: PV and PV/T [[5], [6], [7], [8]]; PV and solar thermal [9]; solar thermal, PV and PV/T [[10], [11], [12], [13]]; PV/T [14] and solar assisted heat pump systems [[15], [16], [17]]. Results mainly depend on the end use, the climate and the prices in the country of application. The higher the solar radiation available, the better the financial viability of the project [18]. Low-temperature applications, such as domestic hot water (DHW), show financial improvements [9].

In general, the individual electrical and thermal efficiencies of PV/T systems are lower compared to side-by-side PV and solar water-heating systems [10,19]. According to da Silva and Fernandes [17], to obtain the same thermal and electric yields with side-by-side PV and solar thermal panels, an additional 60% roof area is required.

Good, Andresen and Hestnes [20] addressed the issue of buildings’ limited rooftop area for the installations of RESs for water heating. They compared annual net electricity consumption of three solar energy systems for a Norwegian single-family house. They reported that high-efficiency PV modules had better performance among market-available technologies. The combined use of solar thermal collectors and PV modules came second. The limited rooftop area issue was also addressed by Herrando et al. [8] and Herrando and Markides [5]. Both investigations compared the performance of a PV/T system with a PV system installed in a typical three-bedroom terrace house in London. The PV/T system had more energy output and less GHG emissions. It met more than 50% of the total electricity demand and 36% of the DHW demand, while the PV system met only 49% of the electricity demand. However, PV system had the half the discounted payback time of the PV/T system.

To select among the competing solar technologies for DHW loads of a 40 m² roof area of a house in Montreal, Delisle and Kummert [11] compared PV/T collectors with solar thermal and PV modules, based on the energy produced and financial benefits. They argued that the benefits of the PV/T system depend on the end-use of the thermal and electrical energy and the type of equipment it replaces.

To identify the solar panel area requirement for a 160 L DHW load in Prague, Matuska and Sourek [9] compared a PV water heating and a solar thermal system in terms of the energy yields and financial parameters. The PV water heating system requires 13.2 m² area, while the solar thermal system requires 4.5 m². They found that the solar thermal system achieved 61% solar fraction, supplying 25% more energy than the equivalent PV-DHW system. They reported that solar thermal DHW systems were financially better than PV-DHW systems available at the market. Matuska [12] also compared the performance and cost of a market available PV/T system with a solar thermal system, a PV system and their combination in several configurations, for a multi-residential building in Wurzburg, Germany. The PV/T system outperformed all compared systems in terms of energy production. However, its market price needed to be halved in order to be financially competitive.

Seven solar-assisted water heating systems, with the same solar collector area (6 m²) and the same water storage tank capacity (280 L), were compared by Li and Yang [17] for Hong Kong, in terms of financial viability and total equivalent warming impact. The water heating systems investigated are a conventional electric, a conventional gas, a solar electric boosted, a solar gas boosted, a solar-assisted air-source heat pump (SA-ASHP), a solar-assisted water-source heat pump (SA-WSHP) and a direct expansion solar-assisted heat pump (DX-SAHP). They found that the payback period of the solar water heater boosted by electricity is 3.4 years and that of the solar water heater boosted by gas is 3.8 years, compared to the conventional electric water heater. The SA-ASHP’ payback time was 4.3 years and that of SA-WSHP was 6.9 years. As for the total equivalent warming impact, the conventional electric water heater has the highest, followed by the DX-SAHP and the SA-WSHP. The solar water heater boosted by gas has the lowest, followed by the conventional gas water heater.

Aye, Charters and Chaichana [15] investigated the effects of location and climate on the performance of alternative water heating systems. They compared a thermosyphon solar water heater, an air-source heat pump water heater (HPWH) and a solar heat pump (HP) water heater in several Australian cities, in terms of grid electricity consumption, GHG emissions and life-cycle cost (LCC). The thermosyphon water heater was the most appropriate system for locations of high solar radiation, producing less GHG emissions. In areas of lower solar radiation, a solar HPWH was found to be preferable due to the decreased LCC.

Biaou and Bernier [16] examined four renewable energy systems for producing DHW in two climates (Montreal and Los Angeles), by identifying the required solar panel areas to achieve net zero energy consumption. The study was applied in a detached home of 156 m² floor area. The alternatives were: a regular electric hot water tank, the de-superheater of a ground-source heat pump (GSHP) with electric boosting, thermal solar collectors with electric boosting and a HPWH indirectly coupled with a space conditioning GSHP. The electricity consumed by the electric booster was supplied by PV panels. Based on the simulation results, heating DHW with solar thermal collectors was the best solution for both climates. Optimising the size of the solar thermal and PV panels, the authors concluded that for Los Angeles, 4.5 m² thermal solar collectors and 2.06 m² PV panels were needed. The system's payback period was found to be 11 years. For Montreal, 12 m² of solar thermal collectors and 5.2 m² of PV panels had to be installed. However, this system was not cost-effective, having a payback period of 29 years.

Even though cities’ population density increases and the ratio of available rooftop area per habitable area decreases, the problem of optimising the use of the limited roof area for the on-site production of solar energy has not been sufficiently addressed. The present study responds to this issue by comparing alternative solar driven water heating systems for DHW, meeting hot water loads in a cost-effective way.